27
The Origin and Diversification of
the Eukaryotes
27 The Origin and Diversification of the Eukaryotes
• 27.1 How Do Microbial Eukaryotes Affect the
World Around Them?
• 27.2 How Did the Eukaryotic Cell Arise?
• 27.3 How Did the Microbial Eukaryotes
Diversify?
• 27.4 How Do Microbial Eukaryotes
Reproduce?
• 27.5 What Are the Major Groups of
Eukaryotes?
27.1 How Do Microbial Eukaryotes Affect
the World Around Them?
Eukaryotes that are neither plants,
animals, or fungi are called protists, or
microbial eukaryotes (though not all
are microbial).
They do not constitute a clade, they are
paraphyletic.
Their true phylogeny is the subject of
research and debate.
Table 27.1 Major Eukaryote Clades (Part 1)
Table 27.1 Major Eukaryote Clades (Part 2)
Table 27.1 Major Eukaryote Clades (Part 3)
27.1 How Do Microbial Eukaryotes Affect
the World Around Them?
There is great diversity of microbial
eukaryotes.
Most are microscopic, but some are large
(e.g., giant kelp).
Many are constituents of plankton—free
floating, microscopic, aquatic organisms.
Plankton that are photosynthetic are
called phytoplankton.
27.1 How Do Microbial Eukaryotes Affect
the World Around Them?
In marine food webs, phytoplankton are
the primary producers.
Diatoms (a clade) are dominant in the
phytoplankton. They do one-fifth of the
carbon fixation on Earth.
The primary producers are consumed by
heterotrophs.
Figure 27.1 Architecture in Miniature: A Photosynthetic Diatom
27.1 How Do Microbial Eukaryotes Affect
the World Around Them?
Endosymbiosis, in which one organism
lives inside another, is common in
microbial eukaryotes.
Dinoflagellates are common
endosymbionts in animals and other
microbial eukaryotes; some are
photosynthetic.
27.1 How Do Microbial Eukaryotes Affect
the World Around Them?
Many radiolarians have photosynthetic
endosymbionts. Often, both organisms
benefit from the relationship.
Some dinoflagellates live as
endosymbionts in corals.
Figure 27.2 Two Microbial Eukaryotes in an Endosymbiotic Relationship
27.1 How Do Microbial Eukaryotes Affect
the World Around Them?
Pathogens:
Plasmodium—cause of malaria. Part of
its life cycle is spent as a parasite in red
blood cells.
Female Anopheles mosquito is the
vector; takes up Plasmodium gametes
with the blood, zygotes form in mosquito
gut. Plasmodium is passed to another
human.
Figure 27.3 The Life Cycle of the Malarial Parasite
27.1 How Do Microbial Eukaryotes Affect
the World Around Them?
Plasmodium’s complex life cycle makes it
difficult to control.
Best strategy—remove stagnant water
where mosquitoes breed. Insecticides
are also used.
The genomes of Plasmodium falciparum,
and Anopheles gambiae have been
sequenced.
27.1 How Do Microbial Eukaryotes Affect
the World Around Them?
Trypanosomes (kinetoplastids) are some
of the most deadly organisms on Earth,
causing sleeping sickness,
leishmaniasis, and Chagas’ disease.
27.1 How Do Microbial Eukaryotes Affect
the World Around Them?
Some chromalveolates, including
diatoms, dinoflagellates, and
haptophytes, can form “red tides.”
Color is from pigments in dinoflagellates.
Cell concentrations are extremely high.
Some produce neurotoxins that kill fish.
Gonyaulax produces a toxin that
accumulates in shellfish.
Figure 27.4 Chromalveolates Can Bloom in the Oceans
27.1 How Do Microbial Eukaryotes Affect
the World Around Them?
Coccolithophores (haptophytes) can also
form immense blooms in the ocean.
Blooms can reduce the amount of
sunlight that penetrates deeper waters.
Emiliania huxleyi—one of smallest
unicellular eukaryotes. May contribute
to global warming through its
metabolism.
27.1 How Do Microbial Eukaryotes Affect
the World Around Them?
Diatoms store oil as an energy reserve.
Over millions of years, diatoms have died
and sunk to the ocean floor, and
through chemical and physical changes
form petroleum and natural gas
deposits.
27.1 How Do Microbial Eukaryotes Affect
the World Around Them?
Foraminiferans secrete shells of calcium
carbonate.
Discarded shells make up extensive deposits
of limestone. Some beach sands are made of
fragments of foram shells.
Foram shells are also used to date and
characterize sedimentary rocks, and are used
to infer temperatures from the past.
Figure 27.5 Foraminiferan Shells Are Building Blocks
27.2 How Did the Eukaryotic Cell Arise?
Eukaryotic cells arose as the
environment was changing
dramatically—from anaerobic to
aerobic.
Major events that occurred in the
evolution of eukaryote cells are still
conjectural—a framework for thinking
about this challenging problem.
27.2 How Did the Eukaryotic Cell Arise?
The main events:
• Origin of a flexible cell surface
• Origin of a cytoskeleton
• Origin of a nuclear envelope
• Appearance of digestive vesicles or
vacuoles
• Endosymbiotic acquisition of some
organelles
27.2 How Did the Eukaryotic Cell Arise?
Flexible cell surface: Prokaryotic cell wall
was lost; cells can grow larger.
As cell size increases, surface area-tovolume ratio decreases, but with a
flexible surface, infolding can occur,
creating more surface area.
A flexible cell surface also allowed
endocytosis to develop.
Figure 27.6 Membrane Infolding
27.2 How Did the Eukaryotic Cell Arise?
A cytoskeleton provided cell support,
allowed cells to change shape, and
move materials around the cell,
including daughter chromosomes.
In some cells microtubules gave rise to
flagella.
27.2 How Did the Eukaryotic Cell Arise?
The nuclear envelope may have
developed from the plasma membrane.
The DNA of a prokaryote is attached to
the plasma membrane; infolding of the
membrane could have been the first
step in development of the nucleus.
Figure 27.7 From Prokaryotic Cell to Eukaryotic Cell
27.2 How Did the Eukaryotic Cell Arise?
The next step was probably
phagocytosis—the ability to engulf and
digest other cells.
The first true eukaryotes had a
cytoskeleton and nuclear envelope; they
probably had ER, Golgi apparatus, and
perhaps flagella.
27.2 How Did the Eukaryotic Cell Arise?
Cyanobacteria were producing oxygen;
at some point, some Eukarya
incorporated proteobacteria that
evolved into mitochondria—the
endosymbiotic theory.
The function of mitochondria initially
might have been to detoxify O2 by
reducing it to water. Later this became
associated with ATP production.
27.2 How Did the Eukaryotic Cell Arise?
Some eukaryotes incorporated a
prokaryote related to today’s
cyanobacteria, which developed into
chloroplasts.
Evolution of chloroplasts probably
occurred in a series of endosymbiotic
events. Evidence comes from nucleic
acid sequencing and electron
microscopy.
27.2 How Did the Eukaryotic Cell Arise?
Primary endosymbiosis: All
chloroplasts descended from a gramnegative cyanobacterium with an inner
and outer membrane.
A small amount of peptidoglycan from the
bacterial cell wall is found today in the
glaucophytes—the first group to branch
off.
27.2 How Did the Eukaryotic Cell Arise?
Primary endosymbiosis gave rise to
chloroplasts of green algae
(chlorophytes and charophytes) and the
red algae.
Photosynthetic land plants arose from a
green algal ancestor.
Red algal chloroplasts retain some
pigments that were present in the
original cyanobacterium.
27.2 How Did the Eukaryotic Cell Arise?
Secondary and tertiary endosymbiosis
gave rise to chloroplasts in the other
microbial eukaryote groups.
The euglenid ancestor engulfed a
chlorophyte, retaining the chloroplasts.
Euglenid chloroplasts have the same
pigments as green algae and land
plants, and has a third membrane.
Figure 27.8 Endosymbiotic Events in the Family Tree of Chloroplasts
27.2 How Did the Eukaryotic Cell Arise?
The cryptophytes (a clade of
chromalveolates) engulfed a red algal
cell that became the chloroplast.
These chloroplasts contain reduced red
algal nuclei, and appear to be a sister
clade to all other chromalveolate
chloroplasts.
27.2 How Did the Eukaryotic Cell Arise?
Dinoflagellates engaged in tertiary
endosymbiosis:
Karenia brevis lost its chloroplast and
took up a haptophyte (a result of
secondary endosymbiosis).
One case of sequential secondary
endosymbiosis—a dinoflagellate lost its
red algal chloroplast and took up a
chlorophyte.
27.2 How Did the Eukaryotic Cell Arise?
Uncertainties remain about the origins of
eukaryotic cells.
Lateral gene transfer complicates the
study of relationships.
Endosymbiosis does not account for all
bacterial genes in eukaryotes.
A recent suggestion is that Eukarya
arose from the fusion of a gramnegative bacterium and an archaean.
27.3 How Did the Microbial Eukaryotes Diversify?
Microbial eukaryotes have evolved a
diversity of lifestyles.
Most are aquatic, marine and freshwater;
but also damp soils and decaying
organic matter.
Some are photosynthetic, some are
heterotrophs, some can switch between
modes.
27.3 How Did the Microbial Eukaryotes Diversify?
Some used to be considered animals,
and are called protozoans. But this term
lumps phylogenetically unrelated
groups. Most protozoans are ingestive
heterotrophs.
The term algae also lumps many groups
of photosynthetic microbial eukaryotes
and does not reflect phylogeny.
27.3 How Did the Microbial Eukaryotes Diversify?
Locomotion
Amoeboid motion—cells form
pseudopods that are extensions of the
cell. A network of cytoskeletal
microfilaments squeezes the cytoplasm
forward.
Figure 27.9 An Amoeba
27.3 How Did the Microbial Eukaryotes Diversify?
Cilia and flagella developed from
microtubules.
Cilia beat in a coordinated fashion; move
cell forward or backward.
Flagella have whip-like movement.
Some pull, some push the cell forward.
Flagella have a 9 + 2 arrangement of
microtubules.
Figure 4.22 Sliding Microtubules Cause Cilia to Bend
27.3 How Did the Microbial Eukaryotes Diversify?
Vacuoles increase effective surface area
in large cells.
Contractile vacuoles in freshwater
microbial eukaryotes such as
Paramecium are used to excrete excess
water.
Figure 27.10 Contractile Vacuoles Bail Out Excess Water
27.3 How Did the Microbial Eukaryotes Diversify?
Food vacuoles are formed by
Paramecium and others when solid food
particles are ingested by endocytosis.
The food is digested in the vacuole.
Smaller vesicles pinch off—increasing
surface area for products of digestion to
be absorbed by the rest of the cell.
Figure 27.11 Food Vacuoles Handle Digestion and Excretion
27.3 How Did the Microbial Eukaryotes Diversify?
Cell surfaces
Many microbial eukaryotes have diverse
means of strengthening their surfaces.
Paramecium has a covering of surface
proteins called a pellicle, making it
flexible but resilient.
Other groups secrete a “shell,” such as
foraminiferans.
27.3 How Did the Microbial Eukaryotes Diversify?
Some amoebas make a “shell” or test
from bits of sand beneath the plasma
membrane.
Diatoms form glassy cell walls of silica.
These walls are exceptionally strong,
and perhaps enhanced defense against
predators.
Figure 27.12 Cell Surfaces in the Microbial Eukaryotes
27.4 How Do Microbial Eukaryotes Reproduce?
Most microbial eukaryotes have both
sexual and asexual reproduction.
Asexual processes:
• Binary fission—equal splitting; mitosis
followed by cytokinesis.
• Multiple fission—splitting into more than
two cells.
27.4 How Do Microbial Eukaryotes Reproduce?
• Budding—outgrowth of a new cell from
the surface of an old cell.
• Spores—specialized cells that are
capable of growing into a new
individual.
27.4 How Do Microbial Eukaryotes Reproduce?
The ciliate clade (such as Paramecium)
have a single macronucleus and one to
several micronuclei.
The macronucleus contains many copies
of the genetic information, packaged
into units; regulates the life of the cell.
Micronuclei are essential for genetic
recombination.
27.4 How Do Microbial Eukaryotes Reproduce?
In conjugation, two Paramecia line up
together, the oral groove regions fuse,
and nuclear material is exchanged and
reorganized.
Each cell gets two haploid nuclei, one
from each cell. These fuse to form a
new diploid micronucleus.
Conjugation is a sexual process, but it is
not reproductive. Asexual clones must
periodically conjugate.
Figure 27.13 Paramecia Achieve Genetic Recombination by Conjugating
27.4 How Do Microbial Eukaryotes Reproduce?
In alternation of generations, a diploid
spore-forming organism gives rise to a
haploid gamete-forming organism.
When haploid gametes fuse (fertilization
or syngamy) a diploid individual is
formed.
The haploid or diploid organism, or both,
may reproduce asexually.
Figure 27.14 Alternation of Generations
27.4 How Do Microbial Eukaryotes Reproduce?
In multicellular organisms, the
sporophyte is the multicellular diploid
generation; the gametophyte is the
multicellular haploid generation.
The generations may be different
morphologically—heteromorphic.
Isomorphic—the generations have
similar morphology.
27.4 How Do Microbial Eukaryotes Reproduce?
Specialized cells in the sporophyte
(sporocytes) divide meiotically to
produce haploid spores.
The spores germinate and divide
mitotically to produce the haploid
gametophyte generation.
Gametes produced by the gametophyte
generation must fuse to form a new
sporophyte generation.
27.4 How Do Microbial Eukaryotes Reproduce?
Life cycles in chlorophytes:
Ulva lactuca (sea lettuce) has alternation
of generations.
The haploid spores have four flagella—
zoospores. They lose flagella and divide
mitotically to form the sporophyte.
Isomorphic—both generations look alike.
Figure 27.15 An Isomorphic Life Cycle
27.4 How Do Microbial Eukaryotes Reproduce?
Isogamous—the gametes are the same
morphologically.
Anisogamous—female and male
gametes are different.
27.4 How Do Microbial Eukaryotes Reproduce?
In haplontic life cycles, a multicellular
haploid individual produces gametes
that fuse to form a zygote.
The zygote undergoes meiosis to form
haploid spores, these develop into a
new haploid individual.
The zygote is the only diploid part of the
life cycle.
Figure 27.16 A Haplontic Life Cycle
27.4 How Do Microbial Eukaryotes Reproduce?
In diplontic life cycles, meiosis of diploid
sporocytes produces haploid gametes.
Gametes fuse to form a diploid zygote
that develops mitotically into a new
diploid individual.
Gametes are the only haploid part of the
life cycle.
Most animals have this type of life cycle.
27.4 How Do Microbial Eukaryotes Reproduce?
Many microbial eukaryote life cycles
require participation of different host
species.
Examples: Plasmodium and the
trypanosomes
Advantages of such a life cycle are not
clear. The sexual part of the life cycle
takes place in the insect vectors.
27.5 What Are the Major Groups of Eukaryotes?
Phylogeny of microbial eukaryotes is the
subject of much research. Electron
microscopy and gene sequencing are
revealing new information.
Most eukaryotes can be divided into five
groups: chromalveolates, Plantae,
excavates, Rhizaria, and unikonts.
Figure 27.17 Major Eukaryote Groups in an Evolutionary Context (Part 1)
Figure 27.17 Major Eukaryote Groups in an Evolutionary Context (Part 2)
27.5 What Are the Major Groups of Eukaryotes?
Chromalveolates
Includes
haptophytes—
many are
“armored” with
elaborate scales.
27.5 What Are the Major Groups of Eukaryotes?
Alveolates: synapomorphy that
distinguishes this clade is presence of
alveoli or sacs beneath surface of
plasma membrane.
All unicellular; includes dinoflagellates,
apicomplexans, and ciliates.
27.5 What Are the Major Groups of Eukaryotes?
Most dinoflagellates are marine and are
important primary producers.
Mixture of pigments give them a golden
brown color.
Some are endosymbionts in corals and
other invertebrates and microbial
eukaryotes.
Some are nonphotosynthetic parasites.
27.5 What Are the Major Groups of Eukaryotes?
Dinoflagellates have two flagella, one in
an equatorial groove, the other in a
longitudinal groove.
Some can take different forms, including
amoeboid, (e.g., Pfiesteria piscicida).
When present in large numbers, they
can stun fish and feed on them.
Figure 27.18 A Dinoflagellate
27.5 What Are the Major Groups of Eukaryotes?
Apicomplexans are all parasites. They
have a mass of organelles at one tip—
the apical complex.
The organelles help the parasite enter
the host’s cells.
They have complex life cycles, often with
two different hosts, (e.g., Plasmodium).
They lack contractile vacuoles; have a
reduced, nonfunctional chloroplast.
27.5 What Are the Major Groups of Eukaryotes?
Ciliates have numerous cilia, the
structure is identical to flagella.
Most are heterotrophic; very diverse
group.
Have complex body form; two types of
nuclei.
Figure 27.19 Diversity among the Ciliates
27.5 What Are the Major Groups of Eukaryotes?
Paramecium has a pellicle composed of
an outer membrane and an inner layer
of membrane-enclosed sacs (alveoli)
that surround bases of cilia.
Trichocysts in the pellicle are defensive
organelles, they are like sharp darts on
the tip of an expanding filament.
Locomotion by cilia is more precise than
by flagella.
Figure 27.20 Anatomy of Paramecium
27.5 What Are the Major Groups of Eukaryotes?
Stramenopiles: synapomorphy that
defines them is rows of tubular hairs on
the longer of the two flagella.
Some stramenopiles lack flagella but are
descended from ancestors that had
them.
Includes diatoms, brown algae,
oomycetes, and slime nets.
27.5 What Are the Major Groups of Eukaryotes?
Diatoms are unicellular, but many
associate in filaments.
Have carotenoids and appear yellow or
brown.
All make chrysolaminarin (a
carbohydrate) and oils as storage
products.
Only male gametes have flagella.
27.5 What Are the Major Groups of Eukaryotes?
Diatoms deposit silicon in their cell walls.
They are constructed in two pieces, like
a petri plate.
They are either bilaterally or radially
symmetrical.
Asexual reproduction is by binary fission.
Both top and bottom of the “petri plate”
become the tops of the new daughter
cells.
27.5 What Are the Major Groups of Eukaryotes?
Sexual reproduction results in larger
cells: gametes fuse to form a zygote
which grows substantially before a new
cell wall is laid down.
Diatoms are major primary producers in
the ocean, and also in fresh waters.
Figure 27.21 Diatom Diversity
27.5 What Are the Major Groups of Eukaryotes?
Some diatoms form large blooms in the
ocean that are not grazed by copepods,
the usual predator. The cells die and
sink to the ocean floor.
When diatoms make up a large
proportion of the copepod diet, they
become toxic, preventing copepod
populations from taking advantage of
the food available in a diatom bloom.
Figure 27.22 Why Don’t Copepods Flourish During Diatom Blooms? (Part 1)
Figure 27.22 Why Don’t Copepods Flourish During Diatom Blooms? (Part 2)
27.5 What Are the Major Groups of Eukaryotes?
Diatom cell walls resist decomposition,
and become fossilized in sedimentary
rock.
Diatomaceous earth is from rock
composed almost entirely of diatom cell
walls. It is used in insulation, filtration,
metal polishing, and as an insecticide.
27.5 What Are the Major Groups of Eukaryotes?
Brown algae are multicelluar; some get
very large (e.g., the giant kelp).
The carotenoid fucoxanthin imparts the
brown color.
Almost exclusively marine. Sargassum
forms dense mats in the Sargasso Sea
in the mid-Atlantic.
Figure 27.23 Brown Algae
27.5 What Are the Major Groups of Eukaryotes?
Most brown algae attach to rocks by a
holdfast that glues it to the rock.
The “glue” is alginic acid—a gummy
polymer of sugars. Also holds cells and
filaments together. It is harvested and
used as an emulsifier in ice cream,
cosmetics, and other products.
27.5 What Are the Major Groups of Eukaryotes?
Some brown algae have specialized
organs—stem-like stalks and leaf-like
blades, and gas-filled bladders that act
as floats.
Some of the larger species have tissue
differentiation. Giant kelps have tubular
cells that resemble nutrient-conducting
tissue of land plants, called trumpet
cells.
27.5 What Are the Major Groups of Eukaryotes?
Oomycetes: water molds and downy
mildews; nonphotosynthetic.
Water molds are absorptive heterotrophs
(e.g., Saprolegnia).
Once were classed as fungi, but are
unrelated.
Some are coenocytes—many nuclei
enclosed in one plasma membrane.
27.5 What Are the Major Groups of Eukaryotes?
Oomycetes are diploid throughout most
of their life cycle, and have flagellated
reproductive cells.
Water molds are saprobic (feed on dead
organic matter). A few are terrestrial,
and a few of those are parasites on
plants.
Figure 27.24 An Oomycete
27.5 What Are the Major Groups of Eukaryotes?
The Plantae consist of several clades; all
chloroplasts trace back to a single
incidence of endosymbiosis.
27.5 What Are the Major Groups of Eukaryotes?
Glaucophytes are unicellular, freshwater
organisms; probably first group to
diverge.
The chloroplast retains a bit of
peptidoglycan between the inner and
outer membrane.
They probably resemble the common
ancestor of all Plantae.
27.5 What Are the Major Groups of Eukaryotes?
Most red algae are marine and
multicellular.
Red pigment is phycoerythrin. Also have
phycocyanin, chlorophyll a, and
carotenoids. They can vary the relative
amounts of pigments depending on light
conditions.
In deep water, low light conditions, they
increase the amount of phycoerythrin
and look more red.
Figure 27.25 Red Algae
27.5 What Are the Major Groups of Eukaryotes?
Red algae storage product is floridean
starch—very small glucose chains.
They have no flagellated cells at any time
in the life cycle.
Some species secrete calcium
carbonate, and enhance growth of coral
reefs.
27.5 What Are the Major Groups of Eukaryotes?
Some red algae produce mucilaginous
polysaccharides, which form solid gels
and are the source of agar.
A red alga was the ancestor to the
chloroplasts of photosynthetic
chromalveolates by secondary
endosymbiosis.
27.5 What Are the Major Groups of Eukaryotes?
The chlorophytes are the sister group to
charophytes and land plants.
Synapomorphies include chlorophyll a
and b, and starch as a storage product.
More than 17,000 species; marine,
freshwater, and terrestrial. Unicellular to
large multicellular forms.
27.5 What Are the Major Groups of Eukaryotes?
Some chlorophytes form colonies of cells
that show the possible first step for cell
and tissue differentiation.
In Volvox colonies, some cells are
specialized for reproduction.
Other species are multicellular, some are
coenocytic; Acetabularia is a single
giant cell a few centimeters long.
Figure 27.26 Chlorophytes
27.5 What Are the Major Groups of Eukaryotes?
Excavates: several clades lack
mitochondria—a derived condition.
27.5 What Are the Major Groups of Eukaryotes?
Diplomonads and parabasalids:
unicellular; lack mitochondria.
Giardia lamblia is a diplomonad. It has
two nuclei bounded by nuclear
envelopes.
Trichomonas vaginalis is a parabasalid
responsible for a sexually transmitted
disease in humans.
Figure 27.27 Some Excavate Groups Lack Mitochondria
27.5 What Are the Major Groups of Eukaryotes?
Heteroloboseans have amoeboid body
form.
The free-living Naegleria can enter
humans and cause a fatal disease of
the nervous system.
27.5 What Are the Major Groups of Eukaryotes?
The euglenids have flagella.
Spiral strips of proteins under the plasma
membrane control cell shape.
Some are photosynthetic, some always
heterotrophic, and some can switch.
Figure 27.28 A Photosynthetic Euglenid
27.5 What Are the Major Groups of Eukaryotes?
The kinetoplastids are unicellular
parasites with two flagella and a single
mitochondrion.
The mitochondrion contains a
kinetoplast, a structure with multiple,
circular DNA molecules and proteins.
Trypanosomes are kinetoplastids.
Table 27.2 A Comparison of Three Kinetoplastid Trypanosomes
27.5 What Are the Major Groups of Eukaryotes?
Rhizaria
Unicellular, aquatic, have long thin
pseudopodia.
27.5 What Are the Major Groups of Eukaryotes?
Some cercozoans are aquatic, others
live in soil.
They have diverse forms and habitats.
One group has chloroplasts derived from
a green alga by secondary
endosymbiosis.
27.5 What Are the Major Groups of Eukaryotes?
Foraminiferans secrete shells of calcium
carbonate.
Some live as plankton, others at the
bottom of the sea.
Thread-like, branched pseudopods
extend through numerous pores in the
shell and form a sticky net that captures
smaller plankton.
27.5 What Are the Major Groups of Eukaryotes?
Radiolarians have thin, stiff pseudopods
reinforced by microtubules.
The pseudopods increase surface area
for exchange of materials; and help the
cell float.
Exclusively marine, most secrete glassy
endoskeletons, many with elaborate
designs.
Figure 27.29 A Radiolarian’s Glass House
27.5 What Are the Major Groups of Eukaryotes?
Unikonts: single flagellum (if present)
27.5 What Are the Major Groups of Eukaryotes?
Animals and fungi arose from a common
ancestor within the opisthokont clade;
sister to the amoebozoans.
Synapomorphy of the opisthokonts: if
flagellum is present it is posterior
(anterior in other eukaryotes).
Choanoflagellates are sister to the
animals. Some are colonial and
resemble a type of cell found in
sponges.
Figure 27.30 A Link to the Animals
27.5 What Are the Major Groups of Eukaryotes?
The amoebozoans have lobe-shaped
pseudopods.
Loboseans, such as Amoeba proteus,
are unicellular and do not aggregate.
Feed by phagocytosis, living as
predators, parasites, or scavengers.
Some secrete shells or glue sand grains
together to form a casing.
27.5 What Are the Major Groups of Eukaryotes?
There are two clades of slime molds.
All are motile, ingest food by endocytosis,
and form spores on stalks called fruiting
bodies.
Slime molds are found in cool, moist
habitats, primarily forests.
27.5 What Are the Major Groups of Eukaryotes?
Plasmodial slime molds
During the vegetative (feeding stage)
they are coenocytes with many diploid
nuclei that streams over the substrate in
a network of strands called a
plasmodium.
27.5 What Are the Major Groups of Eukaryotes?
Movement is by cytoplasmic
streaming—outer cytoplasmic region
becomes more fluid, and cytoplasm
rushes in.
Microfilaments and a contractile protein
called myxomyosin interact to produce
the streaming movement.
As it moves, the plasmodium engulfs
food particles by endocytosis.
Figure 27.31 Plasmodial Slime Molds
27.5 What Are the Major Groups of Eukaryotes?
If there is food, a plasmodium can grow
indefinitely.
When conditions become unfavorable, a
resting form develops with hardened
components, called a sclerotium.
Becomes a plasmodium again when
conditions are favorable.
27.5 What Are the Major Groups of Eukaryotes?
Alternatively, the plasmodium transforms
into spore-bearing fruiting bodies.
In the rigid stalks, walls form and thicken
between nuclei.
Diploid nuclei undergo meiosis,
sporangia form at the tip of the stalk and
haploid nuclei in the sporangia form
spores.
27.5 What Are the Major Groups of Eukaryotes?
The spores germinate into swarm cells—
haploid cells that divide mitotically to
form more swarm cells, or function as
gametes.
Swarm cells can live as individuals,
moving by flagella or pseudopods, or
become walled, resistant cysts.
Two swarm cells can fuse to form a
zygote, which forms a new plasmodium.
27.5 What Are the Major Groups of Eukaryotes?
The vegetative unit of cellular slime
molds is an amoeboid cell.
The myxamoebas have a single haploid
nucleus, engulf food by endocytosis,
and reproduce by fission. This stage
persists if food is available.
When conditions become unfavorable,
the cells aggregate into a slug or
pseudoplasmodium. Individuals retain
their plasma membranes.
27.5 What Are the Major Groups of Eukaryotes?
A slug may migrate before coming to rest
and forming fruiting bodies.
Cells at the top become spores.
Spores germinate and release
myxamoebas when conditions are
favorable.
Two myxamoebas may also fuse in
sexual reproduction.
Figure 27.32 A Cellular Slime Mold