19 The Origin and Diversification of Eukaryotes © Oxford University Press Chapter The Origin and Diversification of Eukaryotes Key Concepts 19.1 Eukaryotes Acquired Features from Both Archaea and Bacteria 19.2 Major Lineages of Eukaryotes Diversified in the Precambrian 19.3 Protists Reproduce Sexually and Asexually 19.4 Protists Are Critical Components of Many Ecosystems © Oxford University Press Concept 19.1 Eukaryotes Acquired Features from Both Archaea and Bacteria Eukaryotes that are not plants, animals, or fungi have traditionally been called protists, which is not a formal taxonomic group—it is a convenience term. Eukaryotes are monophyletic and are thought to be more closely related to Archaea than to Bacteria. But mitochondria and chloroplasts are clearly derived from bacterial lineages. © Oxford University Press Concept 19.1 Eukaryotes Acquired Features from Both Archaea and Bacteria Events in the origin of the eukaryotic cell: • Origin of a flexible cell surface • Origin of a cytoskeleton • Nuclear envelope developed • Digestive vacuoles appeared • Acquisition of mitochondria and chloroplasts by endosymbiosis. © Oxford University Press Figure 19.1 Evolution of the Eukaryotic Cell Concept 19.1 Eukaryotes Acquired Features from Both Archaea and Bacteria Flexible Cell Surface: Loss of the cell wall (which occurs in some modern prokaryotes) opens possibilities: • A flexible cell surface allows infolding and increased surface area (cells can be larger). • Endocytosis is possible—pinching off bits of the environment and bringing them into the cell. © Oxford University Press Concept 19.1 Eukaryotes Acquired Features from Both Archaea and Bacteria Increased compartmentalization and complexity: • Development of a more complex cytoskeleton • Formation of ribosome-studded internal membranes • Enclosure of the DNA in a nucleus • Formation of flagella from microtubules of the cytoskeleton • Evolution of digestive vacuoles © Oxford University Press Concept 19.1 Eukaryotes Acquired Features from Both Archaea and Bacteria Simple cytoskeletons evolved in prokaryotes. In eukaryotes, greater development of microfilaments and microtubules gives support and allows changes in shape, distribution of daughter chromosomes, and movement of materials. Microtubules allowed some cells to develop eukaryotic flagella. © Oxford University Press Concept 19.1 Eukaryotes Acquired Features from Both Archaea and Bacteria A nuclear envelope developed early in eukaryote evolution. • May have arisen from DNA attached to the membrane of an infolded vesicle • Prokaryote DNA is attached to the inner plasma membrane © Oxford University Press Concept 19.1 Eukaryotes Acquired Features from Both Archaea and Bacteria Phagocytosis: the ability to engulf and digest other cells © Oxford University Press Concept 19.1 Eukaryotes Acquired Features from Both Archaea and Bacteria Phagocytosis: the ability to engulf and digest other cells Endosymbiosis: a proteobacterium was incorporated and evolved into the mitochondrion Original function of mitochondria might have been to detoxify the O2 that was being produced by cyanobacteria, reducing it to water. Later, this became coupled with formation of ATP. © Oxford University Press Concept 19.1 Eukaryotes Acquired Features from Both Archaea and Bacteria Chloroplast development occurred in a series of endosymbioses. Primary endosymbiosis: a cyanobacterium was engulfed by a larger eukaryotic cell. Remnants of a peptidoglycan cell wall can be found in glaucophytes. Also gave rise to chloroplasts of red algae, green algae, and land plants. Their chloroplasts have two membranes. © Oxford University Press Figure 19.2 Endosymbiotic Events in the Evolution of Chloroplasts Concept 19.1 Eukaryotes Acquired Features from Both Archaea and Bacteria Other photosynthetic eukaryotes are the result of additional rounds of endosymbiosis. Secondary endosymbiosis: a eukaryote engulfed a green alga cell, which became a chloroplast. Euglenid chloroplasts have three membranes, and they have the same pigments as land plants and green algae. © Oxford University Press Concept 19.1 Eukaryotes Acquired Features from Both Archaea and Bacteria Tertiary endosymbiosis: A dinoflagellate lost its chloroplast and took up another protist that had acquired its chloroplast through secondary endosymbiosis. © Oxford University Press Endosymbiosis © Oxford University Press Concept 19.2 Major Lineages of Eukaryotes Diversified in the Precambrian There are eight major clades of eukaryotes that began to diversify about 1.5 billion years ago. The five major clades of protistan eukaryotes have enormous diversity. • Most are unicellular and microscopic (microbial eukaryotes). • Some are multicellular, and some are quite large (e.g., giant kelp). © Oxford University Press Figure 19.3 Precambrian Divergence of Major Eukaryotic Groups Concept 19.2 Major Lineages of Eukaryotes Diversified in the Precambrian Multicellularity has arisen dozens of times in the eukaryotes. Experimental studies show that artificial selection for multicellularity can produce repeated, convergent evolution of multicellular forms in normally unicellular species. Many unicellular eukaryotes associate in colonies; there is a continuum from unicellular to fully multicellular. © Oxford University Press Concept 19.2 Major Lineages of Eukaryotes Diversified in the Precambrian Alveolates Sacs called alveoli lie just beneath the cell membrane. All are unicellular, most are photosynthetic. • Dinoflagellates • Apicomplexans • Ciliates © Oxford University Press Concept 19.2 Major Lineages of Eukaryotes Diversified in the Precambrian Dinoflagellates: mostly marine; photosynthetic; important primary producers in the oceans. • Some species cause red tides. • Some are endosymbionts with invertebrates (e.g., corals). • Some are nonphotosynthetic parasites within various marine organisms. © Oxford University Press Figure 19.4 A Dinoflagellate Concept 19.2 Major Lineages of Eukaryotes Diversified in the Precambrian Apicomplexans: obligate parasites • Apical complex—organelles at the tip of the cell that help it invade host tissue • Elaborate life cycles feature asexual and sexual reproduction and life stages in different hosts • Plasmodium is the causative agent of malaria • Toxoplasma alternates between cats and rats. An infected rat loses its fear of cats. © Oxford University Press Concept 19.2 Major Lineages of Eukaryotes Diversified in the Precambrian Ciliates have numerous hairlike cilia (identical to eukaryotic flagella), which allow more precise locomotion than flagella. • Complex body forms and two types of nuclei • Heterotrophic; some have photosynthetic endosymbionts © Oxford University Press en.wikipedia.org/wiki/Ciliate Figure 19.5 Diversity among the Ciliates Concept 19.2 Major Lineages of Eukaryotes Diversified in the Precambrian Paramecium is covered by a flexible pellicle with trichocysts—defensive organelles that can explode as sharp darts. Lives in fresh water: contractile vacuoles excrete excess water taken in by osmosis. Also has digestive vacuoles, where food is digested. © Oxford University Press Concept 19.2 Major Lineages of Eukaryotes Diversified in the Precambrian Stramenopiles Rows of tubular hairs on the longer of their two flagella. Some lack flagella but are descended from ancestors that possessed them. • Diatoms • Brown algae • Oomycetes © Oxford University Press Concept 19.2 Major Lineages of Eukaryotes Diversified in the Precambrian Diatoms: unicellular; some species associate in filaments; carotenoids give them a yellow or brownish color. • Lack flagella except in male gametes • Deposit silica in two-piece cell walls; intricate patterns are unique to each species. • Reproduce both sexually and asexually • Abundant in oceans and fresh waters and are major photosynthetic producers. © Oxford University Press Figure 19.9 Diatom Diversity Concept 19.4 Protists Are Critical Components of Many Ecosystems Diatoms store energy as oil. Over millions of years, diatoms have died and sunk to the ocean floor, ultimately becoming petroleum and natural gas. Diatomaceous earth is sedimentary rock composed mostly of the silica cell walls of diatoms; it is used for insulation, filtration, metal polishing, and to kill insects (it clogs their breathing tubes Concept 19.2 Major Lineages of Eukaryotes Diversified in the Precambrian Brown algae: brown color comes from the carotenoid fucoxanthin. • All are multicellular, marine • Attached forms develop holdfasts with alginic acid to glue them to rocks. Alginic acid is used as an emulsifier in ice cream, cosmetics, and other products. • Giant kelps may be up to 60 meters long. © Oxford University Press Figure 19.10 Brown Algae Concept 19.2 Major Lineages of Eukaryotes Diversified in the Precambrian Oomycetes: water molds, downy mildews • Once classed as fungi • All are absorptive heterotrophs—secrete enzymes that digest large food molecules into smaller molecules that they can absorb. © Oxford University Press Concept 19.2 Major Lineages of Eukaryotes Diversified in the Precambrian Oomycetes: water molds, downy mildews • Once classed as fungi • All are absorptive heterotrophs—secrete enzymes that digest large food molecules into smaller molecules that they can absorb. • Water molds—all aquatic and saprobic (feed on dead organic matter) • Some downy mildews attack crop plants. © Oxford University Press Concept 19.2 Major Lineages of Eukaryotes Diversified in the Precambrian Rhizaria Unicellular and mostly aquatic; have long, thin pseudopods Make up a large component of ocean sediments • Cercozoans • Foraminiferans • Radiolarians © Oxford University Press Concept 19.2 Major Lineages of Eukaryotes Diversified in the Precambrian Cercozoans: soil and aquatic organisms One group has chloroplasts derived from a green alga by secondary endosymbiosis—and that chloroplast contains a trace of the alga’s nucleus. © Oxford University Press Concept 19.2 Major Lineages of Eukaryotes Diversified in the Precambrian Foraminiferans: external shells of calcium carbonate • Threadlike, branched pseudopods extend through microscopic holes in the shell and form a sticky net, used to catch smaller plankton. • Accumulations of shells have produced much of the world’s limestone. © Oxford University Press Concept 19.4 Protists Are Critical Components of Many Ecosystems Foraminiferan shells make up extensive limestone deposits and some sandy beaches. The shells are preserved as fossils in marine sediments. The chemical makeup of the shells can be used to estimate temperatures prevalent at the time the shells were formed. Fossil shells in rocks are used in stratigraphy and dating of the rocks. © Oxford University Press Concept 19.2 Major Lineages of Eukaryotes Diversified in the Precambrian Radiolarians: radial symmetry; thin, stiff pseudopods reinforced by microtubules; marine • Secrete glassy endoskeletons • Pseudopods increase surface area of the cell, and help it stay afloat © Oxford University Press Figure 19.13 Radiolarians Exhibit Distinctive Pseudopods and Radial Symmetry Concept 19.2 Major Lineages of Eukaryotes Diversified in the Precambrian Excavates: diverse group; some have lost their mitochondria • Diplomonads • Parabasalids • Heteroloboseans • Euglenids • Kinetoplastids © Oxford University Press Figure 19.14 Some Excavate Groups Lack Mitochondria Concept 19.2 Major Lineages of Eukaryotes Diversified in the Precambrian Diplomonads and parabasalids: unicellular and lack mitochondria (a derived condition) • Giardia lamblia causes the intestinal disease giardiasis. Parabasalids have undulating membranes that aid in locomotion. • Trichomonas vaginalis causes a sexually transmitted disease (trichomoniasis). © Oxford University Press Concept 19.2 Major Lineages of Eukaryotes Diversified in the Precambrian Heteroloboseans: amoeboid body form • Naegleria has two stages, one with amoeboid cells and the other with flagellated cells. • Some species can enter the human body and cause a fatal disease of the nervous system. © Oxford University Press Concept 19.2 Major Lineages of Eukaryotes Diversified in the Precambrian Euglenids and kinetoplastids: mitochondria with disc-shaped cristae, and flagella with a crystalline rod Some euglenids are always heterotrophic; some are photosynthetic but can loose their pigments and feed on organic matter. © Oxford University Press Figure 19.15 A Photosynthetic Euglenid Concept 19.2 Major Lineages of Eukaryotes Diversified in the Precambrian Kinetoplastids: parasites with two flagella • Mitochondrion has a kinetoplast that contains multiple circular DNA molecules. • Trypanosomes are pathogens that can change cell surface recognition molecules frequently, making them hard to control. © Oxford University Press https://en.wikipedia.org/wiki/Trypanosoma Concept 19.2 Major Lineages of Eukaryotes Diversified in the Precambrian Amoebozoans Amoeboid body form; lobe-shaped pseudopods • Loboseans • Plasmodial slime molds • Cellular slime molds © Oxford University Press Concept 19.2 Major Lineages of Eukaryotes Diversified in the Precambrian Loboseans: feed by phagocytosis, engulfing smaller organisms and particles with pseudopods. • Many are adapted to living on the bottoms of lakes and ponds. • Testate amoebas live in shells made from sand grains or secreted by the organism. © Oxford University Press Figure 19.17 Life in a Glass House Concept 19.2 Major Lineages of Eukaryotes Diversified in the Precambrian Plasmodial slime molds: • Vegetative state (the plasmodium) is a mass of cytoplasm with no cell walls and many diploid nuclei (a coenocyte). • Moves by cytoplasmic streaming and engulfs food particles by endocytosis. © Oxford University Press Concept 19.2 Major Lineages of Eukaryotes Diversified in the Precambrian If conditions become unfavorable, the plasmodium can: • Form a hardened mass, which can re-grow into a plasmodium • Transform into spore-bearing fruiting structures; haploid spores form by meiosis; they germinate to form swarm cells Swarm cells can live independently, form resting cysts, or fuse to form a diploid zygote, which divides to become a new plasmodium. © Oxford University Press Concept 19.2 Major Lineages of Eukaryotes Diversified in the Precambrian Cellular slime molds: amoeboid cells (myxamoebas) are the vegetative state. Myxamoebas have one haploid nuclei, engulf food particles by endocytosis, and reproduce by mitosis and binary fission. If conditions become unfavorable, they aggregate into a slug or pseudoplasmodium, which can migrate, then form a fruiting structure. Spores germinate to form new myxamoebas. © Oxford University Press Figure 19.19 A Cellular Slime Mold Concept 19.3 Protists Reproduce Sexually and Asexually Asexual reproduction among the protists: • Equal splitting of one cell into two by mitosis followed by cytokinesis • Splitting of one cell into multiple cells • Budding—outgrowth of a new cell from the surface of an old one • Sporulation—formation of specialized cells that can develop into new individuals © Oxford University Press Concept 19.3 Protists Reproduce Sexually and Asexually Offspring from asexual reproduction are genetically identical to their parents. Such asexually reproduced groups are known as clonal lineages. © Oxford University Press Concept 19.3 Protists Reproduce Sexually and Asexually Reproduction in Paramecium: • Two types of nuclei—one macronucleus and one to several micronuclei • Asexual reproduction—all nuclei are copied before the cell divides • Conjugation—two individuals fuse and exchange micronuclei; a sexual process, but not reproductive. © Oxford University Press Figure 19.20 Conjugation in Paramecia Concept 19.3 Protists Reproduce Sexually and Asexually Alternation of generations: • A multicellular, diploid, spore-producing organism gives rise to a multicellular, haploid, gameteproducing organism. • The haploid organism, the diploid organism, or both may also reproduce asexually. • Occurs in many multicellular protists, all land plants, and some fungi. © Oxford University Press Concept 19.3 Protists Reproduce Sexually and Asexually Heteromorphic—the two generations differ morphologically Isomorphic—the two generations are similar Both types occur among the brown algae. © Oxford University Press Concept 19.3 Protists Reproduce Sexually and Asexually Gametes are not produced by meiosis. Specialized cells of the diploid spore-producing organism, called sporocytes, divide meiotically to produce four haploid spores. Spores divide mitotically to produce the multicellular haploid generation, which then produces gametes by mitosis and cytokinesis. © Oxford University Press © Oxford University Press Concept 19.4 Protists Are Critical Components of Many Ecosystems Phytoplankton are important primary producers in aquatic ecosystems. The diatoms perform about 1/5 of all carbon fixation on Earth—about the same amount as the rainforests. Phytoplankton includes many other protists that also contribute to global photosynthesis. © Oxford University Press Concept 19.4 Protists Are Critical Components of Many Ecosystems Some microbial eukaryotes are pathogens. Plasmodium (apicomplexans) are parasites in human red blood cells and cause malaria, one of the world’s most serious diseases. Plasmodium has a complex life cycle that includes Anopheles mosquitoes as an alternate host. Plasmodium is an extracellular parasite in the mosquito and an intracellular parasite in the human host. © Oxford University Press Figure 19.21 Life Cycle of the Malarial Parasite Concept 19.4 Protists Are Critical Components of Many Ecosystems Some species of diatoms and dinoflagellates can reproduce in enormous numbers when conditions are favorable. The result is called a red tide, and toxins produced by these organisms can kill or harm vertebrates, including fish and humans. © Oxford University Press Concept 19.4 Protists Are Critical Components of Many Ecosystems Many microbial eukaryotes live as endosymbionts in animal cells and radiolarians. Some photosynthetic dinoflagellates are endosymbionts in corals. If the dinoflagellates die or are expelled (e.g., by rising temperature) the coral is said to be bleached. If the corals do not acquire new endosymbionts, they usually die or are damaged due to reduced food supply. © Oxford University Press
