Among prokaryotes, obligate aerobes require O2, obligate 3. Which of the following statements is not true?
anaerobes are poisoned by O2, and facultative anaerobes (A) Archaea and bacteria have different membrane lipids.
can survive with or without O2. (B) The cell walls of archaea lack peptidoglycan.
(C) Only bacteria have histones associated with DNA.
Unlike eukaryotes, prokaryotes can metabolize nitrogen in many (D) Only some archaea use CO2 to oxidize H2, releasing methane.
different forms. Some can convert atmospheric nitrogen to
ammonia, a process called nitrogen fixation. 4. Which of the following involves metabolic cooperation among
prokaryotic cells?
Prokaryotic cells and even species may cooperate metabolically. (A) binary fission (C) biofilms
Metabolic cooperation also occurs in surface-coating biofilms (B) endospore formation (D) photoautotrophy
that include different species.
5. Bacteria perform the following ecological roles. Which role
? Describe the range of prokaryotic metabolic adaptations. typically does not involve symbiosis?
(A) skin commensalist (C) gut mutualist
Concept 27.4 (B) decomposer (D) pathogen
Prokaryotes have radiated into a diverse 6. Plantlike photosynthesis that releases O2 occurs in
set of lineages (pp. 581–585) (A) cyanobacteria. (C) gram-positive bacteria.
(B) archaea. (D) chemoautotrophic bacteria.
Molecular systematics is helping biologists classify prokaryotes
and identify new clades. Level 2: Application/Analysis
Diverse nutritional types are scattered among the major groups 7. EVOLUTION CONNECTION In patients with nonresistant
of bacteria. The two largest groups are the proteobacteria and strains of the tuberculosis bacterium, antibiotics can relieve
gram-positive bacteria. symptoms in a few weeks. However, it takes much longer to halt
the infection, and patients may discontinue treatment while
Some archaea, such as extreme thermophiles and extreme bacteria are still present. Explain how this could result in the
halophiles, live in extreme environments. Other archaea live evolution of drug-resistant pathogens.
in moderate environments such as soils and lakes.
Level 3: Synthesis/Evaluation
? How have molecular data informed prokaryotic phylogeny?
8. SCIENTIFIC INQUIRY • INTERPRET THE DATA The nitrogen-
Concept 27.5 fixing bacterium Rhizobium infects the roots of some plant
species, forming a mutualism in which the bacterium provides
Prokaryotes play crucial roles in the biosphere nitrogen, and the plant provides carbohydrates. Scientists
(pp. 585–586) measured the 12-week growth of one such plant species (Acacia
irrorata) when infected by six different Rhizobium strains.
Decomposition by heterotrophic prokaryotes and the synthetic (a) Graph the data. (b) Interpret your graph.
activities of autotrophic and nitrogen-fixing prokaryotes contrib-
ute to the recycling of elements in ecosystems. Rhizobium strain 1 2 3 4 5 6
Many prokaryotes have a symbiotic relationship with a host; the Plant mass (g) 0.91 0.06 1.56 1.72 0.14 1.03
relationships between prokaryotes and their hosts range from
mutualism to commensalism to parasitism. Data from J. J. Burdon et al., Variation in the effectiveness of symbiotic associa-
tions between native rhizobia and temperate Australian Acacia: within species
? In what ways are prokaryotes key to the survival of many species? interactions, Journal of Applied Ecology 36:398–408 (1999).
Note: Without Rhizobium, after 12 weeks, Acacia plants have a mass of about 0.1 g.
Concept 27.6
9. WRITE ABOUT A THEME: ENERGY In a short essay (about
Prokaryotes have both beneficial and harmful 100–150 words), discuss how prokaryotes and other members of
impacts on humans (pp. 586–589) hydrothermal vent communities transfer and transform energy.
People depend on mutualistic prokaryotes, including hundreds 10. SYNTHESIZE YOUR KNOWLEDGE
of species that live in our intestines and help digest food.
Pathogenic bacteria typically cause disease by releasing exotoxins
or endotoxins. Horizontal gene transfer can spread genes associ-
ated with virulence to harmless species or strains.
Prokaryotes can be used in bioremediation and production of
plastics, vitamins, antibiotics, and other products.
? Describe beneficial and harmful impacts of prokaryotes on humans.
Test Your Understanding
Level 1: Knowledge/Comprehension
1. Genetic variation in bacterial populations cannot
result from
(A) transduction. (C) mutation.
(B) conjugation. (D) meiosis. PRACTICE Explain how the small size and rapid reproduction rate of bacteria
TEST (such as the population shown here on the tip of a pin) contribute
goo.gl/CUYGKD to their large population sizes and high genetic variation.
2. Photoautotrophs use
(A) light as an energy source and CO2 as a carbon source. For selected answers, see Appendix A.
(B) light as an energy source and methane as a carbon source.
(C) N2 as an energy source and CO2 as a carbon source. For additional practice questions, check out the Dynamic Study
(D) CO2 as both an energy source and a carbon source. Modules in MasteringBiology. You can use them to study on
your smartphone, tablet, or computer anytime, anywhere!
590 Unit five The Evolutionary History of Biological Diversity
Protists 28
1 μm
Figure 28.1 Which of these organisms are prokaryotes and which are eukaryotes?
Key Concepts Living Small
28.1 Most eukaryotes are single-celled Knowing that most prokaryotes are extremely small organisms, you might assume
that Figure 28.1 depicts six prokaryotes and one much larger eukaryote. But in fact,
organisms the only prokaryote is the organism immediately above the scale bar. The other six
organisms are members of diverse, mostly unicellular groups of eukaryotes informally
28.2 Excavates include protists with known as protists. These very small eukaryotes have intrigued biologists for more
than 300 years, ever since the Dutch scientist Antoni van Leeuwenhoek first laid eyes
modified mitochondria and on them under a light microscope. Some protists change their forms as they creep along
protists with unique flagella using blob-like appendages, while others resemble tiny trumpets or miniature jewelry.
Recalling his observations, van Leeuwenhoek wrote, “No more pleasant sight has met
28.3 SAR is a highly diverse group my eye than this, of so many thousands of living creatures in one small drop of water.”
of protists defined by DNA The protists that fascinated van Leeuwenhoek continue to surprise us today.
similarities Metagenomic studies have revealed a treasure trove of previously unknown protists
within the world of microscopic life. Many of these newly discovered organisms are
28.4 Red algae and green algae are just 0.5–2 µm in diameter—as small as many prokaryotes. Genetic and morphologi-
cal studies have also shown that some protists are more closely related to plants,
the closest relatives of plants fungi, or animals than they are to other protists. As a result, the kingdom in which
all protists once were classified, Protista, has been abandoned, and various protist
28.5 Unikonts include protists that lineages are now recognized as major groups in their own right. Most biologists still
are closely related to fungi
and animals
28.6 Protists play key roles in
ecological communities
Trumpet-shaped protists (Stentor
coeruleus)
When you see this blue icon, log in Get Ready for This Chapter
to MasteringBiology and go to the
Study Area for digital resources.
591
use the term protist, but only as a convenient way to refer to cellular level, many protists are very complex—the most elabo-
eukaryotes that are not plants, animals, or fungi. rate of all cells. In multicellular organisms, essential biological
functions are carried out by organs. Unicellular protists carry
In this chapter, you will become acquainted with some out the same essential functions, but they do so using subcel-
of the most significant groups of protists. You will learn lular organelles, not multicellular organs. The organelles that
about their structural and biochemical adaptations as well as protists use are mostly those discussed in Figure 6.8, includ-
their enormous impact on ecosystems, agriculture, industry, ing the nucleus, endoplasmic reticulum, Golgi apparatus, and
and human health. lysosomes. Certain protists also rely on organelles not found in
most other eukaryotic cells, such as contractile vacuoles that
HHMI Video: Seeing the Invisible: Van Leeuwenhoek’s pump excess water from the protistan cell (see Figure 7.13).
First Glimpses of the Microbial World
Protists are also very diverse in their nutrition. Some pro-
Concept 28.1 tists are photoautotrophs and contain chloroplasts. Some
are heterotrophs, absorbing organic molecules or ingesting
Most eukaryotes are single-celled larger food particles. Still other protists, called mixotrophs,
organisms combine photosynthesis and heterotrophic nutrition.
Photoautotrophy, heterotrophy, and mixotrophy have all
Protists, along with plants, animals, and fungi, are classified arisen independently in many different protist lineages.
as eukaryotes; they are in domain Eukarya, one of the three
domains of life. Unlike the cells of prokaryotes, eukaryotic Reproduction and life cycles also are highly varied among
cells have a nucleus and other membrane-enclosed organelles, protists. Some protists are only known to reproduce asexu-
such as mitochondria and the Golgi apparatus. Such organelles ally; others can also reproduce sexually or at least employ the
provide specific locations where particular cellular functions sexual processes of meiosis and fertilization. All three basic
are accomplished, making the structure and organization of types of sexual life cycles (see Figure 13.6) are represented
eukaryotic cells more complex than those of prokaryotic cells. among protists, along with some variations that do not quite
fit any of these types. We will examine the life cycles of
Eukaryotic cells also have a well-developed cytoskeleton several protist groups later in this chapter.
that extends throughout the cell (see Figure 6.20). The cyto-
skeleton provides the structural support that enables eukary- Four Supergroups of Eukaryotes
otic cells to have asymmetric (irregular) forms, as well as to
change in shape as they feed, move, or grow. In contrast, pro- Our understanding of the evolutionary history of eukaryotic
karyotic cells lack a well-developed cytoskeleton, thus limiting diversity has been in flux in recent years. Not only has king-
the extent to which they can maintain asymmetric forms or dom Protista been abandoned, but other hypotheses have been
change shape over time. discarded as well. For example, many biologists once thought
that the first lineage to have diverged from all other eukary-
We’ll survey the diversity of eukaryotes throughout the otes was the amitochondriate protists, organisms without con-
rest of this unit, beginning in this chapter with the protists. ventional mitochondria and with fewer membrane-enclosed
As you explore this material, bear in mind that organelles than other protist groups. But recent structural
and DNA data have undermined this hypothesis. Many of the
the organisms in most eukaryotic lineages are protists, and so-called amitochondriate protists have been shown to have
most protists are unicellular. mitochondria—though reduced ones—and some of these
organisms are now classified in distantly related groups.
Thus, life differs greatly from how most of us commonly
think of it. The large, multicellular organisms that we know The ongoing changes in our understanding of the phylog-
best (plants, animals, and fungi) are the tips of just a few eny of protists pose challenges to students and instructors
branches on the great tree of life (see Figure 26.21). alike. Hypotheses about these relationships are a focus of
scientific activity, changing rapidly as new data cause previ-
Structural and Functional Diversity ous ideas to be modified or discarded. We’ll focus here on
in Protists one current hypothesis: the four supergroups of eukaryotes
shown in Figure 28.2. Because the root of the eukaryotic tree
Given that they are classified in a number of different groups, is not known, all four supergroups are shown as diverging
it isn’t surprising that few general characteristics of protists simultaneously from a common ancestor. We know that this
can be cited without exceptions. In fact, protists exhibit more is not correct, but we do not know which supergroup was the
structural and functional diversity than the eukaryotes with first to diverge from the others. In addition, while some of the
which we are most familiar—plants, animals, and fungi. groups in Figure 28.2 are well supported by morphological
and DNA data, others are more controversial. As you read this
For example, most protists are unicellular, although there are chapter, it may be helpful to focus less on the specific names
some colonial and multicellular species. Single-celled protists
are justifiably considered the simplest eukaryotes, but at the
592 Unit five The Evolutionary History of Biological Diversity
of groups of organisms and more on why the organisms are and DNA sequence data indicate that mitochondria and plas-
important and how ongoing research is elucidating their tids are derived from prokaryotes that were engulfed by the
evolutionary relationships. ancestors of early eukaryotic cells. The evidence also suggests
that mitochondria evolved before plastids. Thus, a defining
Endosymbiosis in Eukaryotic Evolution moment in the origin of eukaryotes occurred when a host cell
engulfed a bacterium that would later become an organelle
What gave rise to the enormous diversity of protists that exist found in all eukaryotes—the mitochondrion.
today? There is abundant evidence that much of protistan
diversity has its origins in endosymbiosis, a relationship To determine which prokaryotic lineage gave rise to mito-
between two species in which one organism lives inside the chondria, researchers have compared the DNA sequences of
cell or cells of another organism (the host). In particular, mitochondrial genes (mtDNA) to those found in major clades
as we discussed in Concept 25.3, structural, biochemical, of bacteria and archaea. In the Scientific Skills Exercise, you
will interpret one such set of DNA sequence comparisons.
Scientific Skills Exercise Wheat, used
as the source of
Interpreting Comparisons of Genetic mitochondrial RNA
Sequences
positions from the gene sequences. Each value in the table is the
Which Prokaryotes Are Most Closely Related to percentage of the 617 nucleotide positions for which the pair of
Mitochondria? Early eukaryotes acquired mitochondria by endo- organisms have the same base. Any positions that were identical
symbiosis: A host cell engulfed an aerobic prokaryote that persisted across the rRNA genes of all six organisms were omitted from this
within the cytoplasm to the mutual benefit of both cells. In studying comparison matrix.
which living prokaryotes might be most closely related to mito-
chondria, researchers compared ribosomal RNA (rRNA) sequences. Interpret The Data
Ribosomes perform critical cell functions. Hence, rRNA sequences 1. First, make sure you understand how to read the comparison matrix.
are under strong selection and change slowly over time, making
them suitable for comparing even distantly related species. In this Find the cell that represents the comparison of C. testosteroni and
exercise, you will interpret some of the research data to draw E. coli. What value is given in this cell? What does that value signify
conclusions about the phylogeny of mitochondria. about the comparable rRNA gene sequences in those two organ-
isms? Explain why some cells have a dash rather than a value. Why
How the Research Was Done Researchers isolated and cloned are some cells shaded gray, with no value?
nucleotide sequences from the gene that codes for the small-subunit 2. Why did the researchers choose one plant mitochondrion and five
rRNA molecule for wheat (a eukaryote) and five bacterial species: bacterial species to include in the comparison matrix?
3. Which bacterium has an rRNA gene that is most similar to that of
Wheat, used as the source of mitochondrial rRNA genes the wheat mitochondrion? What is the significance of this similarity?
Agrobacterium tumefaciens, an alpha proteobacterium that lives
Instructors: A version of this Scientific Skills Exercise can be
within plant tissue and produces tumors in the host assigned in MasteringBiology.
Comamonas testosteroni, a beta proteobacterium
Escherichia coli, a well-studied gamma proteobacterium that
inhabits human intestines
Mycoplasma capricolum, a gram-positive mycoplasma, which is
the only group of bacteria lacking cell walls
Anacystis nidulans, a cyanobacterium
Data from the Research Cloned rRNA gene sequences for the six
organisms were aligned and compared. The data table below, called
a comparison matrix, summarizes the comparison of 617 nucleotide
Wheat mitochondrion Wheat mitochondrion A. tumefaciens C. testosteroni E. coli M. capricolum A. nidulans
A. tumefaciens – 48 38 35 34 34
C. testosteroni – 55 57 52 53
E. coli – 61 52 52
M. capricolum – 48 52
A. nidulans – 50
–
Data from D. Yang et al., Mitochondrial origins, Proceedings of the National Academy of Sciences USA 82:4443–4447 (1985).
chapter 28 Protists 593
Figure 28.2 Exploring Protistan Diversity Excavata Excavata
The tree below represents a phylogenetic hypothesis for the relationships among eukaryotesStramenopiles Alveolates RhizariansSAR Some members of this supergroup have
on Earth today. The eukaryotic groups at the branch tips are related in larger “supergroups,” an “excavated” groove on one side of
labeled vertically at the far right of the tree. Groups that were formerly classified in the king- the cell body. Two major clades (the
dom Protista are highlighted in yellow. Dotted lines indicate evolutionary relationships that parabasalids and diplomonads) have
are uncertain and proposed clades that are under active debate. For clarity, this tree only highly reduced mitochondria; members
includes representative clades from each supergroup. In addition, the recent discoveries of of a third clade (the euglenozoans) have
many new groups of eukaryotes indicate that eukaryotic diversity is actually much greater flagella that differ in structure from
than shown here. those of other organisms. Excavates in-
clude parasites such as Giardia, as well
Diplomonads as many predatory and photosynthetic
Parabasalids species.
Euglenozoans
5 μm
Diatoms
Golden algae
Brown algae
Dinoflagellates
Apicomplexans
Ciliates
Radiolarians
Forams
Cercozoans
Green Red algae Archaeplastida
algae Chlorophytes
Charophytes
Plants
Slime moldsAmoebozoans OpisthokontsUnikontaGiardia intestinalis, a diplomonad
Tubulinids parasite. This diplomonad (colorized
Entamoebas SEM), which lacks the characteristic surface
Nucleariids groove of the Excavata, inhabits the
Fungi intestines of mammals. It can infect people
Choanoflagellates when they drink water contaminated with
Animals feces containing Giardia cysts. Drinking
such water—even from a seemingly
DRAW IT Draw a simplified version of this phylogenetic tree that shows pristine stream—can cause severe diarrhea.
only the four supergroups of eukaryotes. Now sketch how the tree would look Boiling the water kills the parasite.
if the unikonts were the sister group to all other eukaryotes.
594 Unit five The Evolutionary History of Biological Diversity
SAR Archaeplastida
This supergroup contains (and is named after) three large and This supergroup of eukaryotes includes red algae and green algae,
very diverse clades: Stramenopila, Alveolata, and Rhizaria. Stra- along with plants. Red algae and green algae include unicellular
menopiles include some of the most important photosynthetic or- species, colonial species, and multicellular species (including the
ganisms on Earth, such as the diatoms shown here. Alveolates also green alga Volvox). Many of the large algae known informally
include photosynthetic species, as well as important pathogens, as “seaweeds” are multicellular red or green algae. Protists in
such as Plasmodium, which causes malaria. According to one Archaeplastida include key photosynthetic species that form the
current hypothesis, stramenopiles and alveolates originated by base of the food web in many aquatic communities.
secondary endosymbiosis when a heterotrophic protist engulfed
a red alga. 20 μm 25 μm
50 μm
Diatom diversity. These beautiful single-celled protists are Volvox, a multicellular freshwater green alga. This alga has two
important photosynthetic organisms in aquatic communities (LM). types of differentiated cells, and so it is considered multicellular rather
than colonial. It resembles a hollow ball whose wall is composed of
The rhizarian subgroup of SAR includes many species of amoe- hundreds of biflagellated cells (see inset LM) embedded in a gelatinous
bas, most of which have pseudopodia that are threadlike in shape. extracellular matrix; if isolated, these cells cannot reproduce. However,
Pseudopodia are extensions that can the alga also contains cells that are specialized for either sexual or
bulge from any portion of the cell; asexual reproduction. The large algae shown here will eventually
they are used in movement and in release the small “daughter” algae that can be seen within them (LM).
the capture of prey.
Video: Volvox
100 μm
Unikonta
This supergroup of eukaryotes includes amoebas that have
lobe- or tube-shaped pseudopodia, as well as animals, fungi, and
non-amoeba protists that are closely related to animals or fungi.
According to one current hypothesis, the unikonts were the first
eukaryotic supergroup to diverge from all other eukaryotes;
however, this hypothesis has yet to be widely accepted.
Globigerina, a rhizarian in SAR. This species is a foram, a group 100 μm
whose members have threadlike pseudopodia that extend through
pores in the shell, or test (LM). The inset shows a foram test, which is A unikont amoeba. This amoeba, the tubulinid Amoeba
hardened by calcium carbonate. proteus, is using its pseudopodia to move.
Video: Amoeba Pseudopodium
chapter 28 Protists 595
Collectively, such studies indicate that mitochondria arose Plastid Evolution: A Closer Look
from an alpha proteobacterium (see Figure 27.16). Results
from mtDNA sequence analyses also indicate that the mito- As you’ve seen, current evidence indicates that mitochondria
chondria of protists, animals, fungi and plants descended are descended from a bacterium that was engulfed by a host
from a single common ancestor, thus suggesting that mito- cell that was an archaean (or a close relative of the archaeans).
chondria arose only once over the course of evolution. Similar This event gave rise to the eukaryotes. There is also much evi-
analyses provide evidence that plastids descended from a sin- dence that later in eukaryotic history, a lineage of heterotro-
gle common ancestor—a cyanobacterium that was engulfed phic eukaryotes acquired an additional endosymbiont—a pho-
by a eukaryotic host cell. tosynthetic cyanobacterium—that then evolved into plastids.
According to the hypothesis illustrated in Figure 28.3, this
Progress has also been made toward identifying the host plastid-bearing lineage gave rise to two lineages of photosyn-
cell that engulfed an alpha proteobacterium, thereby setting thetic protists, or algae: red algae and green algae.
the stage for the origin of eukaryotes. In 2015, for example,
researchers reported the discovery of a new group of archaea, Let’s examine some of the steps in Figure 28.3 more closely.
the lokiarchaeotes. In phylogenomic analyses, this group was First, recall that cyanobacteria are gram-negative and that
identified as the sister group of the eukaryotes and its genome gram-negative bacteria have two cell membranes, an inner
was found to encode many eukaryote-specific features. Was plasma membrane and an outer membrane that is part of the
the host cell that engulfed an alpha proteobacterium a lokiar- cell wall (see Figure 27.3). Plastids in red algae and green algae
chaeote? While this may have been the case, it is also possible are also surrounded by two membranes. Transport proteins
that the host was closely related to the archaeans (but was in these membranes are homologous to proteins in the inner
not itself an archaean). Either way, current evidence indicates and outer membranes of cyanobacteria, providing further
that the host was a relatively complex cell in which certain support for the hypothesis that plastids originated from a
features of eukaryotic cells had evolved, such as a cytoskeleton cyanobacterial endosymbiont.
that enabled it to change shape (and thereby engulf the alpha
proteobacterium). On several occasions during eukaryotic evolution, red algae
and green algae underwent secondary endosymbiosis,
Figure 28.3 Diversity of plastids produced by endosymbiosis. Plastid
Studies of plastid-bearing eukaryotes suggest that plastids evolved from Stramenopiles
a cyanobacterium that was engulfed by an ancestral heterotrophic
eukaryote (primary endosymbiosis). That ancestor then Alveolates
diversified into red algae and green algae, some of
which were subsequently engulfed by other
eukaryotes (secondary endosymbiosis).
Secondary
endosymbiosis
Membranes Red alga
are represented
Cyanobacterium as dark lines in
the cell.
1 23
Primary
endosymbiosis
Nucleus One of these Secondary Plastid
Heterotrophic membranes was endosymbiosis Euglenids
eukaryote lost in red and
green algal Secondary Chlorarachniophytes
descendants. endosymbiosis
Green alga
VISUAL SKILLS Based on this diagram, which of the following
groups are likely to be more closely related: stramenopiles and
alveolates, or euglenids and chlorarachniophytes? Explain.
596 Unit five The Evolutionary History of Biological Diversity
Figure 28.4 Excavata cytoskeleton. Some members of this diverse group also have
Nucleomorph an “excavated” feeding groove on one side of the cell body.
within a plastid of a The excavates include the diplomonads, parabasalids, and
chlorarachniophyte. euglenozoans. Molecular data indicate that each of these
three groups is monophyletic, and recent genomic studies
Inner plastid support the monophyly of the excavate supergroup.
membrane
Diplomonads and Parabasalids
Nucleomorph
The protists in these two groups lack plastids and have highly
Outer plastid reduced mitochondria (until recently, they were thought to
membrane lack mitochondria altogether). Most diplomonads and para-
basalids are found in anaerobic environments.
Nuclear pore-like gap
Diplomonads have reduced mitochondria called
meaning they were ingested in the food vacuoles of hetero- mitosomes. These organelles lack functional electron trans-
trophic eukaryotes and became endosymbionts themselves. port chains and hence cannot use oxygen to help extract
For example, protists known as chlorarachniophytes likely energy from carbohydrates and other organic molecules.
evolved when a heterotrophic eukaryote engulfed a green alga. Instead, diplomonads get the energy they need from anaero-
Evidence for this process can be found within the engulfed cell, bic biochemical pathways. Many diplomonads are parasites,
which contains a tiny vestigial nucleus, called a nucleomorph including the infamous Giardia intestinalis (see Figure 28.2),
(Figure 28.4). Genes from the nucleomorph are still transcribed, which inhabits the intestines of mammals.
and their DNA sequences indicate that the engulfed cell was a
green alga. Structurally, diplomonads have two equal-sized nuclei and
multiple flagella. Recall that eukaryotic flagella are extensions
Concept Check 28.1 of the cytoplasm, consisting of bundles of microtubules cov-
ered by the cell’s plasma membrane (see Figure 6.24). They are
1. Cite at least four examples of structural and functional quite different from prokaryotic flagella, which are filaments
diversity among protists. composed of globular proteins attached to the cell surface
(see Figure 27.7).
2. Summarize the role of endosymbiosis in eukaryotic
evolution. Parabasalids also have reduced mitochondria; called
hydrogenosomes, these organelles generate some energy anaero-
3. MAKE CONNECTIONS After studying Figure 28.3, bically, releasing hydrogen gas as a by-product. The best-known
predict how many distinct genomes are contained parabasalid is Trichomonas vaginalis, a sexually transmitted par-
within the cell of a chlorarachniophyte. Explain. asite that infects some 5 million people each year. T. vaginalis
(See Figures 6.17 and 6.18). travels along the mucus-coated lining of the human reproduc-
For suggested answers, see Appendix A. tive and urinary tracts by moving its flagella and by undulating
part of its plasma membrane (Figure 28.5). In females, if the
Concept 28.2 vagina’s normal acidity is disturbed, T. vaginalis can outcom-
pete beneficial microorganisms there and infect the vagina.
Excavates include protists with (Trichomonas infections also can occur in the urethra of males,
modified mitochondria and protists though often without symptoms.) T. vaginalis has a gene that
with unique flagella allows it to feed on the vaginal lining, promoting infection.
Studies suggest that the protist acquired this gene by horizontal
Diplomonads gene transfer from bacterial parasites in the vagina.
Parabasalids
Euglenozoans Figure 28.5 The parabasalid parasite Trichomonas
vaginalis (colorized SEM).
SAR
Archaeplastida Flagella
Unikonta
Undulating 5 μm
Now that we have examined some of the broad patterns in membrane
eukaryotic evolution, we will look more closely at the four
main groups of protists shown in Figure 28.2.
We begin with Excavata (the excavates), a clade that was
originally proposed based on morphological studies of the
chapter 28 Protists 597
Flagella
0.2 μm
Tyrpanosome
8 μm Crystalline rod
(cross section)
Ring of microtubules Red blood
(cross section) cells
Figure 28.6 Euglenozoan flagellum. Most euglenozoans have
a crystalline rod inside one of their flagella (the TEM is a flagellum shown 9 μm
in cross section). The rod lies alongside the 9 + 2 ring of microtubules Figure 28.7 Trypanosoma, the kinetoplastid that causes
found in all eukaryotic flagella (compare with Figure 6.24). sleeping sickness (colorized SEM).
Euglenozoans
Protists called euglenozoans belong to a diverse clade that Trypanosomes evade immune responses with an effective
includes predatory heterotrophs, photosynthetic autotrophs, “bait-and-switch” defense. The surface of a trypanosome is
mixotrophs, and parasites. The main morphological feature coated with millions of copies of a single protein. However,
that distinguishes protists in this clade is the presence of a before the host’s immune system can recognize the pro-
rod with either a spiral or a crystalline structure inside each tein and mount an attack, new generations of the parasite
of their flagella (Figure 28.6). The two best-studied groups switch to another surface protein with a different molecular
of euglenozoans are the kinetoplastids and the euglenids. structure. Frequent changes in the surface protein prevent
Kinetoplastids the host from developing immunity. (The Scientific Skills
Exercise in Chapter 43 explores this topic further.) About
Protists called kinetoplastids have a single, large mitochon- a third of Trypanosoma’s genome is dedicated to producing
drion that contains an organized mass of DNA called a kineto- these surface proteins.
plast. These protists include species that feed on prokaryotes Euglenids
in freshwater, marine, and moist terrestrial ecosystems, as well
as species that parasitize animals, plants, and other protists. A euglenid has a pocket at one end of the cell from which
For example, kinetoplastids in the genus Trypanosoma infect one or two flagella emerge (Figure 28.8). Some euglenids are
humans and cause sleeping sickness, a neurologi-
cal disease that is invariably fatal if not treated. Long flagellum
The infection occurs via the bite of a vector
(carrier) organism, the African tsetse fly Eyespot: pigmented Light detector:
(Figure 28.7). Trypanosomes also cause organelle that swelling near the base
Chagas’ disease, which is transmitted functions as a light of the long flagellum;
by bloodsucking insects and can lead shield, allowing light detects light that is
to congestive heart failure. from only a certain not blocked by the
direction to strike eyespot. As a result,
Figure 28.8 the light detector Euglena moves
Euglena, toward light of
a euglenid Short flagellum appropriate intensity,
commonly found Contractile vacuole an important
in pond water. Nucleus adaptation that
Chloroplast enhances
Video: Euglena Plasma membrane photosynthesis.
Pellicle: protein bands beneath
5 μm the plasma membrane that
Euglena (LM) provide strength and flexibility
598 Unit five The Evolutionary History of Biological Diversity
mixotrophs: They perform photosynthesis when sunlight is represents the best current hypothesis for the phylogeny
available, but when it is not, they can become heterotrophic, of the three large protist clades to which we now turn.
absorbing organic nutrients from their environment. Many
other euglenids engulf prey by phagocytosis. Stramenopiles
Concept Check 28.2 One major subgroup of SAR, the stramenopiles, includes
some of the most important photosynthetic organisms on the
1. Why do some biologists describe the mitochondria of planet. Their name (from the Latin stramen, straw, and pilos,
diplomonads and parabasalids as “highly reduced”? hair) refers to their characteristic flagellum, which has numer-
ous fine, hairlike projections. In most stramenopiles, this
2. WHAT IF? DNA sequence data for a diplomonad, a “hairy” flagellum is paired with a shorter “smooth” (nonhairy)
euglenid, a plant, and an unidentified protist suggest flagellum (Figure 28.9). Here we’ll focus on three groups of
that the unidentified species is most closely related stramenopiles: diatoms, golden algae, and brown algae.
to the diplomonad. Further studies reveal that the
unknown species has fully functional mitochondria. Figure 28.9 Stramenopile flagella. Most stramenopiles,
Based on these data, at what point on the phylogenetic such as Synura petersenii, have two flagella: one covered with fine,
tree in Figure 28.2 did the mystery protist’s lineage prob- stiff hairs and a shorter one that is smooth.
ably diverge from other eukaryote lineages? Explain.
For suggested answers, see Appendix A.
Concept 28.3 Hairy
flagellum
SAR is a highly diverse group of
protists defined by DNA similarities
Diatoms Excavata Smooth
Golden algae flagellum
Brown algae Archaeplastida
Stramenopiles SARUnikonta 5 μm
Dinoflagellates Alveolates
Apicomplexans Rhizarians Diatoms
Ciliates
A key group of photosynthetic protists, diatoms are unicel-
Radiolarians lular algae that have a unique glass-like wall made of silicon
Forams dioxide embedded in an organic matrix (Figure 28.10). The
Cercozoans wall consists of two parts that overlap like a shoe box and its
lid. These walls provide effective protection from the crush-
Our second supergroup, referred to as SAR, was proposed ing jaws of predators: Live diatoms can withstand pressures
recently based on whole-genome DNA sequence analyses. as great as 1.4 million kg/m2, equal to the pressure under each
These studies have found that three major clades of protists— leg of a table supporting an elephant!
the stramenopiles, alveolates, and rhizarians—form a
monophyletic supergroup. This supergroup contains a With an estimated 100,000 living species, diatoms are a
large, extremely diverse collection of protists. To date, this highly diverse group of protists (see Figure 28.2). They are
supergroup has not received a formal name but is instead among the most abundant
known by the first letters of its major clades: SAR. photosynthetic organisms
both in the ocean and
Some morphological and DNA sequence data suggest that in lakes: One bucket of
two of these groups, the stramenopiles and alveolates, origi- water scooped from the
nated more than a billion years ago, when a common ancestor surface of the sea may
of these two clades engulfed a single-celled, photosynthetic contain millions of these
red alga. Because red algae are thought to have originated by microscopic algae. The
primary endosymbiosis (see Figure 28.3), such an origin for abundance of diatoms in
the stramenopiles and alveolates is referred to as secondary the past is also evident in
endosymbiosis. Others question this idea, noting that some
species in these groups lack plastids or their remnants (includ- Figure 28.10 The 40 μm
ing any trace of plastid genes in their nuclear DNA). diatom Triceratium
morlandii (colorized SEM).
As its lack of a formal name suggests, SAR is one of the
most controversial of the four supergroups we describe in
this chapter. Even so, for many scientists, this supergroup
chapter 28 Protists 599
the fossil record, where massive accumulations of fossilized Many golden algae are components of freshwater and marine
diatom walls are major constituents of sediments known as plankton, communities of mostly microscopic organisms that
diatomaceous earth. These sediments are mined for their qual- drift in currents near the water’s surface. While all golden algae
ity as a filtering medium and for many other uses. are photosynthetic, some species are mixotrophic. These mixo-
trophs can absorb dissolved organic compounds or ingest food
Diatoms are so widespread and abundant that their photo- particles, including living cells, by phagocytosis. If environmen-
synthetic activity affects global carbon dioxide (CO2) levels. tal conditions deteriorate, many species form protective cysts
Diatoms have this effect in part because of events that occur that can survive for decades.
during episodes of rapid population growth, or blooms, when
ample nutrients are available. Typically, diatoms are eaten Brown Algae
by a variety of protists and invertebrates, but during a bloom,
many escape this fate. When these uneaten diatoms die, their The largest and most complex algae are brown algae. All are
bodies sink to the ocean floor. It takes decades, or even cen- multicellular, and most are marine. Brown algae are especially
turies, for diatoms that sink to the ocean floor to be broken common along temperate coasts that have cold-water cur-
down by bacteria and other decomposers. As a result, the car- rents. They owe their characteristic brown or olive color to
bon in their bodies remains there for some time, rather than the carotenoids in their plastids.
being released immediately as CO2 as the decomposers respire.
The overall effect of these events is that CO2 absorbed by dia- Many of the species commonly called “seaweeds” are
toms during photosynthesis is transported, or “pumped,” brown algae. Some brown algal seaweeds have specialized
to the ocean floor. structures that resemble organs in plants, such as a rootlike
holdfast, which anchors the alga, and a stemlike stipe,
With an eye toward reducing global warming by lowering which supports the leaflike blades (Figure 28.12). Unlike
atmospheric CO2 levels, some scientists advocate promoting plants, however, brown algae lack true tissues and organs.
diatom blooms by fertilizing the ocean with essential nutri- Moreover, morphological and DNA data show that these
ents such as iron. In a 2012 study, researchers found that CO2 similarities evolved independently in the algal and plant
was indeed pumped to the ocean floor after iron was added lineages and are thus analogous, not homologous. In addi-
to a small region of the ocean. Further tests are planned to tion, while plants have adaptations (such as rigid stems) that
examine whether iron fertilization has undesirable side effects provide support against gravity, brown algae have adapta-
(such as oxygen depletion or the production of nitrous oxide, tions that enable their main photosynthetic surfaces (the leaf-
a more potent greenhouse gas than CO2). like blades) to be near the water surface. Some brown algae
accomplish this task with gas-filled, bubble-shaped floats.
Golden Algae
Figure 28.12 Seaweeds: adapted to life at the ocean’s
The characteristic color of golden algae results from their margins. The sea palm (Postelsia) lives on rocks along the coast of
yellow and brown carotenoids. The cells of golden algae are the northwestern United States and western Canada. The body of this
typically biflagellated, with both flagella attached near one brown alga is well adapted to maintaining a firm foothold despite
end of the cell. Most species are unicellular, but some are the crashing surf.
colonial (Figure 28.11).
Blade
Figure 28.11 Dinobryon, a colonial golden alga found
in fresh water (LM).
Flagellum
Outer container
Living cell
Stipe
Holdfast
25 μm
600 Unit five The Evolutionary History of Biological Diversity
Giant brown algae known as kelps that live in deep waters of generations, the alternation of multicellular haploid and
have such floats in their blades, which are attached to stipes diploid forms. Although haploid and diploid conditions alter-
that can rise as much as 60 m from the seafloor—more than nate in all sexual life cycles—human gametes, for example,
half the length of a football field. are haploid—the term alternation of generations applies only
to life cycles in which both haploid and diploid stages are
Brown algae are important commodities for humans. Some multicellular. As you will read in Concept 29.1, alternation
species are eaten, such as Laminaria (Japanese “kombu”), of generations also evolved in plants.
which is used in soups. In addition, the cell walls of brown
algae contain a gel-forming substance, called algin, which is The complex life cycle of the brown alga Laminaria pro-
used to thicken many processed foods, including pudding vides an example of alternation of generations (Figure 28.13).
and salad dressing. The diploid individual is called the sporophyte because it pro-
duces spores. The spores are haploid and move by means of
Alternation of Generations flagella; they are called zoospores. The zoospores develop into
haploid, multicellular male and female gametophytes, which
A variety of life cycles have evolved among the multicellular produce gametes. The union of two gametes (fertilization)
algae. The most complex life cycles include an alternation
Figure 28.13 The life cycle of the brown alga Laminaria: an example of alternation
of generations.
1 The sporophytes are usually
found in water just below the
line of the lowest tides, attached
to rocks by branching holdfasts.
Sporangia 2 Cells on the surface
of the blade develop
into sporangia.
10 cm 3 Sporangia produce
zoospores by meiosis.
Sporophyte MEIOSIS
(2n) Zoospore
7 The zygotes Developing Female 4 The zoospores are all
grow into new sporophyte Gametophytes structurally alike, but
(n) about half of them develop
sporophytes Zygote into male gametophytes
while attached (2n) and half into female
to the remains gametophytes. The
gametophytes are short,
of the female branched laments that
gametophyte. grow on subtidal rocks.
Male
FERTILIZATION Egg
Mature female Sperm 5 Male gametophytes release sperm,
gametophyte and female gametophytes produce eggs,
(n) which remain attached to the female
gametophyte. Eggs secrete a chemical
Key signal that attracts sperm of the same
species, thereby increasing the
Haploid (n) 6 Sperm fertilize probability of fertilization in the ocean.
Diploid (2n) the eggs.
VISUAL SKILLS Based on this diagram, are the sperm shown in 5 genetically identical to one
another? Explain.
chapter 28 Protists 601
results in a diploid zygote, which matures and gives Flagella
rise to a new multicellular sporophyte. (a) Dinoflagellate flagella.
Beating of the spiral
In Laminaria, the two generations are flagellum, which lies in a
heteromorphic, meaning that the sporo- groove that encircles the
phytes and gametophytes are structurally cell, makes this specimen
different. Other algal life cycles have an of Pfiesteria shumwayae
alternation of isomorphic generations, in spin (colorized SEM).
which the sporophytes and gametophytes
look similar to each other, although they
differ in chromosome number.
Alveolates 3 μm
Members of the next subgroup of SAR, the alveolates,
have membrane-enclosed sacs (alveoli) just under the plasma
membrane (Figure 28.14). Alveolates are abundant in many
habitats and include a wide range of photosynthetic and het-
erotrophic protists. We’ll discuss three alveolate clades here:
a group of flagellates (the dinoflagellates), a group of parasites
(the apicomplexans), and a group of protists that move using
cilia (the ciliates).
Figure 28.14 Alveoli. These sacs under the plasma membrane (b) Red tide in the Gulf
are a characteristic that distinguishes alveolates from other of Carpentaria in
eukaryotes (TEM). northern Australia.
The red color is due to
Flagellum Alveoli high concentrations of
a carotenoid-containing
Alveolate0.2 μm dinoflagellate.
Dinoflagellates Figure 28.15 Dinoflagellates.
The cells of many dinoflagellates are reinforced by cellu Video: Dinoflagellate
lose plates. Two flagella located in grooves in this “armor”
make dinoflagellates (from the Greek dinos, whirling) spin as (Figure 28.15b). The blooms make coastal waters appear
they move through the waters of their marine and freshwater brownish red or pink because of the presence of carotenoids,
communities (Figure 28.15a). Although their ancestors may the most common pigments in dinoflagellate plastids. Toxins
have originated by secondary endosymbiosis (see Figure 28.3), produced by certain dinoflagellates have caused massive
roughly half of all dinoflagellates are now purely heterotro- kills of invertebrates and fishes. Humans who eat molluscs
phic. Others are important species of phytoplankton (photo- that have accumulated the toxins are affected as well, some-
synthetic plankton, which include photosynthetic bacteria times fatally.
as well as algae); many photosynthetic dinoflagellates
are mixotrophic. Apicomplexans
Periods of explosive population growth (blooms) in dino- Nearly all apicomplexans are parasites of animals—and
flagellates sometimes cause a phenomenon called “red tide” virtually all animal species examined so far are attacked by
these parasites. The parasites spread through their host as
tiny infectious cells called sporozoites. Apicomplexans are so
named because one end (the apex) of the sporozoite cell con-
tains a complex of organelles specialized for penetrating host
cells and tissues. Although apicomplexans are not photosyn-
thetic, recent data show that they retain a modified plastid
(apicoplast), most likely of red algal origin.
602 Unit five The Evolutionary History of Biological Diversity
Figure 28.16 The two-host life cycle of 1 An infected Anopheles 2 The sporozoites enter the person’s
Plasmodium, the apicomplexan that causes mosquito bites a person, liver cells. After several days, the sporozoites
injecting Plasmodium undergo multiple divisions and become
malaria. sporozoites in its saliva. merozoites, which use their apical complex
Inside mosquito to penetrate red blood cells (see TEM below).
? Are morphological differences between
sporozoites, merozoites, and gametocytes
caused by different genomes
or by differences in gene Inside human
Liver
expression? Explain.
Animation: Life Cycle Merozoite
of a Malaria Parasite
Sporozoites
(n)
8 An oocyst develops Liver cell
from the zygote in the wall
of the mosquito’s gut. The Oocyst Apex
oocyst releases thousands
of sporozoites, which
migrate to the mosquito’s
salivary gland.
MEIOSIS Merozoite Red blood 0.5 μm
(n) cell
Zygote Red blood 3 The merozoites divide
(2n) cells asexually inside the red
blood cells. At intervals of
7 Fertilization occurs FERTILIZATION 48 or 72 hours (depending
in the mosquito’s on the species), large
Gametes numbers of merozoites
digestive tract, and a break out of the blood
zygote forms. cells, causing periodic chills
and fever. Some of the
Key merozoites infect other
Haploid (n) red blood cells.
Diploid (2n)
Gametocytes
(n)
4 Some merozoites
form gametocytes.
6 Gametes form from gametocytes; each male 5 Another Anopheles mosquito bites the infected person
gametocyte produces several slender male gametes. and picks up Plasmodium gametocytes along with blood.
Most apicomplexans have intricate life cycles with both the sickle-cell allele; for an explanation of this connection,
sexual and asexual stages. Those life cycles often require see Figure 23.18.
two or more host species for completion. For example,
Plasmodium, the parasite that causes malaria, lives in both The search for malarial vaccines has been hampered by
mosquitoes and humans (Figure 28.16). the fact that Plasmodium lives mainly inside cells, hidden
from the host’s immune system. And, like trypanosomes,
Historically, malaria has rivaled tuberculosis as the leading Plasmodium continually changes its surface proteins. Even
cause of human death by infectious disease. The incidence so, significant progress was made in 2015, when European
of malaria was diminished in the 1960s by insecticides that regulators approved the world’s first licensed malarial vaccine.
reduced carrier populations of Anopheles mosquitoes and by However, this vaccine, which targets a protein on the surface
drugs that killed Plasmodium in humans. But the emergence of sporozoites, provides only partial protection against malaria.
of resistant varieties of both Anopheles and Plasmodium has As a result, researchers continue to study other potential vac-
led to a resurgence of malaria. About 200 million people in cine targets, including the apicoplast. This approach may be
the tropics are currently infected, and 600,000 die each year. effective because the apicoplast is a modified plastid; as such,
In regions where malaria is common, the lethal effects of this it descended from a cyanobacterium and hence has different
disease have resulted in the evolution of high frequencies of metabolic pathways from those in the human patients.
chapter 28 Protists 603
Ciliates of tightly packed cilia function collectively in locomotion.
Other ciliates scurry about on leg-like structures constructed
The ciliates are a large and varied group of protists named from many cilia bonded together.
for their use of cilia to move and feed (Figure 28.17a). Most
ciliates are predators, typically of bacteria or of other pro- A distinctive feature of ciliates is the presence of two
tists. Their cilia may completely cover the cell surface or may types of nuclei: tiny micronuclei and large macronuclei. A
be clustered in a few rows or tufts. In certain species, rows cell has one or more nuclei of each type. Genetic variation
Figure 28.17 Structure and function in the ciliate Paramecium caudatum.
Paramecium constantly takes in water Contractile Cilia along a funnel-shaped oral groove
by osmosis from its hypotonic vacuole move food (mainly bacteria) into the
cell mouth, where the food is engulfed
environment. Bladderlike contractile into food vacuoles by phagocytosis.
vacuoles accumulate excess water from
radial canals and periodically expel it
through the plasma membrane.
Oral groove
Thousands of cilia cover the Cell mouth
surface of Paramecium.
50 μm
Micronucleus Food vacuoles fuse with lysosomes (not
Macronucleus shown). As the food is digested, the
vacuoles follow a looping path through
the cell. Wastes are released when the
vacuoles fuse with a specialized region
of the plasma membrane that functions
as an anal pore.
(a) Feeding, waste removal, and water balance.
1 Two cells of compatible 2 Meiosis of micronuclei 3 Three micronuclei in each cell
mating strains align side produces four haploid disintegrate. The remaining micro-
by side and partially fuse. micronuclei in each cell. nucleus in each cell divides by mitosis.
MEIOSIS 4 The cells swap
one micronucleus.
Compatible Diploid Haploid
mates micronucleus micronucleus
The original Diploid
macronucleus micronucleus
disintegrates.
MICRONUCLEAR
FUSION
5 The cells
separate.
9 Two rounds of 8 Four micro- 7 Three rounds 6 The two Key
binary fission yield nuclei become of mitosis produce micronuclei fuse.
four daughter cells. macronuclei. eight micronuclei. Conjugation
Asexual
(b) Conjugation and reproduction. reproduction
MAKE CONNECTIONS The events shown in steps 5 and 6 of this diagram have a similar overall Video: Paramecium
effect to what event in the human life cycle (see Figure 13.5)? Explain.
604 Unit five The Evolutionary History of Biological Diversity
results from conjugation, a sexual process in which two streaming then carries the captured prey into the main part
individuals exchange haploid micronuclei but do not repro- of the cell. After radiolarians die, their skeletons settle to
duce (Figure 28.17b). Ciliates generally reproduce asexually the seafloor, where they have accumulated as an ooze that
by binary fission, during which the existing macronucleus is hundreds of meters thick in some locations.
disintegrates and a new one is formed from the cell’s micro-
nuclei. Each macronucleus typically contains multiple Forams
copies of the ciliate’s genome. Genes in the macronucleus
control the everyday functions of the cell, such as feeding, The protists called foraminiferans (from the Latin foramen,
waste removal, and maintaining water balance. little hole, and ferre, to bear), or forams, are named for their
porous shells, called tests (see Figure 28.2). Foram tests con-
Video: Ciliate Movement in Stentor sist of a single piece of organic material that typically is hard-
ened with calcium carbonate. The pseudopodia that extend
Rhizarians through the pores function in swimming, test formation,
and feeding. Many forams also derive nourishment from the
Our next subgroup of SAR is the rhizarians. Many species in photosynthesis of symbiotic algae that live within the tests.
this group are amoebas, protists that move and feed by means
of pseudopodia, extensions that may bulge from almost Forams are found in both the ocean and fresh water. Most
anywhere on the cell surface. As it moves, an amoeba extends species live in sand or attach themselves to rocks or algae,
a pseudopodium and anchors the tip; more cytoplasm then but some live as plankton. The largest forams, though single-
streams into the pseudopodium. Amoebas do not constitute a celled, have tests several centimeters in diameter.
monophyletic group; instead, they are dispersed across many
distantly related eukaryotic taxa. Most amoebas that are rhiz- Ninety percent of all identified species of forams are known
arians differ morphologically from other amoebas by having from fossils. Along with the calcium-containing remains of
threadlike pseudopodia. Rhizarians also include flagellated other protists, the fossilized tests of forams are part of marine
(non-amoeboid) protists that feed using threadlike pseudopodia. sediments, including sedimentary rocks that are now land
formations. Foram fossils are excellent markers for correlating
We’ll examine three groups of rhizarians here: radiolarians, the ages of sedimentary rocks in different parts of the world.
forams, and cercozoans. Researchers are also studying these fossils to obtain informa-
tion about climate change and its effects on the oceans and
Radiolarians their life (Figure 28.19).
The protists called radiolarians have delicate, intricately Cercozoans
symmetrical internal skeletons that are generally made of
silica. The pseudopodia of these mostly marine protists radi- First identified in molecular phylogenies, the cercozoans are
ate from the central body (Figure 28.18) and are reinforced a large group of amoeboid and flagellated protists that feed
by bundles of microtubules. The microtubules are covered by using threadlike pseudopodia. Cercozoan protists are com-
a thin layer of cytoplasm, which engulfs smaller microorgan- mon inhabitants of marine, freshwater, and soil ecosystems.
isms that become attached to the pseudopodia. Cytoplasmic
Figure 28.19 Fossil forams. By measuring the magnesium
Figure 28.18 A radiolarian. Numerous threadlike pseudopodia content in fossilized forams like these, researchers seek to learn how
radiate from the central body of this radiolarian (LM). ocean temperatures have changed over time. Forams take up more
magnesium in warmer water than in colder water.
Pseudopodia
200 μm
chapter 28 Protists 605
Figure 28.20 A second case of primary endosymbiosis? Concept 28.4
The cercozoan Paulinella conducts photosynthesis in a unique sausage-
shaped structure called a chromatophore (LM). Chromatophores are Red algae and green algae
surrounded by a membrane with a peptidoglycan layer, suggesting are the closest relatives of plants
that they are derived from a bacterium. DNA evidence indicates that
chromatophores are derived from a different cyanobacterium than Excavata
that from which plastids are derived. SAR
Chromatophore Red algae Archaeplastida
5 μm Chlorophytes Green algae
Charophytes
Most cercozoans are heterotrophs. Many are parasites of
plants, animals, or other protists; many others are predators. Plants
The predators include the most important consumers of bac-
teria in aquatic and soil ecosystems, along with species that Unikonta
eat other protists, fungi, and even small animals. One small
group of cercozoans, the chlorarachniophytes (mentioned As described earlier, morphological and molecular
earlier in the discussion of secondary endosymbiosis), are evidence indicates that plastids arose when a heterotrophic
mixotrophic: These organisms ingest smaller protists and protist acquired a cyanobacterial endosymbiont. Later,
bacteria as well as perform photosynthesis. At least one other photosynthetic descendants of this ancient protist evolved
cercozoan, Paulinella chromatophora, is an autotroph, deriving into red algae and green algae (see Figure 28.3), and the lineage
its energy from light and its carbon from CO2. As described that produced green algae then gave rise to plants. Together,
in Figure 28.20, Paulinella appears to represent an intriguing red algae, green algae, and plants make up our third eukaryotic
additional evolutionary example of a eukaryotic lineage supergroup, which is called Archaeplastida. Archaeplastida
that obtained its photosynthetic apparatus directly from is a monophyletic group that descended from the ancient
a cyanobacterium. protist that engulfed a cyanobacterium. We will examine
plants in Chapters 29 and 30; here we will look at the diversity
Concept Check 28.3 of their closest algal relatives, red algae and green algae.
1. Explain why forams have such a well-preserved fossil Red Algae
record.
Many of the 6,000 known species of red algae (rhodo-
2. WHAT IF? Would you expect the plastid DNA of pho- phytes, from the Greek rhodos, red) are reddish, owing to the
tosynthetic dinoflagellates, diatoms, and golden algae photosynthetic pigment phycoerythrin, which masks the
to be more similar to the nuclear DNA of plants (domain green of chlorophyll (Figure 28.21). However, other species
Eukarya) or to the chromosomal DNA of cyanobacteria (those adapted to shallow water) have less phycoerythrin. As
(domain Bacteria)? Explain. a result, red algal species may be greenish red in very shallow
water, bright red at moderate depths, and almost black in
3. MAKE CONNECTIONS Which of the three life cycles deep water. Some species lack pigmentation altogether and
in Figure 13.6 exhibits alternation of generations? How live as heteroptrophic parasites on other red algae.
does it differ from the other two?
Red algae are abundant in the warm coastal waters of tropi-
4. MAKE CONNECTIONS Review Figures 9.2 and 10.6, cal oceans. Some of their photosynthetic pigments, including
and then summarize how CO2 and O2 are both used phycoerythrin, allow them to absorb blue and green light,
and produced by chlorarachniophytes and other which penetrate relatively far into the water. A species of red
aerobic algae. alga has been discovered near the Bahamas at a depth of more
For suggested answers, see Appendix A. than 260 m. There are also a small number of freshwater and
terrestrial species.
Most red algae are multicellular. Although none are as big
as the giant brown kelps, the largest multicellular red algae
are included in the informal designation “seaweeds.” You
may have eaten one of these multicellular red algae, Porphyra
(Japanese “nori”), as crispy sheets or as a wrap for sushi (see
Figure 28.21). Red algae reproduce sexually and have diverse
life cycles in which alternation of generations is common.
However, unlike other algae, red algae do not have flagellated
gametes, so they depend on water currents to bring gametes
together for fertilization.
606 Unit five The Evolutionary History of Biological Diversity
Figure 28.21 Red algae. The second group, the chlorophytes (from the Greek chloros,
▶ Bonnemaisonia green), includes more than 7,000 species. Most live in fresh
hamifera. This red alga water, but there are also many marine and some terrestrial
has a filamentous form. species. The simplest chlorophytes are unicellular organisms
such as Chlamydomonas, which resemble gametes of more
20 cm complex chlorophytes. Various species of unicellular chloro-
phytes live independently in aquatic habitats as phytoplank-
8 mm ton or inhabit damp soil. Some live symbiotically within other
◀ Dulse (Palmaria palmata). eukaryotes, contributing part of their photosynthetic output to
the food supply of their hosts. Still other chlorophytes live in
This edible species has a environments exposed to intense visible and ultraviolet radia-
”leafy” form. tion; these species are protected by radiation-blocking com-
pounds in their cytoplasm, cell wall, or zygote coat.
▼ Nori. The red alga Porphyra is the source of
a traditional Japanese food. Larger size and greater complexity evolved in green algae
by three different mechanisms:
The seaweed is
grown on nets in 1. The formation of colonies of individual cells, as seen in
shallow coastal Zygnema (Figure 28.22a) and other species whose fila-
waters. mentous forms contribute to the stringy masses known
as pond scum.
2. The formation of true multicellular bodies by cell divi-
sion and differentiation, as in Volvox (see Figure 28.2) and
Ulva (Figure 28.22b).
3. The repeated division of nuclei with no cytoplasmic
division, as in Caulerpa (Figure 28.22c).
Figure 28.22 Examples of large chlorophytes.
(a) Zygnema, a
common pond
alga. This
filamentous
charophyte
features two
star-shaped
chloroplasts in
each cell.
Paper-thin, glossy sheets of dried nori 2 cm (b) Ulva, or sea
make a mineral-rich wrap for rice, (c) Caulerpa, an lettuce. This
seafood, and vegetables in sushi. multicellular, edible
intertidal chloro- chlorophyte has
Green Algae phyte. The branched differentiated
filaments lack cross- structures, such as
The grass-green chloroplasts of green algae have a struc- walls and thus are its leaflike blades
ture and pigment composition much like the chloroplasts of multinucleate. In and a rootlike
plants. Molecular systematics and cellular morphology leave effect, the body of holdfast that
little doubt that green algae and plants are closely related. this alga is one huge anchors the alga.
In fact, some systematists now advocate including green algae ”supercell.”
in an expanded “plant” kingdom, Viridiplantae (from the
Latin viridis, green). Phylogenetically, this change makes sense,
since otherwise the green algae are a paraphyletic group.
Green algae can be divided into two main groups, the cha-
rophytes and the chlorophytes. The charophytes include the
algae most closely related to plants, and we will discuss them
along with plants in Chapter 29.
chapter 28 Protists 607
Figure 28.23 The life cycle of Chlamydomonas, a unicellular chlorophyte.
Flagella 1 μm 2 In response to a nutrient shortage,
Cell wall drying of the enviroment, or other
stress, cells develop into gametes.
Nucleus
1 In Chlamydomonas, 3 Gametes of different
mature cells are haploid mating types (designated
and contain a single + and –) fuse (fertilization),
cup-shaped chloroplast. forming a diploid zygote.
–
Cross + Gamete + –
section of (n)
cup-shaped
chloroplast
(TEM)
7 These daughter cells develop Zoospore Mature cell FERTILIZATION
flagella and cell walls and then emerge as ASEXUAL (n) Zygote
swimming zoospores from the parent cell. REPRODUCTION (2n)
SEXUAL
The zoospores develop into mature REPRODUCTION
haploid cells.
+–
Key 6 When a mature cell reproduces +– MEIOSIS
asexually, it resorbs its flagella and then
Haploid (n) undergoes two rounds of mitosis, forming 4 The zygote secretes
Diploid (2n) four cells (more in some species). a durable coat that
protects the cell from
5 After a dormant period, harsh conditions.
meiosis produces four haploid
VISUAL SKILLS Circle the stage(s) in the diagram in which clones are individuals (two of each mating
formed, producing additional new daughter cells that are genetically identical type) that emerge and mature.
to the parent cell(s).
Most chlorophytes have complex life cycles, with both Concept 28.5
sexual and asexual reproductive stages. Nearly all species of
chlorophytes reproduce sexually by means of biflagellated Unikonts include protists that
gametes that have cup-shaped chloroplasts (Figure 28.23). are closely related to fungi and
Alternation of generations has evolved in some chlorophytes, animals
including Ulva.
Slime molds Excavata
Animation: Alternation of Generations in a Protist Tubulinids SAR
Entamoebas Archaeplastida
Concept Check 28.4 Nucleariids
Fungi Unikonta
1. Contrast red algae and brown algae. Choanoflagellates
2. Why is it accurate to say that Ulva is truly multicellular Animals
but Caulerpa is not? Unikonta is an extremely diverse supergroup of eukaryotes
3. WHAT IF? Suggest a possible reason why species in the that includes animals, fungi, and some protists. There are
two major clades of unikonts, the amoebozoans and the
green algal lineage may have been more likely to colonize
land than species in the red algal lineage.
For suggested answers, see Appendix A.
608 Unit five The Evolutionary History of Biological Diversity
opisthokonts (animals, fungi, and closely related protist Figure 28.24
groups). Each of these two major clades is strongly supported
by molecular systematics. The close relationship between Inquiry What is the root of the eukaryotic tree?
amoebozoans and opisthokonts is more controversial.
Support for this close relationship is provided by comparisons Experiment Responding to the difficulty in determining the root of
of myosin proteins and by some (but not all) studies based the eukaryotic phylogenetic tree, Alexandra Stechmann and Thomas
on multiple genes or whole genomes. Cavalier-Smith proposed a new approach. They studied two genes,
one coding for the enzyme dihydrofolate reductase (DHFR) and the
Another controversy involving the unikonts concerns the other coding for the enzyme thymidylate synthase (TS). The scientists’
root of the eukaryotic tree. Recall that the root of a phylogenetic approach took advantage of a rare evolutionary event: In some organ-
tree anchors the tree in time: Branch points close to the root are isms, the genes for DHFR and TS have fused, leading to the produc-
the oldest. At present, the root of the eukaryotic tree is uncer- tion of a single protein with both enzyme activities. Stechmann and
tain; hence, we do not know which supergroup of eukaryotes Cavalier-Smith amplified (using PCR; see Figure 20.8) and sequenced
was the first to diverge from all other eukaryotes. Some hypoth- the genes for DHFR and TS in nine species (one choanoflagellate, two
eses, such as the amitochondriate hypothesis described earlier, amoebozoans, one euglenozoan, one stramenopile, one alveolate,
have been abandoned, but researchers have yet to agree on an and three rhizarians). They combined their data with previously pub-
alternative. If the root of the eukaryotic tree were known, it lished data for species of bacteria, animals, plants, and fungi.
would help scientists infer characteristics of the common ances- Results The bacteria studied all have separate genes coding
tor of all eukaryotes. for DHFR and TS, suggesting that this is the ancestral condition
(red dot on the tree below). Other taxa with separate genes are
In trying to determine the root of the eukaryotic tree, denoted by red type. Fused genes are a derived character, found
researchers have based their phylogenies on different sets in certain members (blue type) of the supergroups Excavata, SAR,
of genes, some of which have produced conflicting results. and Archaeplastida:
Researchers have also tried a different approach, based
on tracing the occurrence of a rare evolutionary event Choanoflagellates
(Figure 28.24). Results from this “rare event” approach
indicate that Excavata, SAR, and Archaeplastida share a Animals Unikonta
more recent common ancestor than any of them does Fungi
with Unikonta. This suggests that the root of the tree is
located between the unikonts and all other eukaryotes, Common Amoebozoans
which implies that the unikonts were the first eukaryotic ancestor
supergroup to diverge from all other eukaryotes. This idea of all Diplomonads Excavata
remains controversial and will require more supporting eukaryotes Euglenozoans
evidence to be widely accepted.
Stramenopiles
Amoebozoans
Alveolates SAR
The amoebozoan clade includes many species of amoebas
that have lobe- or tube-shaped pseudopodia rather than the DHFR-TS Rhizarians
threadlike pseudopodia found in rhizarians. Amoebozoans gene
include slime molds, tubulinids, and entamoebas. fusion
Slime Molds Red algae Archaeplastida
Green algae
Slime molds, or mycetozoans (from the Latin, meaning “fun- Plants
gus animals”), once were thought to be fungi because, like
fungi, they produce fruiting bodies that aid in spore dispersal. Conclusion The results show that Excavata, SAR, and
However, DNA sequence analyses indicate that the resem- Archaeplastida form a clade, which supports the hypothesis that
blance between slime molds and fungi is a case of evolution- the root of the tree is located between the unikonts and all other
ary convergence. DNA sequence analyses also show that slime eukaryotes. Because support for this hypothesis is based on only
molds descended from unicellular ancestors—an example of one trait—the fusion of the genes for DHFR and TS—more data
the independent origin of multicellularity in eukaryotes. are needed to evaluate its validity.
Slime molds have diverged into two main branches, plas- Data from A. Stechmann and T. Cavalier-Smith, Rooting the eukaryote tree by using
modial slime molds and cellular slime molds. We’ll compare a derived gene fusion, Science 297:89–91 (2002).
their characteristics and life cycles.
WHAT IF? Stechmann and Cavalier-Smith wrote that their conclusions are
“valid only if the genes fused just once and were never secondarily split.” Why
is this assumption critical to their approach?
chapter 28 Protists 609
Figure 28.25 A plasmodial slime mold. The photograph shows a mature plasmodium, the feeding
stage in the life cycle of a plasmodial slime mold. When food becomes scarce, the plasmodium forms stalked
fruiting bodies that produce haploid spores that function in sexual reproduction.
7 Repeated mitotic divisions of the 1 The feeding stage is a
zygote’s nucleus, without cytoplasmic multinucleate plasmodium.
division, form the plasmodium.
6 The cells Zygote Feeding
fuse, forming (2n) plasmodium
diploid zygotes.
FERTILIZATION
Mature 4 cm
plasmodium
(preparing to fruit)
Young these organisms consists of solitary
sporangium
Amoeboid Mature cells that function individually, but
cells (n) sporangium
Flagellated when food is depleted, the cells form
cells Germinating
(n) spore a slug-like aggregate that functions as
5 The motile a unit (Figure 28.26). Unlike the feed-
haploid cells are
either amoeboid ing stage (plasmodium) of a plasmo-
or flagellated; the
two forms readily dial slime mold, these aggregated cells
convert from one
to the other. Spores (n) 2 The plasmodium remain separated by their individual
erects stalked plasma membranes. Ultimately, the
MEIOSIS fruiting bodies aggregated cells form an asexual
(sporangia) when fruiting body.
Stalk conditions
become harsh. Dictyostelium discoideum, a cellular
slime mold commonly found on forest
4 The resistant spores 3 In the sporangia, Key floors, has become a model organism
germinate in favorable meiosis produces for studying the evolution of multi-
conditions, releasing haploid spores, which
motile cells. disperse through the air. Haploid (n) cellularity. One line of research has
Diploid (2n) focused on the slime mold’s fruiting
body stage. During this stage, the cells
that form the stalk die as they dry out,
Plasmodial Slime Molds Many plasmodial slime molds while the spore cells at the top survive and have the potential
are brightly colored, often yellow or orange (Figure 28.25). to reproduce (see Figure 28.26). Scientists have found that
As they grow, they form a mass called a plasmodium, which
can be many centimeters in diameter. (Don’t confuse a slime mutations in a single gene can turn individual Dictyostelium
mold’s plasmodium with the genus Plasmodium, which
includes the parasitic apicomplexan that causes malaria.) cells into “cheaters” that never become part of the stalk.
Despite its size, the plasmodium is not multicellular; it is a
single mass of cytoplasm that is undivided by plasma mem- Because these mutants gain a strong reproductive advantage
branes and that contains many nuclei. This “supercell” is the
product of mitotic nuclear divisions that are not followed by over noncheaters, why don’t all Dictyostelium cells cheat?
cytokinesis. The plasmodium extends pseudopodia through
moist soil, leaf mulch, or rotting logs, engulfing food particles Recent discoveries suggest an answer to this question.
by phagocytosis as it grows. If the habitat begins to dry up
or there is no food left, the plasmodium stops growing and Cheating cells lack a specific surface protein, and noncheating
differentiates into fruiting bodies that function in sexual
reproduction. cells can recognize this difference. Noncheaters preferentially
Cellular Slime Molds The life cycle of the protists called aggregate with other noncheaters, thus depriving cheaters of
cellular slime molds can prompt us to question what it the chance to exploit them. Such a recognition system may
means to be an individual organism. The feeding stage of
have been important in the evolution of other multicellular
eukaryotes, such as animals and plants.
Tubulinids
Tubulinids constitute a large and varied group of amoebozoans
that have lobe- or tube-shaped pseudopodia. These unicel-
lular protists are ubiquitous in soil as well as freshwater and
marine environments. Most are heterotrophs that actively seek
and consume bacteria and other protists; one such tubulinid
610 Unit five The Evolutionary History of Biological Diversity
Figure 28.26 The life cycle of Dictyostelium, a cellular slime mold.
9 In favorable 1 In the feeding stage, 2 During sexual repro-
conditions, amoebas solitary haploid amoebas duction, two haploid
emerge from the engulf bacteria; these solitary amoebas fuse and form
spore coats and feed. cells periodically divide by a zygote.
mitosis (asexual reproduction).
8 Spores FERTILIZATION
are released.
7 Other Spores Emerging 3 The zygote
cells crawl (n) amoeba becomes a giant
up the stalk (n) cell by consuming
and develop haploid amoebas
into spores. SEXUAL Zygote (not shown). After
REPRODUCTION (2n) developing a
600 μm resistant wall, the
Solitary amoebas MEIOSIS giant cell undergoes
(feeding stage) meiosis followed by
(n) Amoebas several mitotic
ASEXUAL (n) divisions.
REPRODUCTION
Fruiting 4 The wall ruptures,
bodies Aggregated releasing new
(n) amoebas haploid amoebas.
Migrating 5 When food is depleted,
aggregate hundreds of amoebas
congregate in response to a
6 The aggregate migrates for a chemical attractant and form
while and then stops. Some of the a slug-like aggregate
cells dry up after forming a stalk that (see photo).
supports an asexual fruiting body.
VISUAL SKILLS Suppose cells were removed from the slug-like aggregate shown 200 μm Key
in the photo. Use the information in life cycle to infer whether these cells would be
haploid or diploid. Explain. Haploid (n)
Diploid (2n)
species, Amoeba proteus, is shown in Figure 28.2. Some tubuli- Opisthokonts
nids also feed on detritus (nonliving organic matter).
Opisthokonts are an extremely diverse group of eukaryotes
Entamoebas that includes animals, fungi, and several groups of protists.
We will discuss the evolutionary history of fungi and animals
Whereas most amoebozoans are free-living, those that belong in Chapters 31–34. Of the opisthokont protists, we will discuss
to the genus Entamoeba are parasites. They infect all classes the nucleariids in Chapter 31 because they are more closely
of vertebrate animals as well as some invertebrates. Humans related to fungi than they are to other protists. Similarly, we
are host to at least six species of Entamoeba, but only one, will discuss choanoflagellates in Chapter 32, since they are
E. histolytica, is known to be pathogenic. E. histolytica causes more closely related to animals than they are to other protists.
amebic dysentery and is spread via contaminated drinking The nucleariids and choanoflagellates illustrate why scientists
water, food, or eating utensils. Responsible for up to 100,000 have abandoned the former kingdom Protista: A monophy-
deaths worldwide every year, the disease is the third-leading letic group that includes these single-celled eukaryotes would
cause of death due to eukaryotic parasites, after malaria also have to include the multicellular animals and fungi that
(see Figure 28.16) and schistosomiasis (see Figure 33.11). are closely related to them.
chapter 28 Protists 611
Concept Check 28.5 Figure 28.28 Sudden oak death. Many dead oak trees are
visible in this Monterey County, California landscape. Infected trees
1. Contrast the pseudopodia of amoebozoans and forams. lose their ability to adjust to cycles of wet and dry weather.
2. In what sense is “fungus animal” a fitting description
of a slime mold? In what sense is it not fitting?
3. DRAW IT Recent evidence indicates that the root of
the eukaryotic tree may lie between a clade that includes
unikonts and excavates, and all other eukaryotes. Draw
the tree suggested by this result.
For suggested answers, see Appendix A.
Concept 28.6 malaria-causing protist Plasmodium: Income levels in countries
hard hit by malaria are 33% lower than in similar countries free
Protists play key roles in ecological of the disease. Protists can have devastating effects on other
communities species too. Massive fish kills have been attributed to Pfiesteria
shumwayae (see Figure 28.15), a dinoflagellate parasite that
Most protists are aquatic, and they are found almost anywhere attaches to its victims and eats their skin. Among species that
there is water, including moist terrestrial habitats such as damp parasitize plants, the stramenopile Phytophthora ramorum has
soil and leaf litter. In oceans, ponds, and lakes, many protists emerged as a major new forest pathogen. This species causes
are bottom-dwellers that attach to rocks and other substrates sudden oak death (SOD), a disease that has killed millions
or creep through the sand and silt. As we’ve seen, other protists of oaks and other trees in the United States and Great Britain
are important constituents of plankton. We’ll focus here on (Figure 28.28; also see Concept 54.5). A closely related species,
two key roles that protists play in the varied habitats in which P. infestans, causes potato late blight, which turns the stalks and
they live: that of symbiont and that of producer. stems of potato plants into black slime. Late blight contributed
to the devastating Irish famine of the 19th century, in which a
Symbiotic Protists million people died and at least that many were forced to leave
Ireland. The disease continues to be a major problem today,
Many protists form symbiotic associations with other species. causing crop losses as high as 70% in some regions.
For example, photosynthetic dinoflagellates are food-providing
symbiotic partners of the animals (coral polyps) that build coral Photosynthetic Protists
reefs. Coral reefs are highly diverse ecological communities.
That diversity ultimately depends on corals—and on the mutu- Many protists are important producers, organisms that use
alistic protists that nourish them. Corals support reef diversity energy from light (or in some prokaryotes, inorganic chemi-
by providing food to some species and habitat to many others. cals) to convert CO2 to organic compounds. Producers form
the base of ecological food webs. In aquatic communities, the
Another example is the wood-digesting protists that inhabit main producers are photosynthetic protists and prokaryotes
the gut of many termite species (Figure 28.27). Unaided, ter- (Figure 28.29). All other organisms in the community depend
mites cannot digest wood, and they rely on protistan or pro- on them for food, either directly (by eating them) or indirectly
karyotic symbionts to do so. Termites cause over $3.5 billion (by eating an organism that ate a producer). Scientists estimate
in damage annually to wooden homes in the United States. that roughly 30% of the world’s photosynthesis is performed by
diatoms, dinoflagellates, multicellular algae, and other aquatic
Symbiotic protists also include parasites that have com- protists. Photosynthetic prokaryotes contribute another 20%,
promised the economies of entire countries. Consider the and plants are responsible for the remaining 50%.
Figure 28.27 Because producers form the foundation of food webs, fac-
A symbiotic tors that affect producers can dramatically affect their entire
protist. This community. In aquatic environments, photosynthetic pro-
organism is a tists are often held in check by low concentrations of nitrogen,
hypermastigote, a phosphorus, or iron. Various human actions can increase the
member of a group
of parabasalids that
live in the gut of
termites and certain
cockroaches and
enable the hosts to
digest wood (SEM).
10 μm
612 Unit five The Evolutionary History of Biological Diversity
Figure 28.29 Protists: key producers in aquatic regions as sea surface temperatures have increased. By what
communities. Arrows in this simplified food web lead from mechanism do rising sea surface temperatures reduce the
food sources to the organisms that eat them. growth of marine producers? One hypothesis relates to the
rise or upwelling of cold, nutrient-rich waters from below.
Other Many marine producers rely on nutrients brought to the
consumers surface in this way. However, rising sea surface tempera-
tures can cause the formation of a layer of light, warm water
Herbivorous Carnivorous that acts as a barrier to nutrient upwelling—thus reducing
plankton plankton the growth of marine producers. If sustained, the changes
shown in Figure 28.30 would likely have far-reaching effects
Prokaryotic Protistan on marine ecosystems, fishery yields, and the global carbon
producers producers cycle (see Figure 55.14). Global warming can also affect
producers on land, but there the base of food webs is occu-
concentrations of these elements in aquatic communities. For pied not by protists but by plants, which we will discuss in
example, when fertilizer is applied to a field, some of the fertil- Chapters 29 and 30.
izer may be washed by rainfall into a river that drains into a
lake or ocean. When people add nutrients to aquatic commu- Concept Check 28.6
nities in this or other ways, the abundance of photosynthetic
protists can increase spectacularly. Such increases can have 1. Justify the claim that photosynthetic protists are among
major ecological consequences, including the formation of the biosphere’s most important organisms.
large “dead zones” in marine ecosystems (see Figure 56.23).
2. Describe three symbioses that include protists.
A pressing question is how global warming will affect 3. WHAT IF? High water temperatures and pollution
photosynthetic protists and other producers. As shown in
Figure 28.30, the growth and biomass of photosynthetic can cause corals to expel their dinoflagellate symbionts.
protists and prokaryotes have declined in many ocean How might such “coral bleaching” affect corals and
other species?
4. MAKE CONNECTIONS The bacterium Wolbachia is a
symbiont that lives in mosquito cells and spreads rapidly
through mosquito populations. Wolbachia can make mos-
quitoes resistant to infection by Plasmodium; researchers
are seeking a strain that confers resistance and does not
harm mosquitoes. Compare evolutionary changes that
could occur if malaria control is attempted using such a
Wolbachia strain versus using insecticides to kill mosqui-
toes. (Review Figure 28.16 and Concept 23.4.)
For suggested answers, see Appendix A.
Figure 28.30 Effects of climate change on marine producers.
NP EP A NA Arctic (A)
North Atlantic (NA)
NI EA Equatorial Atlantic (EA)
South Atlantic (SA)
Ocean region
North Indian (NI)
SI SA South Indian (SI)
S SP North Pacific (NP)
Equatorial Pacific (EP)
–2.50 –1.25 0.00 1.25 2.50 South Pacific (SP)
Sea-surface temperature (SST) change (˚C)
Southern (S)
(a) Researchers studied 10 ocean regions, identified with letters
on the map (see (b) for the corresponding names). SSTs have –0.02 –0.01 0.00 0.01 0.02
increased since 1950 in most areas of these regions. Chlorophyll change (mg /[m2 • yr])
(b) The concentration of chlorophyll, an index for the biomass and
growth of marine producers, has decreased over the same time
period in most ocean regions.
chapter 28 Protists 613
28 Chapter Review Go to MasteringBiology™ for Videos, Animations, Vocab Self-Quiz,
Practice Tests, and more in the Study Area.
Summary of Key Concepts Current evidence indicates that eukaryotes originated by
endosymbiosis when an archaeal host (or a host closely
Concept 28.1 related to the archaeans) engulfed an alpha proteobacterium
that would evolve into an organelle found in all eukaryotes,
Most eukaryotes are single-celled the mitochondrion.
organisms (pp. 592–597)
Plastids are thought to be descendants of cyanobacteria that were
Domain Eukarya includes many groups of protists, VOCAB engulfed by early eukaryotic cells. The plastid-bearing lineage
along with plants, animals, and fungi. Unlike pro- SELF-QUIZ eventually evolved into red algae and green algae. Other protist
goo.gl/6u55ks groups evolved from secondary endosymbiotic events in which
red algae or green algae were themselves engulfed.
karyotes, protists and other eukaryotes have a nucleus
and other membrane-enclosed organelles, as well as a cytoskeleton In one hypothesis, eukaryotes are grouped into four supergroups,
that enables them to have asymmetric forms and to change shape each a monophyletic clade: Excavata, SAR, Archaeplastida, and
as they feed, move, or grow. Unikonta.
Protists are structurally and functionally diverse and have a wide ? Describe similarities and differences between protists and other
eukaryotes.
variety of life cycles. Most are unicellular. Protists include photo-
autotrophs, heterotrophs, and mixotrophs.
Key Concept/Eukaryote Supergroup Major Groups Key Morphological Specific Examples
Diplomonads and Characteristics Giardia,
Concept 28.2 parabasalids Trichomonas
Modified mitochondria Trypanosoma,
Excavates include protists with modified Euglenozoans Euglena
mitochondria and protists with unique Kinetoplastids Spiral or crystalline rod
flagella (pp. 597–599) Euglenids inside flagella
? What evidence indicates that the excavates form a clade? Stramenopiles
Diatoms
Concept 28.3 Golden algae Hairy and smooth flagella Phytophthora,
Brown algae Laminaria
SAR is a highly diverse group of protists
defined by DNA similarities (pp. 599–606) Alveolates
Dinoflagellates
? Although they are not photosynthetic, apicomplexan Apicomplexans Membrane-enclosed sacs Pfiesteria,
parasites such as Plasmodium have modified plastids. Ciliates (alveoli) beneath plasma Plasmodium,
membrane Paramecium
Describe a current hypothesis that explains this observation. Rhizarians
Radiolarians
Forams Amoebas with threadlike Globigerina
Cercozoans pseudopodia
Concept 28.4 Red algae Phycoerythrin (photosyn- Porphyra
thetic pigment) Chlamydomonas,
Red algae and green algae are the Green algae Plant-type chloroplasts Ulva
closest relatives of plants (pp. 606–608)
Plants
? On what basis do systematists place plants in the same
supergroup (Archaeplastida) as red and green algae?
(See Chapters 29 and 30.) Mosses, ferns,
conifers,
flowering
plants
Concept 28.5 Amoebozoans Amoebas with lobe- Amoeba, Dictyostelium
Slime molds shaped or tube-shaped Choanoflagellates,
Unikonts include protists that Tubulinids pseudopodia nucleariids,
are closely related to fungi Entamoebas (Highly variable; see animals,
and animals (pp. 608–612) Chapters 31–34.) fungi
Opisthokonts
?
Describe a key feature for each of the main protist subgroups
of Unikonta.
614 Unit five The Evolutionary History of Biological Diversity
Concept 28.6 7. EVOLUTION CONNECTION • DRAW IT Medical researchers
seek to develop drugs that can kill or restrict the growth of
Protists play key roles in ecological human pathogens yet have few harmful effects on patients.
communities (pp. 612–613) These drugs often work by disrupting the metabolism of the
pathogen or by targeting its structural features.
Protists form a wide range of mutualistic and parasitic relationships Draw and label a phylogenetic tree that includes an
that affect their symbiotic partners and many other members of ancestral prokaryote and the following groups of organisms:
the community. Excavata, SAR, Archaeplastida, Unikonta, and, within
Unikonta, amoebozoans, animals, choanoflagellates, fungi,
Photosynthetic protists are among the most important producers and nucleariids. Based on this tree, hypothesize whether it
in aquatic communities. Because they are at the base of the food would be most difficult to develop drugs to combat human
web, factors that affect photosynthetic protists affect many other pathogens that are prokaryotes, protists, animals, or fungi.
species in the community. (You do not need to consider the evolution of drug resistance
by the pathogen.)
? Describe several protists that are ecologically important.
Level 3: Synthesis/Evaluation
Test Your Understanding
8. SCIENTIFIC INQUIRY Applying the “If p then” logic of
Level 1: Knowledge/Comprehension deductive reasoning (see Concept 1.3), what are a few of the
predictions that arise from the hypothesis that plants evolved
1. Plastids that are surrounded by more than two PRACTICE from green algae? Put another way, how could you test this
membranes are evidence of TEST hypothesis?
(A) evolution from mitochondria.
(B) fusion of plastids. goo.gl/CUYGKD 9. WRITE ABOUT A THEME: INTERACTIONS Organisms interact
(C) origin of the plastids from archaea. with each other and the physical environment. In a short
(D) secondary endosymbiosis. essay (100–150 words), explain how the response of diatom
populations to a drop in nutrient availability can affect both
2. Biologists think that endosymbiosis gave rise to mitochondria other organisms and aspects of the physical environment
before plastids partly because (such as carbon dioxide concentrations).
(A) the products of photosynthesis could not be metabolized
without mitochondrial enzymes. 10. SYNTHESIZE YOUR KNOWLEDGE
(B) all eukaryotes have mitochondria (or their remnants),
whereas many eukaryotes do not have plastids. This micrograph shows a single-celled eukaryote, the ciliate
(C) mitochondrial DNA is less similar to prokaryotic DNA than Didinium (left), about to engulf its Paramecium prey, which
is plastid DNA. is also a ciliate. Identify the eukaryotic supergroup to which
(D) without mitochondrial CO2 production, photosynthesis ciliates belong and describe the role of endosymbiosis in the
could not occur. evolutionary history of that supergroup. Are these ciliates
more closely related to all other protists than they are to plants,
3. Which group is incorrectly paired with its description? fungi, or animals? Explain.
(A) diatoms—important producers in aquatic communities For selected answers, see Appendix A.
(B) red algae—eukaryotes that acquired plastids by secondary
endosymbiosis For additional practice questions, check out the Dynamic Study
(C) apicomplexans—unicellular parasites with intricate life cycles Modules in MasteringBiology. You can use them to study on
(D) diplomonads—unicellular eukaryotes with modified your smartphone, tablet, or computer anytime, anywhere!
mitochondria
4. According to the phylogeny presented in this chapter, which
protists are in the same eukaryotic supergroup as plants?
(A) green algae
(B) dinoflagellates
(C) red algae
(D) both A and C
5. In a life cycle with alternation of generations, multicellular
haploid forms alternate with
(A) unicellular haploid forms.
(B) unicellular diploid forms.
(C) multicellular haploid forms.
(D) multicellular diploid forms.
Level 2: Application/Analysis
6. Based on the phylogenetic tree in Figure 28.2, which of the
following statements is correct?
(A) The most recent common ancestor of Excavata is older than
that of SAR.
(B) The most recent common ancestor of SAR is older than that
of Unikonta.
(C) The most basal (first to diverge) eukaryotic supergroup
cannot be determined.
(D) Excavata is the most basal eukaryotic supergroup.
chapter 28 Protists 615
Plant Diversity I: 29
How Plants Colonized Land
Figure 29.1 How did plants change the world?
Key Concepts The Greening of Earth
29.1 Plants evolved from green algae Looking at a lush landscape, such as that shown in Figure 29.1, it is hard to imagine
the land without plants or other organisms. Yet for much of Earth’s history, the land
29.2 Mosses and other nonvascular was largely lifeless. Geochemical analysis and fossil evidence suggest that thin coat-
ings of cyanobacteria and protists existed on land by 1.2 billion years ago. But it was
plants have life cycles dominated only within the last 500 million years that small plants, fungi, and animals joined
by gametophytes them ashore. Finally, by about 385 million years ago, tall plants appeared, leading to
the first forests (which consisted of very different species than those in Figure 29.1).
29.3 Ferns and other seedless vascular
Today, there are more than 290,000 known plant species. Plants inhabit all but
plants were the first plants the harshest environments, such as some mountaintop and desert areas and the
to grow tall polar ice sheets. Although a few plant species, such as sea grasses, returned to aquatic
habitats during their evolution, most present-day plants live on land. In this text,
we distinguish plants from algae, which are photosynthetic protists.
Plants enabled other life-forms to survive on land. For example, plants supply
oxygen and are a key source of food for terrestrial animals. Also, by their very pres-
ence, plants such as the trees of a forest physically create the habitats required by
animals and many other organisms. This chapter traces the first 100 million years
of plant evolution, including the emergence of seedless plants such as mosses and
ferns. Chapter 30 examines the later evolution of seed plants.
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616
Concept 29.1 not submerged. In charophytes, a layer of a durable polymer
called sporopollenin prevents exposed zygotes from drying
Plants evolved from green algae out. A similar chemical adaptation is found in the tough spo-
ropollenin walls that encase plant spores.
As you read in Chapter 28, green algae called charophytes are
the closest relatives of plants. We’ll begin with a closer look at The accumulation of such traits by at least one population
the evidence for this relationship. of charophyte algae (now extinct) probably enabled their
descendants—the first plants—to live permanently above
Morphological and Molecular Evidence the waterline. This ability opened a new frontier: a terrestrial
habitat that offered enormous benefits. The bright sunlight
Many key traits of plants also appear in some algae. For exam- was unfiltered by water and plankton; the atmosphere offered
ple, plants are multicellular, eukaryotic, photosynthetic auto- more plentiful carbon dioxide than did water; and the soil by
trophs, as are brown, red, and certain green algae. Plants have the water’s edge was rich in some mineral nutrients. But these
cell walls made of cellulose, and so do green algae, dinoflagel- benefits were accompanied by challenges: a relative scarcity
lates, and brown algae. And chloroplasts with chlorophylls of water and a lack of structural support against gravity. (To
a and b are present in green algae, euglenids, and a few appreciate why such support is important, picture how the
dinoflagellates, as well as in plants. soft body of a jellyfish sags when taken out of water.) Plants
diversified as new adaptations arose that enabled them to
However, the charophytes are the only present-day algae thrive despite these challenges.
that share the following distinctive traits with plants, suggest-
ing that they are the closest living relatives of plants: Today, what adaptations are unique to plants? The
answer depends on where you draw the boundary divid-
Rings of cellulose-synthesizing proteins. The ing plants from algae (Figure 29.2). Since the placement
cells of both plants and charophytes have distinc- of this boundary is the subject of ongoing debate, this
tive circular rings of proteins (small photo) embed- text uses a traditional definition that equates the king-
ded in the plasma membrane. These protein rings dom Plantae with embryophytes (plants with embryos).
synthesize the cellulose microfibrils of the cell wall. In this context, let’s now examine the derived traits that
In contrast, noncharophyte algae have linear sets of 30 nm separate plants from their closest algal relatives.
proteins that synthesize cellulose.
Derived Traits of Plants
Structure of flagellated sperm. In species of plants
that have flagellated sperm, the structure of the sperm Several adaptations that facilitate survival and reproduction
closely resembles that of charophyte sperm. on dry land emerged after plants diverged from their algal
relatives. Figure 29.3 depicts five such traits that are found in
Formation of a phragmoplast. Particular details plants but not in charophyte algae.
of cell division occur only in plants and certain charo-
phytes. For example, a group of microtubules known as Figure 29.2 Three possible “plant” kingdoms.
the phragmoplast forms between the daughter nuclei of
a dividing cell. A cell plate then develops in the middle Red algae
of the phragmoplast, across the midline of the dividing
cell (see Figure 12.10). The cell plate, in turn, gives rise to ANCESTRAL Chlorophytes Viridiplantae
a new cross wall that separates the daughter cells. ALGA Streptophyta
Studies of nuclear, chloroplast, and mitochondrial DNA Other
from a wide range of plants and algae indicate that certain charophytes
groups of charophytes—such as Zygnema (see Figure 28.22a) and
Coleochaete—are the closest living relatives of plants. Although
this evidence shows that plants arose from within a group of
charophyte algae, it does not mean that plants are descended
from these living algae. Even so, present-day charophytes may
tell us something about the algal ancestors of plants.
Adaptations Enabling the Move to Land Closest Plantae
charophyte
Many species of charophyte algae inhabit shallow waters relative
around the edges of ponds and lakes, where they are subject Embryophytes
to occasional drying. In such environments, natural selection
favors individual algae that can survive periods when they are
chapter 29 Plant Diversity I: How Plants Colonized Land 617
Figure 29.3 Exploring Derived Traits of Plants
Charophyte algae lack the key traits of plants described in this figure: alternation of generations;
multicellular, dependent embryos; walled spores produced in sporangia; multicellular gametangia;
and apical meristems. This suggests that these traits were absent in the ancestor common to plants
and charophytes but instead evolved as derived traits of plants. Not every plant exhibits all of these
traits; certain lineages of plants have lost some traits over time.
Alternation of Generations
The life cycles of all plants alternate between two includes both multicellular haploid organisms and multicellular
generations of distinct multicellular organisms: gametophytes diploid organisms. The multicellular haploid gametophyte
and sporophytes. As shown in the diagram below (using a (“gamete-producing plant”) is named for its production by
fern as an example), each generation gives rise to the other, a mitosis of haploid gametes—eggs and sperm—that fuse during
process that is called alternation of generations. This type fertilization, forming diploid zygotes. Mitotic division of the
of reproductive cycle evolved in various groups of algae but zygote produces a multicellular diploid sporophyte
does not occur in the charophytes, the algae most closely (“spore-producing plant”). Meiosis in a mature
related to plants. Take care not to confuse the 1 The gametophyte produces sporophytes produces haploid spores,
alternation of generations in plants with the haploid gametes by mitosis. reproductive cells that can develop into
haploid and diploid stages in the life cycles a new haploid organism without
of other sexually reproducing organisms Gametophyte Gamete from fusing with another cell. Mitotic
(see Figure 13.6). Alternation of (n) another plant division of the spore cell
generations is distinguished Mitosis Mitosis n produces a new multicellular
by the fact that the life cycle n gametophyte, and the cycle
begins again.
Alternation of generations: n Spore n 2 Two gametes
five generalized steps MEIOSIS Gamete unite (fertilization)
and form a diploid
5 The spores develop FERTILIZATION zygote.
into multicellular
haploid gametophytes.
4 The sporophyte Sporophyte Zygote 3 The zygote Key
produces unicellar (2n) 2n develops into a Haploid (n)
multicellular Diploid (2n)
haploid spores Mitosis diploid sporophyte.
by meiosis.
Multicellular, Dependent Embryos
As part of a life cycle with alternation of Embryo (LM) and placental transfer cell (TEM)
generations, multicellular plant embryos of Marchantia (a liverwort)
develop from zygotes that are retained
within the tissues of the female parent (a Embryo
gametophyte). The parental tissues protect the Maternal
developing embryo from harsh environmental tissue
conditions and provide nutrients such as
sugars and amino acids. The embryo has 10 μm 2 μm Wall ingrowths
specialized placental transfer cells that enhance Placental transfer
the transfer of nutrients to the embryo through cell (blue outline)
elaborate ingrowths of the wall surface (plasma
membrane and cell wall). The multicellular,
dependent embryo of plants is such a signifi-
cant derived trait that plants are also known as
embryophytes.
618 Unit five The Evolutionary History of Biological Diversity Make Connections Review sexual life cycles in Figure 13.6. Identify
which type of sexual life cycle has alternation of generations, and summarize
how it differs from other life cycles.
Walled Spores Produced in Sporangia Spores
Plant spores are haploid reproductive cells that can grow into Sporangium
multicellular haploid gametophytes by mitosis. The polymer spo-
ropollenin makes the walls of plant spores tough and resistant to Longitudinal
harsh environments. This chemical adaptation enables spores to be section of Mnium
dispersed through dry air without harm. sporangium (LM)
Sporophyte
The sporophyte has multicellular organs called sporangia Gametophyte
(singular, sporangium) that produce the spores. Within a sporan-
gium, diploid cells called sporocytes, or spore mother cells, un-
dergo meiosis and generate the haploid spores. The outer tissues
of the sporangium protect the developing spores until they are
released into the air. Multicellular sporangia that produce spores
with sporopollenin-enriched walls are key terrestrial adaptations
of plants. Although charophytes also produce spores, these algae
lack multicellular sporangia, and their flagellated, water-dispersed
spores lack sporopollenin.
Multicellular Gametangia Sporophytes and sporangia of Mnium (a moss)
Another feature distinguishing early plants from their algal Female Archegonia,
ancestors was the production of gametes within multicellular gametophyte each with an
organs called gametangia. The female gametangia are called egg (yellow)
archegonia (singular, archegonium). Each archegonium is a pear-
shaped organ that produces a single nonmotile egg retained within Antheridia
the bulbous part of the organ (the top for the species shown here). (brown),
The male gametangia, called antheridia (singular, antheridium), containing
produce sperm and release them into the environment. In many sperm
groups of present-day plants, the sperm have flagella and swim to
the eggs through water droplets or a film of water. Each egg is fer- Male
tilized within an archegonium, where the zygote develops into an gametophyte
embryo. The gametophytes of seed plants are so reduced in size (as Archegonia and antheridia of Marchantia (a liverwort)
you will see in Chapter 30) that the archegonia and antheridia
have been lost in many lineages.
Apical Meristems
In terrestrial habitats, a photosynthetic organism Apical meristem Developing
finds essential resources in two very different places. of shoot leaves
Light and CO2 are mainly available above ground;
water and mineral nutrients are found mainly in Apical meristems of plant roots and
the soil. Though plants cannot move from place to shoots. The LMs are longitudinal sections
place, most plants have roots and shoots that can at the tips of a root and shoot.
elongate, increasing exposure to environmental
resources. Growth in length is sustained throughout
the plant’s life by the activity of apical meristems,
regions at growing tips of the plant body where
one or more cells divide repeatedly. Cells produced
by apical meristems differentiate into the outer
epidermis, which protects the body, and various types
of internal tissues. Apical meristems of shoots also
generate leaves in most plants. Thus, the complex
bodies of most plants have specialized below- and
aboveground organs.
Apical meristem Root 100 μm Shoot 100 μm
of root
chapter 29 Plant Diversity I: How Plants Colonized Land 619
Additional derived traits that relate to terrestrial life have Figure 29.4 Ancient plant spores and tissue
evolved in many plant species. For example, the epidermis (colorized SEMs).
in many species has a covering, the cuticle, that consists (a) Fossilized spores.
of wax and other polymers. Permanently exposed to the air, The chemical
plants run a far greater risk of desiccation (drying out) than composition and
do their algal relatives. The cuticle acts as waterproofing, wall structure of
helping prevent excessive water loss from the aboveground these 450-million-
plant organs, while also providing some protection from year-old spores
microbial attack. Most plants also have specialized pores match those found
in plants.
called stomata (singular, stoma), which support photo-
synthesis by allowing the exchange of CO2 and O2 between
the outside air and the plant (see Figure 10.4). Stomata are
also the main avenues by which water evaporates from the
plant; in hot, dry conditions, the stomata close, minimizing
water loss. (b) Fossilized 50 μm
The earliest plants lacked true roots and leaves. Without sporophyte tissue.
The spores were
roots, how did these plants absorb nutrients from the soil? embedded in tissue
Fossils dating from 420 million years ago reveal an adaptation that appears to be
that may have aided early plants in nutrient uptake: They from plants.
formed symbiotic associations with fungi. We’ll describe
these associations, called mycorrhizae, and their benefits to
both plants and fungi in more detail in Concept 31.1. For
now, the main point is that mycorrhizal fungi form extensive after the appearance of plant spores in the fossil record. While
networks of filaments through the soil and transfer nutri- the precise age (and form) of the first plants has yet to be dis-
ents to their symbiotic plant partner. This benefit may have covered, those ancestral species gave rise to the vast diversity
helped plants without roots to colonize land. of living plants. Table 29.1 summarizes the ten extant phyla
in the taxonomic scheme used in this text. (Extant lineages
The Origin and Diversification of Plants are those that have surviving members.) As you read the rest
of this section, look at Table 29.1 together with Figure 29.6,
The algae most closely related to plants include many uni- which reflects a view of plant phylogeny that is based on
cellular species and small colonial species. Since it is likely plant morphology, biochemistry, and genetics.
that the first plants were similarly small, the search for the One way to distinguish groups of plants is whether or not
earliest fossils of plants has focused on the microscopic they have an extensive system of vascular tissue, cells joined
world. As mentioned earlier, microorganisms colonized into tubes that transport water and nutrients throughout the
land as early as 1.2 billion years ago. But the microscopic plant body. Most present-day plants have a complex vascular
fossils that document life on land changed dramatically tissue system and are therefore called vascular plants. Plants
470 million years ago with the appearance of spores from that do not have an extensive transport system—liverworts,
early plants. mosses, and hornworts—are described as “nonvascular”
What distinguishes these spores from those of plants, even though some mosses do have simple
algae or fungi? One clue comes from their chemi- vascular tissue. Nonvascular plants are often infor-
cal composition, which matches the composition mally called bryophytes (from the Greek bryon,
of plant spores today but differs from that of the moss, and phyton, plant). Although the term bryo-
spores of other organisms. In addition, the walls phyte is commonly used to refer to all nonvascu-
of these ancient spores have structural features lar plants, molecular studies and morphological
that today are found only in the spores of certain analyses of sperm structure have concluded that
plants (liverworts). And in rocks dating to 450 bryophytes do not form a monophyletic group
million years ago, researchers have discovered (a clade).
similar spores embedded in plant cuticle mate- Vascular plants, which form a clade that
rial that resembles spore-bearing tissue in living comprises about 93% of all extant plant
plants (Figure 29.4). 0.3 mm s pecies, can be categorized further into
Fossils of larger plant structures, such as the smaller clades. Two of these clades are
Cooksonia sporangium in Figure 29.5, date to Figure 29.5 Cooksonia the lycophytes (the club mosses and
425 million years ago, which is 45 million years sporangium fossil their r elatives) and the monilophytes
620 Unit five The Evolutionary History of Biological Diversity
Table 29.1 Ten Phyla of Extant Plants notice in Figure 29.6 that, like bryophytes, seedless
vascular plants do not form a clade.
Common Name Number
of Known A group such as the bryophytes or the seedless vascular
Nonvascular Plants (Bryophytes) plants is sometimes referred to as a grade, a collection of
Species organisms that share key biological features. Grades can be
Phylum Hepatophyta Liverworts informative by grouping organisms according to their fea-
9,000 tures, such as having a vascular system but lacking seeds. But
Phylum Bryophyta Mosses 15,000 members of a grade, unlike members of a clade, do not nec-
essarily share the same ancestry. For example, even though
Phylum Anthocerophyta Hornworts 100 monilophytes and lycophytes are all seedless vascular plants,
monilophytes share a more recent common ancestor with
Vascular Plants Lycophytes 1,200 seed plants. As a result, we would expect monilophytes and
Seedless Vascular Plants Monilophytes 12,000 seed plants to share key traits not found in lycophytes—and
Phylum Lycophyta they do, as you’ll read in Concept 29.3.
Phylum Monilophyta 1
130 A third clade of vascular plants consists of seed plants,
Seed Plants Ginkgo which represent the vast majority of living plant species. A
Gymnosperms Cycads 75 seed is an embryo packaged with a supply of nutrients inside
Phylum Ginkgophyta Gnetophytes 600 a protective coat. Seed plants can be divided into two groups,
Phylum Cycadophyta Conifers gymnosperms and angiosperms, based on the absence or
Phylum Gnetophyta 250,000 presence of enclosed chambers in which seeds mature.
Phylum Coniferophyta Flowering plants Gymnosperms (from the Greek gymnos, naked, and sperm,
Angiosperms seed) are known as “naked seed” plants because their seeds
Phylum Anthophyta are not enclosed in chambers. Living gymnosperm species,
the most familiar of which are the conifers, form a clade.
(ferns and their relatives). The plants in each of these clades Angiosperms (from the Greek angion, container) are a huge
lack seeds, which is why collectively the two clades are often clade consisting of all flowering plants; their seeds develop
informally called seedless vascular plants. However,
Figure 29.6 Highlights of plant evolution. The phylogeny shown here illustrates a leading hypothesis
about the relationships between plant groups.
Origin of plants (about 470 mya) Liverworts Plants
Mosses Nonvascular
ANCESTRAL 1 Hornworts plants
GREEN ALGA (bryophytes)
Origin of vascular plants (about 425 mya) Lycophytes (club mosses, Seedless Vascular plants
2 spikemosses, quillworts) vascular
Monilophytes (ferns, plants
horsetails, whisk ferns)
Origin of seed plants Gymnosperms Seed plants
3 (about 360 mya) Angiosperms
500 450 400 350 300 50 0
Millions of years ago (mya)
MAKE CONNECTIONS The figure identifies which lineages are plants, nonvascular plants, vascular plants, seedless
vascular plants, and seed plants. Which of these categories are monophyletic, and which are paraphyletic? Explain.
(See Figure 26.10 to review these terms.)
chapter 29 Plant Diversity I: How Plants Colonized Land 621
inside chambers that originate within flowers. Nearly 90% of Over the long course of their evolution, liverworts, mosses,
living plant species are angiosperms. and hornworts have acquired many unique adaptations.
Next, we’ll examine some of those features.
Note that the phylogeny depicted in Figure 29.6 focuses
only on the relationships between extant plant lineages. Bryophyte Gametophytes
Paleobotanists have also discovered fossils belonging to
extinct plant lineages. As you’ll read later in the chapter, Unlike vascular plants, in all three bryophyte phyla the hap-
these fossils can reveal intermediate steps in the emergence loid gametophytes are the dominant stage of the life cycle:
of plant groups found on Earth today. They are usually larger and longer-living than the sporo-
phytes, as shown in the moss life cycle in Figure 29.7. The
Concept Check 29.1 sporophytes are typically present only part of the time.
1. Why do researchers identify the charophytes rather than When bryophyte spores are dispersed to a favorable
another group of algae as the closest living relatives of habitat, such as moist soil or tree bark, they may germinate
plants? and grow into gametophytes. Germinating moss spores,
for example, characteristically produce a mass of green,
2. Identify four derived traits that distinguish plants from branched, one-cell-thick filaments known as a protonema
charophyte green algae and facilitate life on land. (plural, protonemata). A protonema has a large surface area
Explain. that enhances absorption of water and minerals. In favorable
conditions, a protonema produces one or more “buds.”
3. WHAT IF? What would the human life cycle be like (Note that when referring to nonvascular plants, we often
if we had alternation of generations? Assume that the use quotation marks for structures similar to the buds, stems,
multicellular diploid stage would be similar in form to an and leaves of vascular plants because the definitions of these
adult human. terms are based on vascular plant organs.) Each of these bud-
For suggested answers, see Appendix A. like growths has an apical meristem that generates a gamete-
producing structure known as a gametophore. Together,
Concept 29.2 a protonema and one or more gametophores make up the
body of a moss gametophyte.
Mosses and other nonvascular
plants have life cycles dominated Bryophyte gametophytes generally form ground-hugging
by gametophytes carpets, partly because their body parts are too thin to sup-
port a tall plant. A second constraint on the height of many
Nonvascular plants (bryophytes) The nonvascular plants bryophytes is the absence of vascular tissue, which would
enable long-distance transport of water and nutrients. (The
Seedless vascular plants (bryophytes) are repre- thin structure of bryophyte organs makes it possible to
distribute materials for short distances without specialized
Gymnosperms sented today by three vascular tissue.) However, some mosses have conducting
Angiosperms phyla of small, herbaceous tissues in the center of their “stems.” A few of these mosses
can grow as tall as 60 cm (2 feet) as a result. Phylogenetic
(nonwoody) plants: liverworts (phylum Hepatophyta), analyses suggest that conducting tissues similar to those
of vascular plants arose independently in these mosses by
mosses (phylum Bryophyta), and hornworts (phylum convergent evolution.
Anthocerophyta). Liverworts and hornworts are named for The gametophytes are anchored by delicate rhizoids,
which are long, tubular single cells (in liverworts and horn-
their shapes, plus the suffix wort (from the Anglo-Saxon for worts) or filaments of cells (in mosses). Unlike roots, which
are found in vascular plant sporophytes, rhizoids are not
“herb”). Mosses are familiar to many people, although some composed of tissues. Bryophyte rhizoids also lack specialized
conducting cells and do not play a primary role in water and
plants commonly called “mosses” are not really mosses at mineral absorption.
all. These include Irish moss (a red seaweed), reindeer moss Gametophytes can form multiple gametangia, each of
which produces gametes and is covered by protective tissue.
(a lichen), club mosses (seedless vascular plants), and Each archegonium produces one egg, whereas each anther-
idium produces many sperm. Some bryophyte gametophytes
Spanish mosses (lichens in some regions and flowering are bisexual, but in mosses the archegonia and antheridia are
plants in others).
Phylogenetic analyses indicate that liverworts, mosses,
and hornworts diverged from other plant lineages early
in the history of plant evolution (see Figure 29.6). Fossil
evidence provides some support for this idea: The earliest
spores of plants (dating from 450 to 470 million years ago)
have structural features found only in the spores of liver-
worts, and by 430 million years ago spores similar to those
of mosses and hornworts also occur in the fossil record. The
earliest fossils of vascular plants date to about 425 million
years ago.
622 Unit five The Evolutionary History of Biological Diversity
Figure 29.7 The life cycle of a moss.
1 Spores 2 The haploid protonemata
develop into produce ”buds” that divide
threadlike by mitosis and grow into
protonemata. gametophores.
Sperm 3 Sperm must
swim through a
”Bud” Antheridia film of moisture
Male to reach the egg.
gametophyte
Key (n)
Haploid (n) Protonemata
Diploid (2n) (n)
”Bud”
Egg
Spores Gametophore
Female Archegonia
gametophyte
Spore (n)
dispersal
Peristome 7 Meiosis occurs and haploid Rhizoid
spores develop in the capsule.
When the capsule is mature,
its lid pops off, and the spores
are released.
Sporangium 5 The sporophyte grows a FERTILIZATION
MEIOSIS long stalk (seta) that emerges (within archegonium)
from the archegonium.
Mature Seta
sporophytes Capsule Zygote
(sporangium) (2n)
Foot Embryo
Young Archegonium
sporophyte 4 The zygote
2 mm (2n) 6 Attached by its foot, the develops into a
sporophyte embryo.
Female sporophyte remains nutritionally
Capsule (LM) gametophytes dependent on the gametophyte. Animation: Moss Life Cycle
VISUAL SKILLS In this diagram, does the sperm cell that fertilizes the egg cell differ genetically
from the egg? Explain.
typically carried on separate female and male gametophytes. Bryophyte sperm typically require a film of water to reach
Flagellated sperm swim through a film of water toward eggs, the eggs. Given this requirement, it is not surprising that
entering the archegonia in response to chemical attractants. many bryophyte species are found in moist habitats. The fact
Eggs are not released but instead remain within the bases that sperm swim through water to reach the egg also means
of archegonia. After fertilization, embryos are retained that in species with separate male and female gametophytes
within the archegonia. Layers of placental transfer cells help (most species of mosses), sexual reproduction is likely to be
transport nutrients to the embryos as they develop into more successful when individuals are located close to one
sporophytes. another.
chapter 29 Plant Diversity I: How Plants Colonized Land 623
Figure 29.8 Exploring Bryophyte Diversity
Liverworts (Phylum Hepatophyta)
This phylum’s common and scientific names (from the Latin he- liver diseases. Some liverworts, including Marchantia, are
paticus, liver) refer to the liver-shaped gametophytes of its mem- described as “thalloid” because of the flattened shape of their
bers, such as Marchantia, shown below. In medieval times, their gametophytes. Marchantia gametangia are elevated on gameto-
shape was thought to be a sign that the plants could help treat phores that look like miniature trees. You would need a mag-
nifying glass to see the sporophytes, which have a short seta
(stalk) with an oval or round capsule. Other liverworts, such as
Thallus Gametophore of Plagiochila, below, are called “leafy” because their stemlike game-
female gametophyte tophytes have many leaflike appendages. There are many more
species of leafy liverworts than thalloid liverworts.
Sporophyte
Foot Plagiochila
Seta deltoidea,
a ”leafy”
Capsule liverwort
(sporangium)
Marchantia polymorpha, 500 μm
a ”thalloid” liverwort
Marchantia sporophyte (LM)
Hornworts (Phylum Anthocerophyta) Mosses (Phylum Bryophyta)
This phylum’s common and scientific names (from the Greek Moss gametophytes, which range in height from less than 1 mm
keras, horn) refer to the long, tapered shape of the sporophyte. A up to 60 cm, are less than 15 cm tall in most species. The familiar
typical sporophyte can grow to about 5 cm high. Unlike a liver- carpet of moss you observe consists mainly of gametophytes. The
wort or moss sporophyte, a hornwort sporophyte lacks a seta and blades of their “leaves” are usually only one cell thick, but more
consists only of a sporangium. The sporangium releases mature complex “leaves” that have ridges coated with cuticle can be found
spores by splitting open, starting at the tip of the horn. The game- on the common hairy-cap moss (Polytrichum, below) and its close
tophytes, which are usually 1–2 cm in diameter, grow mostly hori- relatives. Moss sporophytes are typically elongated and visible to
zontally and often have multiple sporophytes attached. Hornworts the naked eye, with heights ranging up to about 20 cm. Though
are frequently among the first species to colonize open areas with green and photosynthetic when young, they turn tan or brownish
moist soils; a symbiotic relationship with nitrogen-fixing cyano- red when ready to release spores.
bacteria contributes to their ability to do this (nitrogen is often in
short supply in such areas).
An Anthoceros Polytrichum commune,
hornwort species hairy-cap moss
Sporophyte Capsule Sporophyte
Seta
Gametophyte
Gametophyte
624 Unit five The Evolutionary History of Biological Diversity
Many bryophyte species can increase the Annual nitrogen lossfound, they help retain nitrogen in the soil (Figure 29.9).
number of individuals in a local area through (kg/ha)In northern coniferous forests, species such as the feather
various methods of asexual reproduction. For moss Pleurozium harbor nitrogen-fixing cyanobacteria that
example, some mosses reproduce asexually by increase the availability of nitrogen in the ecosystem. Other
forming brood bodies, small plantlets (as shown mosses inhabit such extreme environments as mountain-
here) that detach from the parent plant and grow tops, tundra, and deserts. Many mosses are able to live in
into new, genetically identical copies of their very cold or dry habitats because they can survive the loss of
parent. most of their body water, then rehydrate when moisture is
available. Few vascular plants can survive the same degree of
Bryophyte Sporophytes desiccation. Moreover, phenolic compounds in moss cell
walls absorb damaging levels of UV radiation present in
The cells of bryophyte sporophytes contain plastids that are deserts or at high altitudes.
usually green and photosynthetic when the sporophytes are
young. Even so, bryophyte sporophytes cannot live inde- One wetland moss genus, Sphagnum, or peat moss, is often
pendently. A bryophyte sporophyte remains attached to its a major component of deposits of partially decayed organic
parental gametophyte throughout the sporophyte’s lifetime,
dependent on the gametophyte for supplies of sugars, amino Figure 29.9
acids, minerals, and water.
Inquiry Can bryophytes reduce the rate at which
Bryophytes have the smallest sporophytes of all extant key nutrients are lost from soils?
plant groups, consistent with the hypothesis that larger spo-
rophytes evolved only later, in the vascular plants. A typical Experiment Soils in terrestrial ecosystems are often low in nitro-
bryophyte sporophyte consists of a foot, a seta, and a spo- gen, a nutrient required for normal plant growth. Richard Bowden,
rangium. Embedded in the archegonium, the foot absorbs of Allegheny College, measured annual inputs (gains) and outputs
nutrients from the gametophyte. The seta (plural, setae), or (losses) of nitrogen in a sandy soil ecosystem dominated by the moss
stalk, conducts these materials to the sporangium, also called Polytrichum. Nitrogen inputs were measured from rainfall (dissolved
a capsule, which produces spores by meiosis. ions, such as nitrate, NO3-), biological N2 fixation, and wind deposi-
tion. Nitrogen losses were measured in leached water (dissolved
Bryophyte sporophytes can produce enormous numbers ions, such as NO3-) and gaseous emissions (such as N2O emitted by
of spores. A single moss capsule, for example, can generate bacteria). Bowden measured losses for soils with Polytrichum and for
over 5 million spores. In most mosses, the seta becomes elon- soils where the moss was removed two months before the experi-
gated, enhancing spore dispersal by elevating the capsule. ment began.
Typically, the upper part of the capsule features a ring of
interlocking, tooth-like structures known as the peristome Results A total of 10.5 kg of nitrogen per hectare (kg/ha) entered
(see Figure 29.7). These “teeth” open under dry conditions
and close again when it is moist. This allows moss spores to [t0h.e1e0ckogsy/(shtaem# yer)a].chThyeearer.sLuiltttsleonf ictroomgpenarwinagsnloitsrot gbeynglaossesoeus sbeymleiassciho-ns
be discharged gradually, via periodic gusts of wind that can
carry them long distances. ing are shown below.
Moss and hornwort sporophytes are often larger and more 6
complex than those of liverworts. For example, hornwort
sporophytes, which superficially resemble grass blades, have 5
a cuticle. Moss and hornwort sporophytes also have stomata,
as do all vascular plants (but not liverworts). 4
Figure 29.8 shows some examples of gametophytes and 3
sporophytes in the bryophyte phyla.
2
The Ecological and Economic Importance
of Mosses 1
Wind dispersal of lightweight spores has distributed mosses 0
throughout the world. These plants are particularly com- With moss Without moss
mon and diverse in moist forests and wetlands. Some
mosses colonize bare, sandy soil, where, researchers have Conclusion The moss Polytrichum greatly reduced the loss of
nitrogen by leaching in this ecosystem. Each year, the moss ecosystem
retained over 95% of the 10.5 kg/ha of total nitrogen inputs (only
0.1 kg/ha and 0.3 kg/ha were lost to gaseous emissions and leaching,
respectively).
Data from R. D. Bowden, Inputs, outputs, and accumulation of nitrogen in an early
successional moss (Polytrichum) ecosystem, Ecological Monographs 61:207–223 (1991).
WHAT IF? How might the presence of Polytrichum affect plant species
that typically colonize the sandy soils after the moss?
chapter 29 Plant Diversity I: How Plants Colonized Land 625
material known as peat (Figure 29.10a). Boggy regions with further to global warming. The historical and expected future
thick layers of peat are called peatlands. Sphagnum does not effects of Sphagnum on the global climate underscore the
decay readily, in part because of phenolic compounds embed- importance of preserving and managing peatlands.
ded in its cell walls. The low temperature, pH, and oxygen
level of peatlands also inhibit decay of moss and other organ- Mosses may have a long history of affecting climate change.
isms in these boggy wetlands. As a result, some peatlands have In the Scientific Skills Exercise, you will explore the question
preserved corpses for thousands of years (Figure 29.10b). of whether they did so during the Ordovician period by con-
tributing to the weathering of rocks.
Peat has long been a fuel source in Europe and Asia, and it
is still harvested for fuel today, notably in Ireland and Canada. Concept Check 29.2
Peat moss is also useful as a soil conditioner and for packing
plant roots during shipment because it has large dead cells 1. How do bryophytes differ from other plants?
that can absorb roughly 20 times the moss’s weight in water. 2. Give three examples of how structure fits function in
Peatlands cover 3% of Earth’s land surface and contain bryophytes.
roughly 30% of the world’s soil carbon: Globally, an estimated 3. MAKE CONNECTIONS Review the discussion of feed-
450 billion tons of organic carbon is stored as peat. Current
overharvesting of Sphagnum—primarily for use in peat-fired back regulation in Concept 1.1. Could effects of global
power stations—may contribute to global warming by releas- warming on peatlands alter CO2 concentrations in ways
ing stored CO2. In addition, if global temperatures continue to that result in negative or positive feedback? Explain.
rise, the water levels of some peatlands are expected to drop.
Such a change would expose peat to air and cause it to decom- For suggested answers, see Appendix A.
pose, thereby releasing additional stored CO2 and contributing
Concept 29.3
Figure 29.10 Sphagnum, or peat moss: a bryophyte with
economic, ecological, and archaeological significance. Ferns and other seedless vascular
plants were the first plants to grow tall
Nonvascular plants (bryophytes) During the first 100 million
Seedless vascular plants years of plant evolution,
Gymnosperms bryophytes were prominent
Angiosperms types of vegetation. But it is
vascular plants that dominate most landscapes today. The
earliest fossils of vascular plants date to 425 million years ago.
These plants lacked seeds but had well-developed vascular
systems, an evolutionary novelty that set the stage for vascu-
lar plants to grow taller than their bryophyte counterparts. As
in bryophytes, however, the sperm of ferns and all other seed-
less vascular plants are flagellated and swim through a film of
water to reach eggs. In part because of these swimming sperm,
seedless vascular plants today are most common in damp
environments.
(a) Peat being harvested from a peatland Origins and Traits of Vascular Plants
(b) ”Tollund Man,” a bog mummy dating from 405–100 B.C.E.
Unlike the nonvascular plants, ancient relatives of vascular
The acidic, oxygen-poor conditions produced by Sphagnum can plants had branched sporophytes that were not dependent
preserve human or other animal bodies for thousands of years. on gametophytes for nutrition (Figure 29.11). Although
these early plants were less than 20 cm tall, their branching
enabled their bodies to become more complex and to have
multiple sporangia. As plant bodies became more complex
over time, competition for space and sunlight probably
increased. As we’ll see, that competition may have stimu-
lated still more evolution in vascular plants, eventually lead-
ing to the formation of the first forests.
Early vascular plants had some derived traits of today’s
vascular plants, but they lacked roots and some other adapta-
tions that evolved later. The main traits that characterize liv-
ing vascular plants are life cycles with dominant sporophytes,
626 Unit five The Evolutionary History of Biological Diversity
Scientific Skills Exercise
Making Bar Graphs and Interpreting Interpret The Data
Data 1. Why did the researchers add
Could Nonvascular Plants Have Caused Weathering of filtrate from which macer-
Rocks and Contributed to Climate Change During the ated moss had been removed
Ordovician Period? The oldest traces of terrestrial plants are fossil- to the control microcosms?
ized spores formed 470 million years ago. Between that time and the
end of the Ordovician period 444 million years ago, the atmospheric 2. Make two bar graphs (for
CO2 level dropped by half, and the climate cooled dramatically. granite and andesite) com-
paring the mean amounts of each element weathered from rocks
One possible cause of the drop in CO2 during the Ordovician period in the control and experimental microcosms. (Hint: For an experi-
is the breakdown, or weathering, of rock. As rock weathers, calcium mental microcosm, what sum represents the total amount weath-
silicate (Ca2SiCO3) is released and combines with CO2 from the air, pro- ered from rocks?)
ducing calcium carbonate (CaCO3). Today, the roots of vascular plants
increase rock weathering and mineral release by producing acids that 3. Overall, what is the effect of moss on chemical weathering
break down rock and soil. Although nonvascular plants lack roots, they of rock? Are the results similar or different for granite and
andesite?
require the same mineral nutrients as vascular plants. Could nonvas- 4. Based on their experimental results, the researchers added
cular plants also increase the chemical weathering of rock? If so, they weathering of rock by nonvascular plants to simulation mod-
could have contributed to the decline in atmospheric CO2 during the els of the Ordovician climate. The new models predicted
Ordovician. In this exercise, you will interpret data from a study of the decreased CO2 levels and global cooling sufficient to produce
effects of moss on releasing minerals from two types of rock. the glaciations in the late Ordovician period. What assump-
How the Experiment Was Done The researchers set up experi- tions did the researchers make in using results from their
mental and control microcosms, or small artificial ecosystems, to mea- experiments in climate simulation models?
sure mineral release from rocks. First, they placed rock fragments of
volcanic origin, either granite or andesite, into small glass containers. 5. “Life has profoundly changed the Earth.” Explain whether or not
these experimental results support this statement.
Then they mixed water and macerated (chopped and crushed) moss Instructors: A version of this Scientific Skills Exercise can be
of the species Physcomitrella patens. They added this mixture to assigned in MasteringBiology.
the experimental microcosms (72 granite and 41 andesite). For
the control microcosms (77 granite
and 37 andesite), they filtered out the Ca21 (µmol) Mg21 (µmol) K1 (µmol)
moss and just added the water. After
130 days, they measured the amounts Granite Andesite Granite Andesite Granite Andesite
of various minerals found in the water Mean weathered amount 1.68 1.54 0.42 0.13 0.68 0.60
in the control microcosms and in the released in water in the
water and moss in the experimental control microcosms
microcosms.
Mean weathered amount 1.27 1.84 0.34 0.13 0.65 0.64
Data from the Experiment The released in water in the
moss grew (increased its biomass) in experimental microcosms
the experimental microcosms. The Mean weathered amount 1.09 3.62 0.31 0.56 1.07 0.28
table shows the mean amounts in taken up by moss in the
micromoles (µmol) of several minerals experimental microcosms
measured in the water and the moss
in the microcosms. Data from T. M. Lenton et al., First plants cooled the Ordovician, Nature Geoscience 5:86–89 (2012).
Figure 29.11 Sporophytes of Aglaophyton major, an Sporangia
ancient relative of living vascular plants. This reconstruction
from 405-million-year-old fossils exhibits dichotomous (Y-shaped)
branching with sporangia at the ends of branches. Sporophyte
branching characterizes living vascular plants but is lacking in living
nonvascular plants (bryophytes). Aglaophyton had structures called
rhizoids that anchored it to the ground. The inset shows a fossilized
stoma of A. major (colorized LM).
25 μm
2 cm 627
Rhizoids
transport in vascular tissues called xylem and phloem, and The xylem of most vascular plants includes tracheids, tube-
well-developed roots and leaves, including spore-bearing shaped cells that carry water and minerals up from the roots
leaves called sporophylls. (see Figure 35.10). The water-conducting cells in vascular
plants are lignified; that is, their cell walls are strengthened
Life Cycles with Dominant Sporophytes by the polymer lignin. The tissue called phloem has cells
arranged into tubes that distribute sugars, amino acids, and
As mentioned earlier, mosses and other bryophytes have life other organic products (see Figure 35.10).
cycles dominated by gametophytes (see Figure 29.7). Fossil evi-
dence suggests that a change began to develop in some of the Lignified vascular tissue helped enable vascular plants to
earliest vascular plants, whose gametophytes and sporophytes grow tall. Their stems became strong enough to provide sup-
were about equal in size. Further reductions in gametophyte port against gravity, and they could transport water and min-
size occurred among extant vascular plants; in these groups, eral nutrients high above the ground. Tall plants could also
the sporophyte generation is the larger and more complex outcompete short plants for access to the sunlight needed for
form in the alternation of generations (Figure 29.12). In ferns, photosynthesis. In addition, the spores of tall plants could
for example, the familiar leafy plants are the sporophytes. You disperse farther than those of short plants, enabling tall spe-
would have to get down on your hands and knees and search cies to colonize new environments rapidly. Overall, the abil-
the ground carefully to find fern gametophytes, which are tiny ity to grow tall gave vascular plants a competitive edge over
structures that often grow on or just below the soil surface. nonvascular plants, which typically are less than 5 cm in
height. Competition among vascular plants also would have
Transport in Xylem and Phloem increased, leading to selection for taller growth forms—a pro-
cess that eventually gave rise to the trees that formed the first
Vascular plants have two types of vascular tissue: xylem and forests about 385 million years ago.
phloem. Xylem conducts most of the water and minerals.
Figure 29.12 The life cycle 1 Sporangia release spores. 2 Each gametophyte develops sperm-producing
of a fern. Most fern species produce a single organs called antheridia and egg-producing
type of spore that develops into a organs called archegonia. Although this
WHAT IF? If the ability to disperse sperm bisexual photosynthetic gametophyte. simplified diagram shows a sperm fertilizing an
by wind evolved in a fern, how might its life egg from the same gametophyte, in most fern
cycle be affected? species a gametophyte produces sperm and eggs
at different times. Therefore, typically an egg
Key Spore Young from one gametophyte is fertilized by a sperm
Haploid (n) (n) gametophyte from another gametophyte.
Diploid (2n)
Rhizoid Antheridium
MEIOSIS
Spore
dispersal
Sporangium Mature New Underside Sperm
sporophyte sporophyte of mature
Sporangium (2n) gametophyte Archegonium
Sorus (n) Egg
Zygote FERTILIZATION
(2n)
5 On the underside Gametophyte 3 Sperm use flagella
of the sporophyte‘s to swim to eggs in the
reproductive leaves archegonia. An attractant
are spots called sori. secreted by archegonia
Each sorus is a helps direct the sperm.
cluster of sporangia.
4 A zygote develops into a new sporophyte,
Fiddlehead (young leaf) and the young plant grows out from an
628 Unit five The Evolutionary History of Biological Diversity archegonium of its parent, the gametophyte.
Animation: Fern Life Cycle
Evolution of Roots Figure 29.13 Microphyll and megaphyll leaves.
Microphyll leaves
Vascular tissue also provides benefits below ground. Instead
of the rhizoids seen in bryophytes, roots evolved in the Microphylls
sporophytes of almost all vascular plants. Roots are organs
that absorb water and nutrients from the soil. Roots also Unbranched
anchor vascular plants to the ground, hence allowing the vascular tissue
shoot system to grow taller.
Selaginella kraussiana
Root tissues of living plants closely resemble stem tissues (Krauss’s spikemoss)
of early vascular plants preserved in fossils. This suggests
that roots may have evolved from the lowest belowground Megaphyll leaves
portions of stems in ancient vascular plants. It is unclear
whether roots evolved only once in the common ancestor Megaphylls
of all vascular plants or independently in different lineages.
Although the roots of living members of these lineages of Branched Hymenophyllum tunbrigense
vascular plants share many similarities, fossil evidence hints vascular (Tunbridge filmy fern)
at convergent evolution. The oldest fossils of lycophytes, for tissue
example, already displayed simple roots 400 million years
ago, when the ancestors of ferns and seed plants still had megasporangia, which produce megaspores, spores that
none. Studying genes that control root development in dif- develop into female gametophytes. Microsporophylls have
ferent vascular plant species may help resolve this question. microsporangia, which produce microspores, smaller spores
that develop into male gametophytes. All seed plants and a
Evolution of Leaves few seedless vascular plants are heterosporous. The following
diagram compares the two conditions:
Leaves are structures that serve as the primary photosyn-
thetic organ of vascular plants. In terms of size and com- Homosporous spore production
plexity, leaves can be classified as either microphylls or (most seedless vascular plants)
megaphylls (Figure 29.13). All of the lycophytes—and only
the lycophytes—have microphylls, small, often spine- Sporangium Single Typically a Eggs
shaped leaves supported by a single strand of vascular tissue. on sporophyll type of spore bisexual Sperm
Almost all other vascular plants have megaphylls, leaves gametophyte
with a highly branched vascular system; a few species have
reduced leaves that appear to have evolved from megaphylls.
Megaphylls are typically larger than microphylls and there-
fore support greater photosynthetic productivity than micro-
phylls. Microphylls first appear in the fossil record 410 million
years ago, but megaphylls do not emerge until about 370
million years ago, toward the end of the Devonian period.
Sporophylls and Spore Variations Heterosporous spore production
One milestone in the evolution of plants was the emer- (all seed plants)
gence of sporophylls, modified leaves that bear sporangia.
Sporophylls vary greatly in structure. For example, fern Megasporangium Megaspore Female Eggs
sporophylls produce clusters of sporangia known as sori on megasporophyll gametophyte Sperm
(singular, sorus), usually on the undersides of the sporophylls
(see Figure 29.12). In many lycophytes and in most gymno- Microsporangium Microspore Male
sperms, groups of sporophylls form cone-like structures called on microsporophyll gametophyte
strobili. The sporophylls of angiosperms are called carpels
and stamens (see Figure 30.8). Classification of Seedless Vascular Plants
Most seedless vascular plant species are homosporous: As we noted earlier, biologists recognize two clades of living
They have one type of sporophyll bearing one type of sporan- seedless vascular plants: the lycophytes (phylum Lycophyta)
gium that produces one type of spore, which typically devel- and the monilophytes (phylum Monilophyta). The lycophytes
ops into a bisexual gametophyte, as in most ferns. In contrast, include the club mosses, the spikemosses, and the quillworts.
a heterosporous species has two types of sporophylls: mega- The monilophytes include the ferns, the horsetails, and the
sporophylls and microsporophylls. Megasporophylls have whisk ferns and their relatives. Although ferns, horsetails,
chapter 29 Plant Diversity I: How Plants Colonized Land 629
Figure 29.14 Exploring Seedless Vascular Plant Diversity
Lycophytes (Phylum Lycophyta)
Many lycophytes grow on tropical trees 2.5 cm
as epiphytes, plants that use other plants as
a substrate but are not parasites. Other Selaginella Isoetes Strobili
species grow on temperate forest floors. moellendorffii, gunnii, (clusters of
In some species, the tiny gametophytes a spike moss a quillwort sporophylls)
live above ground and are photosynthetic.
Others live below ground, nurtured by 1 cm Diphasiastrum tristachyum, a club moss
symbiotic fungi.
Sporophytes have upright stems with
many small leaves, as well as ground-
hugging stems that produce dichotomously
branching roots. Spike mosses are usually
relatively small and often grow horizon-
tally. In many club mosses and spike
mosses, sporophylls are clustered into club-
shaped cones (strobili). Quillworts, named
for their leaf shape, form a single genus
whose members live in marshy areas or as
submerged aquatic plants. Club mosses are
all homosporous, whereas spike mosses
and quillworts are all heterosporous. The
spores of club mosses are released in clouds
and are so rich in oil that magicians and
photographers once ignited them to create
smoke or flashes of light.
Monilophytes (Phylum Monilophyta) Equisetum Psilotum
telmateia, nudum,
Matteuccia giant a whisk
struthiopteris horsetail fern
(ostrich fern) Strobilus on
fertile stem
Vegetative stem
2.5 cm 3 cm
4 cm
Ferns Horsetails Whisk Ferns and Relatives
Unlike the lycophytes, ferns have mega- The group’s name refers to the brushy ap- Like primitive vascular plant fossils, the
phylls (see Figure 29.13). The sporophytes pearance of the stems, which have a gritty sporophytes of whisk ferns (genus Psilotum)
typically have horizontal stems that give texture that made them historically useful have dichotomously branching stems but
rise to large leaves called fronds, often as “scouring rushes” for pots and pans. no roots. Stems have scalelike outgrowths
divided into leaflets. A frond grows as its Some species have separate fertile (cone- that lack vascular tissue and may have
coiled tip, the fiddlehead, unfurls. bearing) and vegetative stems. Horsetails resulted from the evolutionary reduction of
are homosporous, with cones releasing leaves. Each yellow knob on a stem consists
Almost all species are homosporous. The spores that typically give rise to bisexual of three fused sporangia. Species of the
gametophyte in some species shrivels and gametophytes. genus Tmesipteris closely related to whisk
dies after the young sporophyte detaches ferns and found only in the South Pacific,
itself. In most species, sporophytes have Horsetails are also called arthrophytes also lack roots but have small, leaflike
stalked sporangia with springlike devices (“jointed plants”) because their stems have outgrowths in their stems, giving them a
that catapult spores several meters. Air- joints. Rings of small leaves or branches vine-like appearance. Both genera are homo-
borne spores can be carried far from their emerge from each joint, but the stem is sporous, with spores giving rise to bisexual
origin. Some species produce more than a the main photosynthetic organ. Large air gametophytes that grow underground and
trillion spores in a plant’s lifetime. canals carry oxygen to the roots, which are only about a centimeter long.
often grow in waterlogged soil.
630 Unit five The Evolutionary History of Biological Diversity
Horsetail Fern
and whisk ferns differ greatly in appearance, recent anatomical Figure 29.15 Artist’s conception of a Carboniferous forest
and molecular comparisons provide convincing evidence that based on fossil evidence. Lycophyte trees, with trunks covered with
these three groups make up a clade. Accordingly, many system- small leaves, thrived in the “coal forests” of the Carboniferous, along
atists now classify them together as the phylum Monilophyta, with giant ferns and horsetails.
as we do in this chapter. Others refer to these groups as three
separate phyla within a clade. Figure 29.14 describes the two are closely related to ferns. This hypothesis suggests that their
main groups of seedless vascular plants. ancestor’s true roots were lost during evolution. Today, plants
in these two genera absorb water and nutrients through
Phylum Lycophyta: numerous absorptive rhizoids.
Club Mosses, Spikemosses, and Quillworts
The Significance of Seedless Vascular Plants
Present-day species of lycophytes are relicts of a far more
impressive past. By the Carboniferous period (359–299 million The ancestors of living lycophytes, horsetails, and ferns,
years ago), the lycophyte evolutionary lineage included small along with their extinct seedless vascular relatives, grew to
herbaceous plants and giant trees with diameters of more great heights during the Devonian and early Carboniferous,
than 2 m and heights of more than 40 m. The giant lycophyte forming the first forests (Figure 29.15). How did their dra-
trees thrived for millions of years in moist swamps, but their matic growth affect Earth and its other life?
diversity declined when Earth’s climate became drier during
the Permian period (299–252 million years ago). The small One major effect was that early forests contributed to a large
lycophytes survived, represented today by about 1,200 species. drop in CO2 levels during the Carboniferous period, causing
Though some are commonly called club mosses and spike- global cooling that resulted in widespread glacier formation.
mosses, they are not true mosses (which, as discussed earlier, The trees of early forests contributed to this drop in CO2 levels
are nonvascular plants). in part by the actions of their roots. The roots of vascular plants
secrete acids that break down rocks, thereby increasing the rate
Phylum Monilophyta: at which calcium and magnesium are released from rocks into
Ferns, Horsetails, and Whisk Ferns and Relatives the soil. These chemicals react with carbon dioxide dissolved
in rain water, forming compounds that ultimately wash into
Ferns radiated extensively from their Devonian origins and the oceans, where they are incorporated into rocks (calcium
grew alongside lycophyte trees and horsetails in the great or magnesium carbonates). The net effect of these processes—
Carboniferous swamp forests. Today, ferns are by far the most which were accelerated by plants—is that CO2 removed from
widespread seedless vascular plants, numbering more than the air is stored in marine rocks. Although carbon stored in
12,000 species. Though most diverse in the tropics, many ferns these rocks can be returned to the atmosphere, it typically takes
thrive in temperate forests, and some species are even adapted millions of years for this to occur (as when geological uplift
to arid habitats. brings the rocks to the surface, exposing them to erosion).
As mentioned earlier, ferns and other monilophytes are In addition, the seedless vascular plants that formed the first
more closely related to seed plants than to lycophytes. As a forests eventually became coal, again removing CO2 from the
result, monilophytes and seed plants share traits that are not atmosphere for long periods of time. In the stagnant waters of
found in lycophytes, including megaphyll leaves and roots Carboniferous swamps, the dead bodies of early trees did not
that can branch at various points along the length of an exist- completely decay. This organic material turned to thick layers
ing root. In lycophytes, by contrast, roots branch only at the of peat, later covered by the sea. Marine sediments piled on
growing tip of the root, forming a Y-shaped structure. top, and over millions of years, heat and pressure converted
the peat to coal. In fact, Carboniferous coal deposits are the
The monilophytes called horsetails were very diverse dur- most extensive ever formed. Coal was crucial to the Industrial
ing the Carboniferous period, some growing as tall as 15 m.
Today, only 15 species survive as a single, widely distributed
genus, Equisetum, often found in marshy places and along
streams.
Psilotum (whisk ferns) and a closely related genus,
Tmesipteris, form a clade consisting mainly of tropical epi-
phytes. Plants in these two genera, the only vascular plants
lacking true roots, once were called “living fossils” because
of their resemblance to fossils of ancient relatives of living
vascular plants (see Figures 29.11 and 29.14). However, much
evidence, including analyses of DNA sequences and sperm
structure, indicates that the genera Psilotum and Tmesipteris
chapter 29 Plant Diversity I: How Plants Colonized Land 631
Revolution, and people worldwide still burn 6 billion tons a Concept Check 29.3
year. It is ironic that coal, formed from plants that contributed
to a global cooling, now contributes to global warming by 1. List the key derived traits found in monilophytes and
returning carbon to the atmosphere (see Figure 56.29). seed plants, but not in lycophytes.
Growing along with the seedless plants in Carboniferous 2. How do the main similarities and differences between
swamps were primitive seed plants. Though seed plants were seedless vascular plants and nonvascular plants affect
not dominant at that time, they rose to prominence after function in these plants?
the swamps began to dry up at the end of the Carboniferous
period. The next chapter traces the origin and diversification of 3. MAKE CONNECTIONS In Figure 29.12, if fertilization
seed plants, continuing our story of adaptation to life on land. occurred between gametes from one gametophyte, how
would this affect the production of genetic variation
from sexual reproduction? See Concept 13.4.
For suggested answers, see Appendix A.
29 Chapter Review Go to MasteringBiology™ for Videos, Animations, Vocab Self-Quiz,
Practice Tests, and more in the Study Area.
Summary of Key Concepts Fossils show that plants arose more than 470 million years ago.
Subsequently, plants diverged into several major groups, includ-
Concept 29.1 ing nonvascular plants (bryophytes); seedless vascular
plants, such as lycophytes and ferns; and the two groups of
Plants evolved from green algae seed plants: gymnosperms and angiosperms.
(pp. 617–622)
? Draw a phylogenetic tree illustrating our current understanding of
plant phylogeny; label the common ancestor of plants and the origins
VOCAB
Morphological and biochemical traits, as well as SELF-QUIZ of multicellular gametangia, vascular tissue, and seeds.
similarities in nuclear and chloroplast genes, indi- goo.gl/6u55ks
cate that certain groups of charophytes are the clos- Concept 29.2
est living relatives of plants.
A protective layer of sporopollenin and other traits allow charo- Mosses and other nonvascular plants have
phytes to tolerate occasional drying along the edges of ponds and life cycles dominated by gametophytes
lakes. Such traits may have enabled the algal ancestors of plants to (pp. 622–626)
survive in terrestrial conditions, opening the way to the coloniza-
tion of dry land. Lineages leading to the three extant clades of nonvascular plants,
Derived traits that distinguish plants from charophytes, their or bryophytes—liverworts, mosses, and hornworts—
closest algal relatives, include cuticles, stomata, multicellular diverged from other plants early in plant evolution.
dependent embryos, and the four shown here:
In bryophytes, the dominant generation consists of haploid
Apical meristem Developing gametophytes, such as those that make up a carpet of moss.
of shoot leaves Rhizoids anchor gametophytes to the substrate on which they
Gametophyte grow. The flagellated sperm produced by antheridia require a
film of water to travel to the eggs in the archegonia.
Mitosis Mitosis
n n The diploid stage of the life cycle—the sporophytes—grow out
of archegonia and are attached to the gametophytes and depen-
n Spore Gamete n dent on them for nourishment. Smaller and simpler than vascular
plant sporophytes, they typically consist of a foot, seta (stalk),
MEIOSIS FERTILIZATION and sporangium.
2n Zygote Sphagnum, or peat moss, is common in large regions known as
peatlands and has many practical uses, including as a fuel.
Mitosis Haploid
Sporophyte Diploid ? Summarize the ecological importance of mosses.
1 Alternation of generations 2 Apical meristems Concept 29.3
Archegonium Antheridium Spores Ferns and other seedless vascular plants were
with egg with sperm the first plants to grow tall (pp. 626–632)
3 Multicellular gametangia Sporangium Fossils of the forerunners of today’s vascular plants date back
4 Walled spores in sporangia about 425 million years and show that these small plants had
independent, branching sporophytes and a vascular system.
Over time, other derived traits of living vascular plants arose,
such as a life cycle with dominant sporophytes, lignified
vascular tissue, well-developed roots and leaves, and
sporophylls.
Seedless vascular plants include the lycophytes (phylum
Lycophyta: club mosses, spikemosses, and quillworts) and
632 Unit FIVE The Evolutionary History of Biological Diversity
the monilophytes (phylum Monilophyta: ferns, horsetails, and Level 3: Synthesis/Evaluation
whisk ferns and relatives). Current evidence indicates that seed-
less vascular plants, like bryophytes, do not form a clade. 8. SCIENTIFIC INQUIRY
Ancient lineages of lycophytes included both small herbaceous INTERPRET THE DATA The feather moss Pleurozium schreberi
plants and large trees. Present-day lycophytes are small herba- harbors species of symbiotic nitrogen-fixing bacteria.
ceous plants. Scientists studying this moss in northern forests found that
Seedless vascular plants formed the earliest forests about the percentage of the ground surface “covered” by the moss
385 million years ago. Their growth may have contributed to a increased from about 5% in forests that burned 35 to 41 years
major global cooling that took place during the Carboniferous ago to about 70% in forests that burned 170 or more years ago.
period. The decaying remnants of the first forests eventually From mosses growing in these forests, they also obtained the
became coal. following data on nitrogen fixation:
? What trait(s) allowed vascular plants to grow tall, and why might Age N fixation rate
increased height have been advantageous? (years after fire) [kg N/(ha · yr)]
35 0.001
Test Your Understanding 41 0.005
Level 1: Knowledge/Comprehension 78 0.08
101 0.3
1. Three of the following are evidence that 124 0.9
charophytes are the closest algal relatives of
plants. Select the exception. 170 2.0
(A) similar sperm structure
(B) the presence of chloroplasts PRACTICE 220 1.3
(C) similarities in cell wall formation during cell TEST
division 244 2.1
(D) genetic similarities in chloroplasts goo.gl/CUYGKD
270 1.6
300 3.0
2. Which of the following characteristics of plants is absent in 355 2.3
their closest relatives, the charophyte algae?
(A) chlorophyll b Data from O. Zackrisson et al., Nitrogen fixation increases with successional
(B) cellulose in cell walls age in boreal forests, Ecology 85:3327–3334 (2006).
(C) sexual reproduction
(D) alternation of multicellular generations (a) Use the data to draw a line graph, with age on the x-axis
and the nitrogen fixation rate on the y-axis.
3. In plants, which of the following are produced by meiosis?
(A) haploid gametes (b) Along with the nitrogen added by nitrogen fixation, about
(B) diploid gametes 1 kg of nitrogen per hectare per year is deposited into
(C) haploid spores northern forests from the atmosphere as rain and small
(D) diploid spores particles. Evaluate the extent to which Pleurozium affects
nitrogen availability in northern forests of different ages.
4. Microphylls are found in which plant group?
(A) lycophytes 9. WRITE ABOUT A THEME: INTERACTIONS Giant lycophyte
(B) liverworts trees had microphylls, whereas ferns and seed plants have
(C) ferns megaphylls. Write a short essay (100–150 words) describing
(D) hornworts how a forest of lycophyte trees may have differed from a forest
of large ferns or seed plants. In your answer, consider how the
Level 2: Application/Analysis type of forest may have affected interactions among small
plants growing beneath the tall ones.
10. SYNTHESIZE YOUR KNOWLEDGE
5. Suppose an efficient conducting system evolved in a moss that
could transport water and other materials as high as a tall tree.
Which of the following statements about “trees” of such a
species would not be true?
(A) Spore dispersal distances would probably increase.
(B) Females could produce only one archegonium.
(C) Unless its body parts were strengthened, such a “tree”
would probably flop over.
(D) Individuals would probably compete more effectively for
access to light.
6. Identify each of the following structures as haploid or diploid.
(A) sporophyte (C) gametophyte
(B) spore (D) zygote These stomata are from the leaf of a common horsetail.
Describe how stomata and other adaptations facilitated life on
7. EVOLUTION CONNECTION land and ultimately led to the formation of the first forests.
DRAW IT Draw a phylogenetic tree that represents our current
understanding of evolutionary relationships between a moss, For selected answers, see Appendix A.
a gymnosperm, a lycophyte, and a fern. Use a charophyte
alga as the outgroup. (See Figure 26.5 to review phylogenetic For additional practice questions, check out the Dynamic Study
trees.) Label each branch point of the phylogeny with at least Modules in MasteringBiology. You can use them to study on
one derived character unique to the clade descended from the your smartphone, tablet, or computer anytime, anywhere!
common ancestor represented by the branch point.
chapter 29 Plant Diversity I: How Plants Colonized Land 633
Superset 30
Plant Diversity II:
The Evolution of Seed Plants
Figure 30.1 How could these plants have reached this remote location?
Key Concepts Transforming the World
30.1 Seeds and pollen grains are key On May 18, 1980, Mount St. Helens erupted with a force 500 times that of the
Hiroshima atomic bomb. Traveling at over 300 miles per hour, the blast destroyed
adaptations for life on land hundreds of hectares of forest, leaving the region covered in ash and devoid of visible
life. Within a few years, however, plants such as fireweed (Chamerion angustifolium)
30.2 Gymnosperms bear “naked” had colonized the barren landscape (Figure 30.1).
seeds, typically on cones Fireweed and other early arrivals reached the blast zone as seeds. A seed con-
sists of an embryo and its food supply, surrounded by a protective coat. When
30.3 The reproductive adaptations mature, seeds are dispersed from their parent by wind or other means, enabling
them to colonize distant locations.
of angiosperms include flowers
and fruits Plants not only have affected the recovery of regions such as Mount St. Helens
but also have transformed Earth. Continuing the saga of how this occurred, this
30.4 Human welfare depends on seed chapter follows the emergence and diversification of the group to which fireweed
belongs, the seed plants. Fossils and comparative studies of living plants offer clues
plants about the origin of seed plants some 360 million years ago. As this new group
became established, it dramatically altered the course of plant evolution. Indeed,
Fireweed seed seed plants have become the dominant producers on land, and they make up the
vast majority of plant biodiversity today.
In this chapter, we will first examine the general features of seed plants. Then
we will look at their evolutionary history and enormous impact on human society.
When you see this blue icon, log in to MasteringBiology Get Ready for This Chapter
and go to the Study Area for digital resources.
634
Concept 30.1 plants have sporophyte-dominated life cycles. The evolu-
tionary trend of gametophyte reduction continued further
Seeds and pollen grains are key in the vascular plant lineage that led to seed plants. While
adaptations for life on land the gametophytes of seedless vascular plants are visible to
the naked eye, the gametophytes of most seed plants are
We begin with an overview of terrestrial adaptations that microscopic.
seed plants added to those already present in nonvascu-
lar plants (bryophytes) and seedless vascular plants (see This miniaturization allowed for an important evolu-
Concept 29.1). In addition to seeds, all seed plants have tionary innovation in seed plants: Their tiny gametophytes
reduced gametophytes, heterospory, ovules, and pollen. can develop from spores retained within the sporangia of
As we’ll see, these adaptations helped seed plants cope with the parental sporophyte. This arrangement can protect the
conditions such as drought and exposure to ultraviolet gametophytes from environmental stresses. For example,
(UV) radiation in sunlight. They also freed seed plants from the moist reproductive tissues of the sporophyte shield the
requiring water for fertilization, enabling reproduction gametophytes from UV radiation and protect them from
under a broader range of conditions than in seedless plants. drying out. This relationship also enables the developing
gametophytes to obtain nutrients from the parental sporo-
Advantages of Reduced Gametophytes phyte. In contrast, the free-living gametophytes of seedless
vascular plants must fend for themselves. Figure 30.2
Mosses and other bryophytes have life cycles dominated by provides an overview of the gametophyte-sporophyte rela-
gametophytes, whereas ferns and other seedless vascular tionships in nonvascular plants, seedless vascular plants,
and seed plants.
Figure 30.2 Gametophyte-sporophyte relationships in different plant groups.
PLANT GROUP
Mosses and other Ferns and other seedless Seed plants (gymnosperms and angiosperms)
nonvascular plants vascular plants
Gametophyte Dominant Reduced, independent Reduced (usually microscopic), dependent on surrounding
(photosynthetic and sporophyte tissue for nutrition
free-living)
Sporophyte Reduced, dependent on Dominant Dominant
gametophyte for nutrition
Sporophyte Gymnosperm Angiosperm
(2n) Microscopic female
gametophytes (n) inside Microscopic
ovulate cone female
gametophytes
Sporophyte (n) inside
(2n) these parts
of flowers
Example Gametophyte Microscopic
(n) male
gametophytes
Gametophyte Microscopic male (n) inside
(n) gametophytes (n) these parts
inside pollen of flowers
cone Sporophyte (2n)
Sporophyte (2n)
MAKE CONNECTIONS In seed plants, how might retaining the gametophyte within the sporophyte
affect embryo fitness? (See Concepts 17.5, 23.1, and 23.4 to review mutagens, mutations, and fitness.)
chapter 30 Plant Diversity II: The Evolution of Seed Plants 635
Heterospory: The Rule Among Seed Plants Pollen and Production of Sperm
You read in Concept 29.3 that most seedless plants are A microspore develops into a pollen grain that consists of
homosporous—they produce one kind of spore, which usu- a male gametophyte enclosed within the pollen wall. (The
ally gives rise to a bisexual gametophyte. Ferns and other wall’s outer layer is made of molecules secreted by sporophyte
close relatives of seed plants are homosporous, suggesting cells, so we refer to the male gametophyte as being in the pol-
that seed plants had homosporous ancestors. At some point, len grain, not equivalent to the pollen grain.) Sporopollenin
seed plants or their ancestors became heterosporous, produc- in the pollen wall protects the pollen grain as it is transported
ing two kinds of spores: Megasporangia on modified leaves by wind or by hitchhiking on an animal. The transfer of pol-
called megasporophylls produce megaspores that give rise len to the part of a seed plant that contains the ovules is called
to female gametophytes, and microsporangia on modified pollination. If a pollen grain germinates (begins growing), it
leaves called microsporophylls produce microspores that gives rise to a pollen tube that discharges sperm into the female
give rise to male gametophytes. Each megasporangium has gametophyte within the ovule, as shown in Figure 30.3b.
one megaspore, whereas each microsporangium has many
microspores. In nonvascular plants and seedless vascular plants such as
ferns, free-living gametophytes release flagellated sperm that
As noted previously, the miniaturization of seed plant swim through a film of water to reach eggs. Given this require-
gametophytes probably contributed to the great success of ment, it is not surprising that many of these species live in
this clade. Next, we’ll look at the development of the female moist habitats. But a pollen grain can be carried by wind or ani-
gametophyte within an ovule and the development of the mals, eliminating the dependence on water for sperm transport.
male gametophyte in a pollen grain. Then we’ll follow the The ability of seed plants to transfer sperm without water likely
transformation of a fertilized ovule into a seed. contributed to their colonization of dry habitats. The sperm of
seed plants also do not require motility because they are carried
Ovules and Production of Eggs to the eggs by pollen tubes. The sperm of some gymnosperm
species (such as cycads and ginkgos, shown in Figure 30.7)
Although a few species of seedless plants are heterosporous, retain the ancient flagellated condition, but flagella have been
seed plants are unique in retaining the megasporangium lost in the sperm of most gymnosperms and all angiosperms.
within the parent sporophyte. A layer of sporophyte tissue
called integument envelops and protects the megaspo- The Evolutionary Advantage of Seeds
rangium. Gymnosperm megasporangia are surrounded by
one integument, whereas those in angiosperms usually have If a sperm fertilizes an egg of a seed plant, the zygote grows
two integuments. The whole structure—megasporangium, into a sporophyte embryo. As shown in Figure 30.3c, the
megaspore, and their integument(s)—is called an ovule ovule develops into a seed: the embryo, with a food supply,
(Figure 30.3a). Inside each ovule (from the Latin ovulum, packaged in a protective coat derived from the integument(s).
little egg), a female gametophyte develops from a megaspore
and produces one or more eggs. Until the advent of seeds, the spore was the only protective
stage in any plant life cycle. Moss spores, for example, may
Figure 30.3 From ovule to seed in a gymnosperm.
Immature Integument (2n) Female Seed coat
ovulate cone Spore wall gametophyte (n) (derived from integument)
Egg nucleus (n)
Megaspore (n) Megasporangium (2n) Spore wall
Discharged (surrounded by
Male gametophyte sperm nucleus (n) megasporangium
(within a germinated Pollen tube remnant)
pollen grain) (n) Food supply
(female gametophyte
Micropyle Pollen grain (n) tissue) (n)
Embryo (2n)
(a) Unfertilized ovule. In this longitudinal (b) Fertilized ovule. A megaspore develops (new sporophyte)
section through the ovule of a pine (a into a female gametophyte, which produces (c) Gymnosperm seed. Fertilization
gymnosperm), a fleshy megasporangium an egg. The pollen grain, which had entered initiates the transformation of the
is surrounded by a protective layer of through the micropyle, contains a male ovule into a seed, which consists of a
tissue called an integument. The gametophyte. The male gametophyte sporophyte embryo, a food supply, and
micropyle, the only opening through the develops a pollen tube that discharges a protective seed coat derived from the
integument, allows entry of a pollen grain. sperm, thereby fertilizing the egg. integument. The megasporangium
dries out and collapses.
VISUAL SKILLS Based on this diagram, a gymnosperm seed contains cells from how many different Figure Walkthrough
plant generations? Identify the cells and whether each is haploid or diploid.
636 Unit five The Evolutionary History of Biological Diversity
Scientific Skills Exercise
Using Natural Logarithms INTERPRET THE DATA
to Interpret Data
A logarithm is the power to which a
How Long Can Seeds Remain Viable in Dormancy? base is raised to produce a given num-
Environmental conditions can vary greatly over time, and they may ber x. For example, if the base is 10 and
not be favorable for germination when seeds are produced. One x = 100, the logarithm of 100 equals 2
way that plants cope with such variation is through seed dormancy. (because 102 = 100). A natural loga-
Under favorable conditions, seeds of some species can germinate rithm (ln) is the logarithm of a number
after many years of dormancy. x to the base e, where e is about 2.718.
Natural logarithms are useful in calcu-
One unusual opportunity to test how long seeds can remain viable lating rates of some natural processes,
occurred when seeds from date palm trees (Phoenix dactylifera) were such as radioactive decay.
discovered under the rubble of a 2,000-year-old fortress near the
Dead Sea. As you saw in the Chapter 2 Scientific Skills Exercise and 1. The equation F = e-kt describes the fraction F of an original
Concept 25.2, scientists use radiometric dating to estimate the ages isotope remaining after a period of t years; the exponent is nega-
of fossils and other old objects. In this exercise, you will estimate the tive because it refers to a decrease over time. The constant k
ages of three of these ancient seeds by using natural logarithms. provides a measure of how rapidly the original isotope decays.
For the decay of carbon-14 to nitrogen-14, k = 0.00012097. To
How the Experiment Was Done Scientists measured the frac- find t, rearrange the equation by following these steps: (a) Take
tion of carbon-14 that remained in three ancient date palm seeds: the natural logarithm of both sides of the equation: ln(F ) =
two that were not planted and one that was planted and germi- ln(e-kt). Rewrite the right side of this equation by applying the
nated. For the germinated seed, the scientists used a seed coat frag- following rule: ln(ex) = x ln(e). (b) Since ln(e) = 1, simplify the
ment found clinging to a root of the seedling. (The seedling grew equation. (c) Now solve for t and write the equation in the form
into the plant in the photo.) “t = ________.”
Data from the Experiment This table shows the fraction of 2. Using the equation you developed, the data from the table,
carbon-14 remaining from the three ancient date palm seeds. and a calculator, estimate the ages of seed 1, seed 2, and seed 3.
Seed 1 (not planted) Fraction of 3. Why do you think there was more carbon-14 in the germinated
Seed 2 (not planted) Carbon-14 Remaining seed?
Seed 3 (germinated)
0.7656 Instructors: A version of this Scientific Skills Exercise can be
0.7752 a ssigned in MasteringBiology.
0.7977
Data from S. Sallon et al., Germination, genetics, and growth of an ancient date seed,
Science 320:1464 (2008).
survive even if the local environment becomes too cold, too critical support for growth as the sporophyte embryo emerges
hot, or too dry for the mosses themselves to live. Their tiny as a seedling. As we explore in the Scientific Skills Exercise,
size enables the spores to be dispersed in a dormant state to a some seeds have germinated after more than 1,000 years.
new area, where they can germinate and give rise to new moss
gametophytes if and when conditions are favorable enough Concept Check 30.1
for them to break dormancy. Spores were the main way that
mosses, ferns, and other seedless plants spread over Earth for 1. Contrast how sperm reach the eggs of seedless plants
the first 100 million years of plant life on land. with how sperm reach the eggs of seed plants.
Although mosses and other seedless plants continue to be 2. What features not present in seedless plants have
very successful today, seeds represent a major evolutionary contributed to the success of seed plants on land?
innovation that contributed to the opening of new ways of life
for seed plants. What advantages do seeds provide over spores? 3. WHAT IF? If a seed could not enter dormancy, how
Spores are usually single-celled, whereas seeds are multicel- might that affect the embryo’s transport or survival?
lular, consisting of an embryo protected by a layer of tissue, For suggested answers, see Appendix A.
the seed coat. A seed can remain dormant for days, months, or
even years after being released from the parent plant, whereas Concept 30.2
most spores have shorter lifetimes. Also, unlike spores, seeds
have a supply of stored food. Most seeds land close to their Gymnosperms bear “naked” seeds,
parent sporophyte plant, but some are carried long distances typically on cones
(up to hundreds of kilometers) by wind or animals. If condi-
tions are favorable where it lands, the seed can emerge from Nonvascular plants (bryophytes) Extant seed plants form two
dormancy and germinate, with its stored food providing Seedless vascular plants sister clades: gymnosperms
Gymnosperms and angiosperms. Recall that
Angiosperms gymnosperms have “naked”
seeds exposed on sporophylls that usually form cones.
chapter 30 Plant Diversity II: The Evolution of Seed Plants 637
(Angiosperm seeds are enclosed in chambers that mature into how these adaptations come into play during the life cycle
fruits.) Most gymnosperms are cone-bearing plants called of a pine, a familiar conifer.
conifers, such as pines, firs, and redwoods.
The pine tree is the sporophyte; its sporangia are located
The Life Cycle of a Pine on scalelike structures packed densely in cones. Like all seed
plants, conifers are heterosporous. As such, they have two
As you read earlier, seed plant evolution has included three types of sporangia that produce two types of spores: micro-
key reproductive adaptations: the miniaturization of their sporangia that produce microspores, and megasporangia that
gametophytes; the advent of the seed as a resistant, dispersible produce megaspores. In conifers, the two types of spores are
stage in the life cycle; and the appearance of pollen as an air- produced by separate cones: small pollen cones and large
borne agent that brings gametes together. Figure 30.4 shows ovulate cones.
Figure 30.4 The life cycle of a pine. 3 An ovulate cone scale has two ovules,
1 In most conifer each containing a megasporangium. Only
species, each tree one ovule is shown.
has both ovulate Ovule
and pollen cones.
Longitudinal section Megasporocyte (2n) 4 Pollination occurs
when a pollen grain
Ovulate cone of ovulate cone reaches the ovule. The
Integument pollen grain then
Pollen cone germinates, forming a
pollen tube that slowly
Microsporangia digests its way through
the megasporangium.
Mature Microsporocytes Pollen Germinating Megasporangium (2n)
sporophyte (2n) grains (n) pollen grain
(2n)
MEIOSIS MEIOSIS
Longitudinal section Microsporangium (2n) Surviving
of pollen cone megaspore (n)
Seedling 2 Microsporocytes divide by meiosis, 5 While the pollen
producing haploid microspores. A tube develops, the
microspore develops into a pollen megasporocyte
grain (a male gametophyte enclosed undergoes meiosis,
within the pollen wall). producing four
haploid cells. One
Seeds on surface Archegonium survives as a
of ovulate scale Female megaspore.
gametophyte 6 The megaspore
Food reserves Seed develops into a female
(gametophyte coat (2n) Discharged gametophyte that
tissue) (n) sperm nucleus (n) contains two or three
Pollen archegonia, each of
tube which will form an egg.
Embryo FERTILIZATION 7 By the time the eggs are mature,
(new sporophyte) sperm cells have developed in the
(2n) pollen tube, which extends to the
female gametophyte. Fertilization occurs
Key 8 Fertilization usually occurs more than a year Egg when sperm and egg nuclei unite.
after pollination. All eggs may be fertilized, nucleus (n)
Haploid (n) but usually only one zygote develops into an
Diploid (2n) embryo. The ovule becomes a seed, consisting MAKE CONNECTIONS What type of cell division occurs as a megaspore
of an embryo, food supply, and seed coat. develops into a female gametophyte? Explain. (See Figure 13.10.)
Animation: Pine Life Cycle
638 Unit five The Evolutionary History of Biological Diversity
Pollen cones have a relatively simple structure: Their scales are Figure 30.6 An ancient pollinator. This 110-million-year-old
modified leaves (microsporophylls) that bear microsporangia. fossil shows pollen on an insect, the thrip Gymnopollisthrips minor.
Within each microsporangium, cells called microsporocytes Structural features of the pollen suggest that it was produced by
undergo meiosis, producing haploid microspores. Each micro- gymnosperms (most likely by species related to extant ginkgos or
spore develops into a pollen grain containing a male gameto- cycads). Although most gymnosperms today are wind-pollinated,
many cycads are insect-pollinated.
phyte. In conifers, the yellow pollen is released in large amounts
and carried by the wind, dusting everything in its path.
Ovulate cones are more complex: their scales are com-
pound structures composed of both modified leaves (mega- Pollen grains
sporophylls bearing megasporangia) and modified stem
tissue. Within the each megasporangium, megasporocytes
undergo meiosis and produce haploid megaspores inside
the ovule. Surviving megaspores develop into female game-
tophytes, which are retained within the sporangia.
In most pine species, each tree has both types of cones.
From the time pollen and ovulate cones appear on the tree,
it takes nearly three years for the male and female gameto-
phytes to be produced and brought together and for mature climate became much drier. As a result, the lycophytes, horse-
seeds to form from fertilized ovules. The scales of each ovu- tails, and ferns that dominated Carboniferous swamps were
late cone then separate, and seeds are dispersed by the wind. largely replaced by gymnosperms, which were better suited to
A seed that lands in a suitable environment germinates, its the drier climate.
embryo emerging as a pine seedling. Gymnosperms thrived as the climate dried, in part
because they have the key terrestrial adaptations found in
Early Seed Plants and all seed plants, such as seeds and p ollen. In addition, some
the Rise of Gymnosperms gymnosperms were particularly well suited to arid condi-
tions because of the thick cuticles and relatively small
The origins of characteristics found in pines and other living surface areas of their needle-shaped leaves.
seed plants date back to the late Devonian period (380 million Gymnosperms dominated terrestrial ecosystems through-
years ago). Fossils from that time reveal that some plants had out much of the Mesozoic era, which lasted from 252 to
acquired features that are also present in seed plants, such as 66 million years ago. In addition to serving as the food sup-
megaspores and microspores. For example, Archaeopteris ply for giant herbivorous dinosaurs, these gymnosperms
was a heterosporous tree with a woody stem. But it did not were involved in many other interactions with animals.
bear seeds and therefore is not classified as Recent fossil discoveries, for example, show
a seed plant. Growing up to 20 m tall, it had Figure 30.5 A fossil of the that some gymnosperms were pollinated by
fernlike leaves. early seed plant Elkinsia. insects more than 100 million years ago—
the earliest evidence of insect pollination in
The earliest evidence of seed plants comes
from 360-million-year-old fossils of plants any plant group (Figure 30.6). Late in the
in the genus Elkinsia (Figure 30.5). These Mesozoic, angiosperms began to replace gym-
and other early seed plants lived 55 million nosperms in some ecosystems.
years before the first fossils classified as gym- Gymnosperm Diversity
nosperms and more than 200 million years
before the first fossils of angiosperms. These Although angiosperms now dominate most
early seed plants became extinct, and we terrestrial ecosystems, gymnosperms remain
don’t know which extinct lineage gave rise to Ovule an important part of Earth’s flora. For example,
the gymnosperms. vast regions in northern latitudes are covered
The oldest fossils of species from an extant by forests of conifers (see Figure 52.12).
lineage of gymnosperms are 305 million Of the ten plant phyla (see Table 29.1),
years old. These early gymnosperms lived four are gymnosperms: Cycadophyta,
in moist Carboniferous ecosystems that Ginkgophyta, Gnetophyta, and
were dominated by lycophytes, horsetails, Coniferophyta. It is uncertain how the four
ferns, and other seedless vascular plants. As phyla of gymnosperms are related to each
the Carboniferous period gave way to the other. Figure 30.7 surveys the diversity of
Permian (299 to 252 million years ago), the extant gymnosperms.
chapter 30 Plant Diversity II: The Evolution of Seed Plants 639
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