Protists and the Dawn of the Eukarya

28

Protists and the Dawn of the Eukarya

• Protists Defined • The Origin of the Eukaryotic Cell • General Biology of the Protists • Protist Diversity  Diplomonads and Parabasalids    Euglenozoans Alveolates Stramenopiles    Red Algae Chlorophytes Choanoflagellates • A History of Endosymbiosis • Some Recurrent Body Forms

28

Protists Defined

• Many members of the Eukarya do not fit into the three familiar kingdoms of the Plantae, Animalia, and Fungi.

• The eukaryotes that are neither plants, animals, nor fungi are called protists.

• The protists are a polyphyletic group; some are more closely related to the animals than they are to other protists.

Figure 28.1

Three Protists

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The Origin of the Eukaryotic Cell

• The eukaryotic cell differs in many ways from the prokaryotic cell.

• The nature of the evolutionary process dictates that these differences could not have arisen simultaneously.

• The global environment underwent an enormous change —from anaerobic to aerobic—during the course of the evolution of the eukaryotes.

• We can make only reasonable guesses as to what the steps in this evolutionary process were.

28

The Origin of the Eukaryotic Cell

• The evolution of eukaryotic cells included the following components:  The origin of a flexible cell surface  The origin of a cytoskeleton  The origin of a nuclear envelope  The appearance of digestive vesicles  The endosymbiotic acquisition of certain organelles

28

The Origin of the Eukaryotic Cell

• The first step toward the eukaryotic condition may have been the loss of the cell wall by an ancestral prokaryotic cell.

• A surface that is flexible enough to allow for infolding lets the cell exchange materials with its environment rapidly enough to sustain a larger volume and more rapid metabolism. • A flexible surface also allows endocytosis.

• An infolded plasma membrane attached to a chromosome within an ancestral prokaryote may have led to the formation of the nuclear envelope.

Figure 28.2

Membrane Infolding

28

The Origin of the Eukaryotic Cell

• The early steps in the evolution of the eukaryotic cell likely included three advances:  The formation of ribosome-studded internal membranes, some of which surrounded the DNA  The appearance of a cytoskeleton  The evolution of digestive vesicles

Figure 28.3

From Prokaryotic Cell to Eukaryotic Cell

(Part 1)

28

The Origin of the Eukaryotic Cell

• A cytoskeleton allowed the now much larger cell to manage changes in its shape, distribute daughter chromosomes, and move materials from one part of the cell to another.

• The origin of the cytoskeleton is a mystery; the genes that encode it are found in neither bacteria nor archaea.

• A controversial hypothesis suggests that these genes may have originated in a long-extinct fourth domain of life that transferred them laterally to an ancestor of the early eukaryotes.

28

The Origin of the Eukaryotic Cell

• From an intermediate kind of cell, the next advance was likely to have been a motile phagocyte.

• The first true eukaryotic cell possessed a cytoskeleton and a nuclear envelope; it also may have had an associated endoplasmic reticulum and Golgi apparatus and perhaps one or more flagella.

28

The Origin of the Eukaryotic Cell

• During the early stages of eukaryotic evolution, the O 2 levels in the atmosphere were increasing as a result of the photosynthetic activities of the cyanobacteria.

• Most living things were unable to tolerate this new aerobic, oxidizing environment, but some prokaryotes and ancient phagocytes were able to survive.

• One hypothesis suggests that the key to the survival of the early phagocytes was the ingestion of a prokaryote that became symbiotic and evolved into the peroxisomes of today.

28

The Origin of the Eukaryotic Cell

•

Peroxisomes

are organelles that are able to disarm the toxic products of oxygen, such as hydrogen peroxide.

• The crucial endosymbiotic event that marked the completion of the modern eukaryotic cell was the incorporation of a proteobacterium that evolved into the mitochondrion.

28

The Origin of the Eukaryotic Cell

• There are still several uncertainties surrounding the origins of eukaryotic cells.

• Lateral gene transfer may not have been extensive enough to account for the increasing number of genes of bacterial origin that are found in eukaryotes.

• The endosymbiotic origin of the mitochondria and chloroplasts accounts for the presence of bacterial genes that encode enzymes for respiration and photosynthesis, but it does not explain the presence of many other bacterial genes.

28

The Origin of the Eukaryotic Cell

• It is clear that the eukaryotic genome is a mixture of genes with two distinct origins.

• Recently, it has been suggested that the Eukarya may have arisen from the mutualistic fusion of a Gram-negative bacterium and an archaean.

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General Biology of the Protists

• Most protists are aquatic, occupying a variety of environments including marine and fresh waters, the body fluids of other organisms, and soil water.

• Most are unicellular, but some are multicellular, and a few are very large.

• Some protists are heterotrophs, some are autotrophs, and some switch between these two modes of nutrition.

• The terms protozoan and algae actually lump together many phylogenetically distant protist groups.

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General Biology of the Protists

• Most protist groups include motile cells. •

Amoeboid motion

involves the formation of

pseudopods

, extensions of the cell’s constantly changing body mass. • The coordinated beating of tiny, hairlike organelles called

cilia

can move cells forward or backward.

• The eukaryotic

flagella

move like a whip; some flagella push the cell, while others pull the cell.

Figure 28.4

An Amoeba

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General Biology of the Protists

• One reason cells are small is that they need a high surface area-to-volume ratio to support the exchange of materials required for their existence.

• The presence of membrane-enclosed vesicles of various types increases the effective surface area in large, unicellular protists.

• Several protists that are hypertonic to their environments have contractile vacuoles that excrete excess water. • Food vacuoles are vesicles in which ingested food is digested.

Figure 28.5

Contractile Vacuoles Bail Out Excess Water

Figure 28.6

Food Vacuoles Handle Digestion and Excretion

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General Biology of the Protists

• The cell surfaces of protists are diverse.

• Some protists are surrounded only by a plasma membrane, such as an amoeba.

• Most have stiffer surfaces to maintain the structural integrity of the cell.

• Some protists have complex cell walls.

• Some protists have internal “shells”.

Figure 28.7

Diversity among Protist Cell Surfaces (Part 1)

Figure 28.7

Diversity among Protist Cell Surfaces (Part 2)

28

General Biology of the Protists

• Many protists contain

endosymbionts

.

• Endosymbiosis is very common in the protists, and in some cases both the host and the endosymbiont are protists.

Figure 28.8

Protists within Protists

28

General Biology of the Protists

• Most protists practice both asexual and sexual reproduction; some groups practice only asexual.

• Asexual reproductive processes in the protists include binary fission, multiple fission, budding, and the formation of spores.

• Sexual reproduction in the protists also takes various forms.

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Protist Diversity

• The diversity found among the protists reflects the diversity of avenues pursued during the early evolution of the eukaryotes.

• Molecular biology techniques, such as rRNA sequencing, are making it possible to explore the evolutionary relationships among the protists in greater detail.

Figure 28.9

Major Protist Groups in an Evolutionary Context

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Diplomonads and Parabasalids

• The

diplomonads

and the

parabasalids

appear to represent the earliest surviving branches in today’s tree of eukaryotic life.

• Both clades are unicellular organisms that lack mitochondria. Their ancestors possessed mitochondria, but they were lost in the course of evolution.

•

Giardia lamblia

is a parasitic diplomonad that contaminates water supplies and causes giardiasis. •

Trichomonas vaginalis

is a parabasilid responsible for a sexually transmitted disease in humans.

Figure 28.10

Two Protist Groups Lack Mitochondria

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Euglenozoans

• The

euglenozoans

are a clade of unicellular protists with flagella.

• The euglenoids and the kinetoplastids are the two subgroups of the euglenozoans.

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Euglenozoans

• The

euglenoids

possess flagella arising from a pocket at the anterior end of the cell. • The

Euglena

propels itself through the water with one of its two flagella. • Many species of

Euglena

are heterotrophic, whereas others are photoautotrophs.

• These autotrophic

Euglena

can become heterotrophic when kept in the dark, and they resume their autotrophic behavior when returned to light.

Figure 28.11

A Photosynthetic Euglenoid

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Euglenozoans

• The

kinetoplastids

are unicellular, parasitic flagellates with a single, large mitochondrion.

• The mitochondrion contains a

kinetoplast

, a unique structure that houses multiple, circular DNA molecules and associated proteins.

• The trypanosomes are human pathogens that cause sleeping sickness, leishmaniasis, Chagas’ disease, and East Coast fever.

Figure 28.12

A Parasitic Kinetoplastid

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Alveolates

• The

alveolates

are a clade of unicellular organisms characterized by the possession of cavities called

alveoli

just below their plasma membranes.

• Alveolates include the dinoflagellates, apicomplexans, and the ciliates.

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Alveolates

• The

dinoflagellates

are unicellular, aquatic organisms; they are among the most important primary producers in the oceans.

• Many dinoflagellates are endosymbionts, while some live as parasites within other marine organisms.

• The dinoflagellates have a distinctive appearance with two flagella.

• They are responsible for toxic “red tides.” • Many are bioluminescent.

Figure 28.13

A Red Tide of Dinoflagellates

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Alveolates

• The

apicomplexans

are exclusively parasitic.

• The apical complex is a mass of organelles contained within the apical end of their spores. These organelles help the apicomplexan spore invade its host tissue.

• Apicomplexans of the genus

Plasmodium

cause of malaria.

are the • This parasite enters the human circulatory system by way of the

Anopheles

mosquito.

• It is an extracellular parasite in the insect vector and an intracellular parasite in the human host.

Figure 28.14

The Life Cycle of an Apicomplexan

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Alveolates

• The

ciliates

are named for their characteristic hairlike cilia. • Almost all are heterotrophic, and they have a complex body form.

• The ciliates possess two types of nuclei: a single macronucleus and one or more micronuclei.

• The micronuclei are typical eukaryotic nuclei.

• The macronuclei are derived from the micronuclei and contain DNA that is transcribed and translated to regulate the life of the cell.

Figure 28.15

Diversity among the Ciliates (Part 1)

Figure 28.15

Diversity among the Ciliates (Part 2)

28

Alveolates

•

Paramecium

is a frequently studied ciliate.

• The cell is covered by an elaborate

pellicle

composed of an outer membrane and an inner layer of membrane-enclosed alveoli that surround the bases of the cilia.

• Defensive

trichocysts

in the pellicle are expelled in response to threat.

• The cilia on a

Paramecium

provide a form of locomotion that is more precise than locomotion by flagella or pseudopods.

Figure 28.16

Anatomy of

Paramecium

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Alveolates

• Paramecia reproduce asexually by

binary fission

, in which the micronuclei divide mitotically and the macronuclei divide by an unknown mechanism.

• Paramecia also exhibit a form of genetic recombination called

conjugation

. It is not a reproductive process; no new cells are created.

• Each member of a pair of cells gets two haploid micronuclei, which fuse to form a new diploid micronucleus.

• Experiments have shown that in species not permitted to conjugate, the clones can survive only a limited number of divisions.

Figure 28.17

Paramecia Achieve Genetic Recombination by Conjugating

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Stramenopiles

• The

stramenopiles

typically have two flagella of unequal length at some point in their life cycle.

• The longer of the two flagella bears rows of tubular hairs.

• There are photosynthetic and nonphotosynthetic stramenopile groups.

• Some stramenopiles lack flagella, but are presumed to be descended from ancestors that had flagella.

• The stramenopiles include the diatoms, the brown algae, and the oomycetes.

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Stramenopiles

•

Diatoms

are single-celled organisms, but some species form filaments. Diatoms have carotenoids in their chloroplasts to give them a yellow or brownish color.

• Diatoms deposit silicon in their cells walls, which gives them their characteristically intricate appearance.

• Certain sedimentary rocks are almost entirely composed of diatom skeletons, called diatomaceous earth.

• Diatoms reproduce both sexually and asexually. • Diatoms are major photosynthetic producers in coastal waters and in fresh waters.

Figure 28.18

Diatom Diversity

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Stramenopiles

• The

brown algae

are multicellular organisms composed of either branched filaments or leaflike growths called

thalli

. • The carotenoid fucoxanthin in the chloroplasts gives brown algae their color.

• The brown algae are exclusively marine, and most are attached to rocks near the shore.

• The holdfast is a specialized structure that glues the attached forms to rocks. • Some brown algae have stalks and blades, and some develop gas-filled cavities or bladders.

Figure 28.20

Brown Algae

Figure 28.21

Brown Algae in a Turbulent Environment

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Stramenopiles

• The

oomycetes

are a nonphotosynthetic group that consists largely of the water molds and their terrestrial relatives, such as the downy mildews. • The oomycetes are

coenocytes

(many nuclei enclosed in a single plasma membrane).

• The oomycetes are diploid for most of their life cycle and have flagellated reproductive cells.

• The water molds are aquatic and saprobic.

• Most terrestrial oomycetes are decomposers, although some are serious plant parasites.

•

Phytophthora infestans

water mold was the cause of the Irish potato famine.

Figure 28.23

An Oomycete

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Red Algae

• Almost all

red algae

are multicellular. • Their characteristic red color results from the photosynthetic pigment phycoerythrin.

• Most species of red algae are marine-dwelling, from shallow tide pools to deep in the ocean.

• The red algae have the ability to change the relative amounts of their various photosynthetic pigments depending on the light conditions.

Figure 28.24

Red Algae

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Red Algae

• The red algae have characteristics that make them unique among protists.

• They contain the pigments phycoerythrin and phycocyanin and store the products of photosynthesis as floridean starch.

• They produce no motile, flagellated cells at any stage in their life cycle.

• Some produce a mucilaginous polysaccharide substance which is the source of agar.

• Certain red algae became endosymbionts long ago within the cells of other, nonphotosynthetic protists, eventually giving rise to chloroplasts.

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Chlorophytes

• The

chlorophytes

are a monophyletic group with a sister lineage consisting of other green algal lineages and the plant kingdom.

• Like plants, the chlorophytes contain chlorophylls

a

and

b

, and store photosynthetic products as starch in plastids.

• There are terrestrial, marine, and freshwater chlorophyte species.

• There is an incredible variety in shape and construction of the algal body within the chlorophytes.

Figure 28.25

Chlorophytes (Part 1)

Figure 28.25

Chlorophytes (Part 2)

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Chlorophytes

• There is great diversity within the life cycles of the chlorophytes.

• The sea lettuce

Ulva lactuca

exhibits an isomorphic life cycle. • Most species of

Ulva

have structurally indistinguishable male and female gametes, and are categorized as

isogamous

.

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Chlorophytes

• Other chlorophytes are

anisogamous

, having female gametes that are distinctly larger than male gametes.

• Many other chlorophytes have a

heteromorphic

life cycle, with some exhibiting a variation of the heteromorphic life cycle called the

haplontic

life cycle. • Other chlorophytes have a

diplontic

life cycle like that of many animals, where every cell except the gametes is diploid.

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Chlorophytes

• There are green algae other than chlorophytes.

• The chlorophytes are the largest lineage of green algae, but there are other lineages as well.

• These lineages are branches of a lineage that also includes the charophytes and the plant kingdom.

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Choanoflagellates

• The

choanoflagellates

are a group of colonial, flagellated protists that are thought to comprise the closest relatives of the animals. • Choanoflagellates bear a striking resemblance to the most characteristic type of cell found in the sponges.

Figure 28.28

A Link to the Animal Kingdom

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A History of Endosymbiosis

• Chloroplasts are found in many distantly related protist lineages.

• Some of these groups differ from others in terms of the photosynthetic pigments in their chloroplasts and the number of membranes surrounding their chloroplasts.

• These differences can be traced back to whether the group acquired its chloroplast through primary, secondary, or tertiary endosymbiosis.

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Some Recurrent Body Forms

• The amoeboid body plan includes pseudopods for locomotion.

• Amoebas appear in many protist groups.

• Amoebas are specialized protists: many are adapted for life on the bottoms of lakes, ponds, and other bodies of water.

• Most are predators, parasites, or scavengers. A few are photosynthetic.

• Some have two-stage life cycles.

• Some amoebas have shells.

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Some Recurrent Body Forms

• The

actinopods

are have thin, stiff pseudopods, reinforced by microtubules.

• The pseudopods increase the surface area of the cell, help the cell float, provide locomotion in some species, and are the cell’s feeding organs.

•

Radiolarians

are exclusively marine and secrete a glassy endoskeleton. •

Heliozoans

are primarily freshwater actinopods that lack an endoskeleton.

Figure 28.30

Two Actinopods

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Some Recurrent Body Forms

•

Foraminiferans

are marine protists that secrete shells of calcium carbonate.

• The parent shell is abandoned after foraminiferan reproduction. The discarded skeletons of ancient foraminiferans make up extensive limestone deposits.

• The shells of individual foraminiferan species have been preserved as fossils in marine sediments and are valuable as indicators in the classification and dating of sedimentary rocks.

Figure 28.7 (a)

Diversity among Protist Cell Surfaces

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Some Recurrent Body Forms

• Initially, the three groups of slime molds were seen as so similar they were placed in a single phylum.

• In actuality, they are so different that some biologists now classify them in separate kingdoms.

• Slime molds share only general characteristics:  All are motile.

 All ingest particulate food by endocytosis.

 All form spores on erect fruiting bodies.

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Some Recurrent Body Forms

•

Acellular slime molds

form a multinucleate mass with diploid nuclei (a coeonocyte) during the vegetative phase.

• This mass moves over its substrate in a network of strands called a

plasmodium

. • Changes in the fluidity of the outer cytoplasmic regions within acellular slime molds allow them to move by cytoplasmic streaming.

Figure 28.3 (a)

Acellular Slime Molds