27. The Origin and Diversification of the Eukaryotes

The Origin and Diversification of the Eukaryotes27 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 Diatom27.1 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 Relationship27.1 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 Parasite27.1 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 Oceans27.1 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 Blocks27.2 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 surfaceOrigin of a cytoskeletonOrigin of a nuclear envelopeAppearance of digestive vesicles or vacuolesEndosymbiotic acquisition of some organelles27.2 How Did the Eukaryotic Cell Arise?Flexible cell surface: Prokaryotic cell wall was lost; cells can grow larger.As cell size increases, surface area-to-volume 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 Infolding27.2 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 Cell27.2 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 gram-negative 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 Chloroplasts27.2 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 gram-negative 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?LocomotionAmoeboid motion—cells form pseudopods that are extensions of the cell. A network of cytoskeletal microfilaments squeezes the cytoplasm forward.Figure 27.9 An Amoeba27.3 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 Bend27.3 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 Water27.3 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 surfacesMany 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 Eukaryotes27.4 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 Conjugating27.4 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 Generations27.4 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 Cycle27.4 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 Cycle27.4 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 trypanosomesAdvantages 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?ChromalveolatesIncludes 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 Dinoflagellate27.5 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 Ciliates27.5 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 Paramecium27.5 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 Diversity27.5 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 Algae27.5 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 Oomycete27.5 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 Algae27.5 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 Chlorophytes27.5 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 Mitochondria27.5 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 Euglenid27.5 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 Trypanosomes27.5 What Are the Major Groups of Eukaryotes?RhizariaUnicellular, 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 House27.5 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 Animals27.5 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 moldsDuring 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 Molds27.5 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