PROTISTA The paraphyletic, nonfungi, non-animal, nonplant Eucarya + Even MORE new words to remember! Key Points Origin of eukaryotes via symbiosis Origin of classification based on functional (ecological) traits Current classification based on phylogenetic principles Alternation of generations prominent General 1. Eukaryotes are mostly unicellular. 2. Mixed history of classification: Protista an informal term of convenience for non-fungal, non-plant, non-animal eukaryotes. 3. From amoeba to giant kelp, arranged functionally. Functional arrangements a. Animal-like heterotrophic protists: Protozoa. b. Absorptive or fungus-like protists: Pseudopodians. c. Plant-like photosynthetic protists: Algae d. Mixotrophs All of these groups are polyphyletic 1
1. the earliest Eukaryotes a. True nucleus b. Cytoplasmic organelles c. ~2.2 bya 2. always associated with water, dampness, or body fluids a. Plankton, parasitic Protists are 3. aerobic (almost all) and have mitochondria for cellular respiration. 4. photoautotrophs, chemoheterotrophs, or both (mixotrophs). a. Note NO photoheterotrophs nor chemoautotrophs. Protists are Protists are 5. motile: most have flagella or cilia or pseudopodia at some stage. 6. asexual or truly sexual (true meiosis and mitosis) The Origin of Eukaryotes How to make a Eukaryote About 2.5 bya prokaryotes had diversified into many types. But the small size and limited genome of prokaryotes constrained their evolution. So how did Eukaryotes become possible? flagella 2
The Origin of Eukaryotes How to make a Eukaryote Eukaryotic cell: true nucleus, cytoplasmic organelles Membrane-enclosed structures with specialized function Some with own genome (mitochondria, chloroplasts) This compartmentalization allowed the evolution of larger cells. But how? Endosymbiosis A sequence of events in which specialized prokaryotes live within larger prokaryotes in symbiotic relationship. Some became mitochondria and some chloroplasts. Both were important in an increasingly aerobic world. Aerobic α-proteobacterium è mitochondrion Eukaryotic chemoheterotroph Ancestral Archaean & evolution of nucleus Cyanobacterium è chloroplast Eukaryotic photoautotroph Endosymbiosis Model supported by similarity in structure and RNA between certain prokaryotes and corresponding eukaryote organelles. By alternative genetic code (DNA sequence translation to amino acids). Mitochondria: α- proteobacteria are relatives. Plastids (chloroplast and some non-photosynthetic): cyanobacteria are relatives. Aerobic α-proteobacterium è mitochondrion Eukaryotic chemoheterotroph Ancestral Archaean & evolution of nucleus Cyanobacterium è chloroplast Eukaryotic photoautotroph Protist Diversity Note that the vast majority of eukaryotic diversity is protistan and unicellular. Is Protista monophyletic, paraphyletic, or polyphyletic? How does this phylogeny indicate that the difference between paraphyletic and polyphyletic is fuzzy? 3
Protist Diversity Protozoans Excavata I. Alveolata II. Opisthokonts (not covered now) Algal Protists IV. Stramenopiles V. Archaeplastids Pseudopodians VI. Rhizarians VII. Amoebozoans Protist Diversity Protozoans Excavata I. Alveolata II. Opisthokonts (not covered now) Algal Protists IV. Stramenopiles V. Archaeplastids Pseudopodians VI. Rhizarians VII. Amoebozoans Protist Diversity Protozoans Excavata I. Alveolata II. Opisthokonts (not covered now) Algal Protists IV. Stramenopiles V. Archaeplastids Pseudopodians VI. Rhizarians VII. Amoebozoans Protist Diversity Protozoans Excavata I. Alveolata II. Opisthokonts (not covered now) Algal Protists IV. Stramenopiles V. Archaeplastids Pseudopodians VI. Rhizarians VII. Amoebozoans 4
Diplomonads Parabasalids Euglenozoans Excavata Often anaerobic What eukaryotic feature could be modified? Excavata: Parasites Giardia can be a severe intestinal parasite Excavata: Parasites Diplomonads: Small, simple mitochondria (mitosomes) Not involved in cellular respiration Involved in maturation of iron-sulfur proteins Two separate nuclei Function unclear, NOT duplicated genomes Giardia can be a severe intestinal parasite Excavata: Parasites Parabasilids with reduced mitochondria: hydrogenosomes Responsible for some anaerobic metabolism Anaerobic, flagellated protozoa Include Trichomonas vaginalis, the most common protistan STD 5
Excavata: Euglenozoans All characterized by spiral, crystalline rod inside flagella. Kinetoplastids: Heterotrophs including Trypanosoma Kinetoplastid: organelle housing extraneous DNA Euglenids: Often mixotrophs Photosynthesize in light Heterotrophic via phagocytosis in dark What is its sister-clade? Members of the Chromalveolata probably can photosynthesize because of the secondary endosymbiosis of a red alga. Characterized by small, membrane-bound cavities, alveoli Alveolata Likely originated by secondary endosymbiosis Figure 28.2 Plastid Dinoflagellates Alveolata: Dinoflagellates Membranes are represented as dark lines in the cell. Cyanobacterium 1 2 3 Primary endosymbiosis Heterotrophic eukaryote One of these membranes was lost in red and green algal descendants. Red alga Green alga Secondary endosymbiosis Secondary endosymbiosis Secondary endosymbiosis Apicomplexans Stramenopiles Plastid Euglenids Chlorarachniophytes Marine & freshwater photosynthetic (~50%) phytoplankton Some predators Some parasitic on fish Most unicellular Cellulose armor and paired flagella produce spinning movement. Explosive population blooms result in red tides. 6
Alveolata: Dinoflagellates Zooxanthellae: Important mutualists with corals (also jellyfish, clams, sea slugs, and other protists). Obligate mutualism for many coral: provide carbohydrates via photosynthesis, get protection. Coral bleaching: Death or expulsion of zooxanthellae leads to death of corals. Alveolata: Apicomplexans Parasites of animals Release tiny infectious cells (sporozoites) that have specialized ability to penetrate into host cells and tissues. Complex life history Sexual & Asexual reproduction Often multiple hosts E.g. Plasmodium, mosquitoes, and humans Alveolata: Ciliates Move and feed by cilia. Very diverse group with complex cells. Manage to be aggressive predators and unicellular Two types of nuclei: macronucleus and micronuclei (convert back and forth). Asexual reproduction via mitosis and cytokinesis. Sexual reproduction via meiosis and conjugation Algal Protists Single-celled, colonial, or truly multicellular ( seaweeds ) Freshwater or marine Important in aquatic food webs All have chlorophyll a (the primary pigment) Accessory pigments: Carotenoids: yellow-orange Xanthophylls: brownish Phycobilin: red and blue Account for 1/2 of global photosynthetic production Various life cycles, but alternation of generations is key 7
Sister-clade to Alveolates Hair-like projections on flagellae Photoautotrophs Chloroplasts derived from eukaryotic symbiont (recall 2 endosymbiosis) Oomycetes have lost chloroplasts and are heterotrophic Stramenopila Stramenopila: Diatoms (Bacillariophyta) Olive-brown or yellow What pigments are responsible for these? Stramenopila: Diatoms (Bacillariophyta) Olive-brown or yellow Xanthophylls & carotenoids Freshwater & marine Distinctive cell structure based on silica wall matrix. Excellent index fossils. Form massive sediments. Stramenopila: Brown algae (Phaeophyta) Carotenoids; xanthophylls Why do so many marine photosynthesizers use auxiliary pigments? Marine, multicellular. Common in cool coastal water. Some giant (100m) have fastest linear growth of any organism (60m/ season); e.g. Macrocystis, giant bladder kelp. 8
Stramenopila: Brown algae (Phaeophyta) Truly multicellular thallus, independently derived separate tissue specialization: Holdfast: rootlike anchor Stipe: stemlike structure Blades: leaflike structure where majority of photosynthesis occurs True alternation of generations Alternation of Generations Life cycles in which both haploid and diploid stages are multicellular. Also evolved independently in plants and fungi. Divided into haploid gametophyte generation and diploid sporophyte generation Stramenopila: Oomycetes Water molds, white rusts, mildews Heterotrophs, lack chloroplasts Important in organic decomposition in aquatic environments Some (especially mildews) harmful plant pathogens. Potato blight Phytophthora infestans Archaeplastids (the non-plant ones) Rhodophyta (Red Algae) Chlorophyta (Green Algae) 9
Archaeplastids: Red Algae (Rhodophyta) (not all red, red to black). Multicellular, most marine (some fresh). Abundant in warm coastal tropics. Some in very deep water (ca 250m). No flagellated stages in life cycle. Chloroplasts from primary cyanobacteria symbiont. Archaeplastids: Green Algae (Chlorophyta) 7,000 species, most diverse Protista after diatoms. Shared common ancestry with plants. Like red algae, chloroplasts from primary cyanobacteria symbiont. Synapomorphy of Archaeplastids Mostly unicellular Mostly fresh water Chlamydomonas, a single-cellular freshwater green alga Green algae life histories But have quite a diversity in life history Can inhabit damp soils. Can live symbiotically with protozoa, invertebrates, fungi Note that lichens can also be associations between fungi and cyanobacteria or brown algae or yellowgreen algae (a stramenopile we didn t cover) Green algae life histories Larger size and greater complexity evolved by three different mechanisms: 1. Colony formation (e.g. Volvox in pond scum) 2. True multicellularity, complete with alternation of generations (e.g. the sea lettuce, Ulva) 3. Supercellularity: repeated division of nuclei with no cytoplasmic division, similar to fungal hyphae or slime molds (e.g. in Caulerpa) 10
Pseudopodians Eukaryotes with Pseudopodia that move and feed by cellular extensions. Pseudopodia is a generic term for extensions that can bulge from any portion of the cell. Much like wing, this does not indicate homology. Rhizarians Originally united by DNA sequence data. Pseudopodia are threadlike. Rhizaria: Foraminiferans Forams All marine, primarily in sand, attached to algae, or occur as plankton. Encased in multichambered, coiled, snail-like shells (tests) made of Calcium Carbonate (CaCO 3 ) More than 90% of known diversity are from fossils Deposition of CaCO 3 tests creates limestones and chalks. Cytoplasm can be uninucleate or multinucleate and extends through tests as pseudopodia. Some tests >5cm in diameter. Rhizaria: Radiolaria ray feet or axopodia: numerous slender pseudopodia reinforced by microtubules. Used for flotation and feeding: food sticks to axopods, engulfed and transported by cytoplasm Important component of plankton: Heliozoans in freshwater Radiolarians in marine Shells of silica often deposited in sediments 11
Amoebozoans root-like foot Pseudopodia are lobe- or tube-shaped Simple, naked, or shelled Unflagellated cells that move via pseudopodia and feed by surrounding and engulfing food (phagocytosis) Sister group of lineages including fungi and animals Amoebozoans : Gymnamoebas & Entamoebas More typical amoebas. Gymnamoeba: Free-living heterotrophs on bacteria or detritus. Entamoebas: All parasitic; No meiosis, reproduce using various asexual modes Include Entamoeba histolytica, responsible for amoebic dysentery Amoebozoans: Slime Molds (Mycetozoans) Superficially resemble fungi In cellular organization and reproduction are obviously amoebozoans. Plasmodial slime molds (Myxomycota) All heterotrophs, often brightly colored. Feeding stage is a large amoeboid mass called the plasmodium (!). Not multicellular, but multinucleated. Via the process of coenocytosis: repeated division of nuclei without cytoplasmic division. 12
Plasmodial slime molds: Alternation of generations Environmental stress Aggregates of individual cells that keep their identity while feeding. Haploid. NOT coenocytic. Reproduce asexually with fruiting bodies. Reproduce sexually as giant cell (grows via consuming other haploid amoebas). Probable inspiration for scene from Terminator 3 Cellular slime molds (Acrasiomycota) Comparative Biology & Cellular Slime molds Researchers at UCSD studying Dictyostelium, a cellular slime mold, found that two genes used to guide the amoeba to food sources are also used used to guide human white blood cells to the sites of infections. 13