Chapter 28
Given that I do not usually give the taxonomy lectures
(the material is addressed via projects and assignments) these notes will not
have any words removed.-Dr. G.
Protists and the
Dawn of the Eukarya
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
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.
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 changefrom anaerobic to aerobicduring the course of the evolution of the eukaryotes.
We can make only reasonable guesses as to what the steps in this evolutionary process were.
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
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.
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
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.
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.
The Origin of the Eukaryotic Cell
During the early stages of eukaryotic evolution, the O2 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.
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.
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.
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.
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.
General Biology of the Protists
Most protist groups include motile cells.
Amoeboid motion involves the formation of pseudopods, extensions of the cells 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.
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.
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.
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.
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.
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.
Diplomonads and Parabasalids
The diplomonads and the parabasalids appear to represent the earliest surviving branches in todays 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.
Euglenozoans
The euglenozoans are a clade of unicellular protists with flagella.
The euglenoids and the kinetoplastids are the two subgroups of the euglenozoans.
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.
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.
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.
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.
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 are the cause of malaria.
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.
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.
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.
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.
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.
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.
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.
Stramenopiles
Brown algae exhibit alternation of generations.
A diploid, spore-producing organism (the sporophyte) gives rise to a haploid, gamete-producing organism (the gametophyte).
In heteromorphic alternation of generations, the generations differ morphologically; in isomorphic alternation of generations, they do not.
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.
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.
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.
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.
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.
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.
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.
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.
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.
A History of Endosymbiosis
Primary endosymbiosis consisted of the engulfment of a cyanobacterium by a larger eukaryotic cell.
It gave rise to the chloroplasts of the green algae and red algae.
A History of Endosymbiosis
Secondary endosymbiosis:
The ancestor of the photosynthetic euglenoids took up a unicellular chlorophyte, retaining the endosymbionts chloroplast and eventually losing the rest of its constituents.
Other photosynthetic protists derived their chloroplasts by secondary endosymbiosis with unicellular red algae.
A History of Endosymbiosis
Tertiary endosymbiosis:
At least one secondary endosymbiosis produced a unicellular protist that became a partner in tertiary endosymbosis.
In that case, a dinoflagellate lost its plastid and took up a haptophyte protist (itself the product of secondary symbiosis), resulting in the dinoflagellate Karenia brevis.
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.
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 cells feeding organs.
Radiolarians are exclusively marine and secrete a glassy endoskeleton.
Heliozoans are primarily freshwater actinopods that lack an endoskeleton.
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.
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.
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.
Some Recurrent Body Forms
One of two things can happen if growth conditions become unfavorable for the acellular slime molds:
The plasmodium can form an irregular mass of hardened cell-like components called a sclerotium.
The plasmodium can transform itself into spore-bearing fruiting structures called sporangiophores.
Acellular slime mold spores germinate into wall-less, flagellated, haploid cells called swarm cells.
Swarm cells can either divide mitotically to produce more swarm cells, or they can function as gametes.
Some Recurrent Body Forms
The basic vegetative unit of the cellular slime molds is an amoeboid cell.
In favorable conditions, large numbers of cells called myxamoebas exist, containing single haploid nuclei, engulfing food by endocytosis, and reproducing by binary fission or mitosis.
When conditions become unfavorable, the cellular slime molds aggregate into a mass called a slug or pseudoplasmodium.
Within these aggregate structures, the individual myxamoebas retain their plasma membranes.
Some Recurrent Body Forms
The pseudoplasmodium may move around on its substrate for several hours.
Eventually, the pseudoplasmodium develops stalked fruiting structures.
The cells at the top of the fruiting structures develop into spores, which germinate under favorable conditions to release myxamoebas.
Animation 28.1 Food Vacuoles Handle Digestion and Excretion
Animation 28.2 Family Tree of Chloroplasts
Video 28.1 Movement in the euglenoid Eutreptia
Video 28.2 Movement in the green alga Volvox
Video 28.3 Movement in a ciliate from the gut of a termite
Video 28.4 Gliding via mucilage secretion in the green alga Micrasterias
Video 28.5 Amoeba with visible organelles
Video 28.6 A leathery cell wall in a Euglena
Video 28.7 Paramecium bursaria with an endosymbiont (green alga)
Video 28.8 Reproduction in the green alga Chlamydomonas
Video 28.9 Reproduction in the green alga Oedogonium
Video 28.10 Conjugation in the filamentous alga Spirogyra
Video 28.11 Motile spores released from the red alga Purpureofilum
Video 28.12 Various euglenoids
Video 28.13 A dinoflagellate, Certium tenue
Video 28.14 A Paramecium uses cilia for feeding
Video 28.15 A colony of the diatom, Bacillaria paradoxa
Video 28.16 A community of brown algae: The marine kelp forest
Video 28.17 A red alga, Audouinella
Video 28.18 A red alga, Spyridia sp., releases carpospores
Video 28.19 Cytokinesis in the green alga Micrasterias
Video 28.20 The choanoflagellate Salpingoeca sp. feeding
Video 28.21 The acellular slime mold Physarum
Video 28.22 The life stages of the cellular slime mold Dictyostelium discoideum