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The Protists and the Origins of Eukaryotes
Chapter 20
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Characteristics of Protists
Eukaryotes Primarily unicellular (some colonial and multicellular exist) Metabolically diverse Structurally complex Asexual reproduction usual; sexual reproduction diverse Basically, catch-all kingdom!! Only real characteristics in common are: Prefer watery environments
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Protist Classification
Phylogeny not well-established Protists do not represent a monophyletic group Some are more closely related to animals than to other protists For convenience, protists grouped by: Mode of nutrition (plant-like, animal-like, fungal-like) Movement Protists are essentially the earliest eukaryotes and 1st steps towards true multicellular organisms
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The Importance of Protists
Origins of eukaryotic cell Origins of multicellularity Origins of sexual reproduction All of these advances are represented in various protists
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Figure 27.1 Major Eukaryote Groups in an Evolutionary Context (Part 1)
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Figure 27.1 Major Eukaryote Groups in an Evolutionary Context (Part 2)
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Origins of Eukaryotic Cell
Modern eukaryotic cell arose in several steps Flexible cell surface Cytoskeleton Nuclear envelope Digestive vesicles Endosymbiotic acquisition of certain Extends from PM to nucleus Probably helped form nucleus Phagocytosis --> formation of organelles Locomotion Increase in cell size Mitosis: allows for eventual meiotic division needed for sexual reproduction Flexible cell surface: can fold inward to increase surface area Probably lead to endocytosis methods and vesicle formation
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Evidence: Giardia Have nucleus Lack mitochondria
No other membrane-enclosed organelles Has cytoskeleton
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Endosymbiotic Hypothesis of Mitochondria and Chloroplasts
Eukaryotes display presence of bacterial genes encoding for energy metabolism Aerobic bacteria gave rise to mitochondrion Cyanobacterium gave rise to chloroplasts of green algae, plants, and red algae
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History of Endosymbiosis
Mitochondria derived from proteobacterium capable of aerobic metabolism Chloroplasts appear in several distantly related protist clades Photosynthetic pigments differ Not all chloroplasts have a pair of membranes Some have three Primary endosymbiosis All chloroplasts trace their ancestry back to engulfment of a cyanobacterium Chlorophyll a present in all!! One membrane from cyanobacterium, second from host Gave rise to chloroplasts of green and red algae Secondary and tertiary endosymbiosis All other photosynthetic protist lineages
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Eukaryotes Acquired Features From Both Archaea and Bacteria
Why aren’t the three rRNA genes of corn one another’s closest relatives? How would you explain the closer relationship of the mito rRNA gene of corn to the rRNA gene of E.coli than to the nuclear rRNA genes of other eukaryotes? Can you explain the relationship of the rRNA gene from the chloroplast of corn to the rRNA gene of the cyanobacterium? If you were to sequence the rRNA genes from human and yeast mito genomes, where would you expect these two sequences to fit on the gene tree?
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Origins of Multicellularity
Major multicellular lineages in eukaryotes: brown algae, plants, animals, fungi, red algae Some protists form colonies or are primitive multicellular Examples: Volvox, colonial; some algae, multicellular True multicellular organisms - there is a division of labor among different kinds of cells and non are independent Multicellularity Increase in size advantageous Less predation Lower metabolic rate --> less food requirement
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Origins of Multicellularity
What is required for multicelluarity? Cohesion Animals = extra cellular matrix and collagen Plants = plasmodesmata and cell walls Communication between cells to allow for cooperation Nervous system and endocrine system in animals Signal transduction and hormones in plants Developmental plan Regulation of gene expression to guide specialization of cells Cells must specialize!! See distinctive organelles become more or less prevalent in different cell types Can’t just increase size b/c SA/V ratio
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Origins of Multicellularity
Cells must specialize!! See distinctive organelles become more or less prevalent in different cell types Can’t just increase size b/c SA/V ratio Flexible membrane surface Change shape Activity near surface Vacuoles for food storage/waster removal Food vacuoles Contractile vacuoles (help eliminate excess water) Multicellularity Eventually increase cell number
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Origins of Sexual Reproduction
Sexual reproduction allows for adaptive radiation and lost of new species Genetic recombination and exchange Protists exhibit both asexual and sexual reproduction Asexual: binary fission, budding, spores Sexual: Conjugation Haploid life cycle, alternation of generation life cycle Gametes haploid Both diploid and haploid cells undergo mitosis Diploid life cycle Only diploid cells undergo mitosis Eukaryotic reproduction requires existence of mitosis and meiosis!!!! Both processes dependent on linear chromosomes and spindle fibers (microtubules of cytoskeleton)
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Figure 27.13 Alternation of Generations
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Life Cycles Isogamy vs oogamy Sporophyte vs gametophyte
Plants Animals Isogamy vs oogamy Gametes look alike; gametes from each gender distinct Sporophyte vs gametophyte Gives rise to spores; gives rise to gametes Isomorphic vs heteromorphic Haploid and diploid individuals distinct; haploid and diploid individuals appear alike
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Haploid Life Cycle: Chlamydomonas
Individual is haploid Gametes form by mitosis Gametes fuse to form zygote Zygote undergoes meiosis to form spores that grow by mitosis to form new individual NO DIPLOID INDIVIDUAL!!!!
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Figure 27.15 A Haplontic Life Cycle
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Alternation of generations: Ulva and Laminaria
Ulva is green algae Isogamic Phenotypically indistinguishable gametes Isomorphic Both generations phenotypically similar but differ in ploidy Alternation of generations Laminaria: brown algae Oogamy Fertilization of egg by sperm (gametes phenotypically distinguished, generally egg predominant) Heteromorphic Generations phenotypically distinct Plants Oogamic (most)
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Figure 27.14 An Isomorphic Life Cycle
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Diploid Life Cycle: Fucus
Fucus: Brown Algae All animals Only individual is diploid Gametes by meiosis
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Survey of the Protists Cell wall or test Absence of cell wall
Contractile vacuoles for maintaining osmotic pressure Type of nutrition Photoautotroph Heterotroph by ingestion Heterotroph by absorption Locomotion Non-motile Flagella Cilia pseudopodia Note - this arrangement does not follow phylogeny - grouped in many textbooks by convenience!
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Animal-like Protists In general Protists with Pseudopodia
Motile Unicellular or colonial Wall-less Heterotrophic Protists with Pseudopodia Rhizopodia (amoebas) Pseudopodia Entamoeba histolytica - amoebic dysentery Foraminifera (forams) Tests made of CaCO3 Long-hairlike pseudopodia that poke out thru holes of test Actinopoda (radiolarians) Silicon test
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Animal-like Protists Ciliophora (cilliates) Complex organization
Asexual reproduction; sexual by conjugation involving micronuclei
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Animal-like Protists Zoomastigophora (zooflagellates)
Generally symbionts or parasites Giardia Trypanosoma Chagas African sleeping sickness Choanoflagellates Closest relative to Animals!!!!!! (rRNA analysis)
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Animal-like Protists Apicomplexa (sporozoans)
Nonmotile, parasitic, spore-forming Complicated life-cycles Toxoplasma Plasmodium: malaria
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Figure 27.27 A Link to the Animals
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Figure 27.1 Major Eukaryote Groups in an Evolutionary Context (Part 1)
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Plant-like Protists Pyrrophyta (dinoflagellates)
2 flagella; one wraps around middle of cell Cell protected by celluose/silica plates Chlorophylls a and c, carotenoids Red-tide Chrysophyta (golden-brown algae; diatoms) Diatoms formally called Bacillariophyta Diatoms have cell wall of silica; major component of phytoplankton Chlorophyll a and c, fucoxathin Euglenophyta (eugleniods) 1/3 have chloroplasts, rest do not Chloroplasts like those of green algae Chlorophyll a, b and carotenoid 2 flagella No cell wall Eyespot to detect light
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Plant-Like Protists Chlorophyta (green algae) Rhodophyta (red algae)
Closest relatives to plants Chlorophyll a, b, carotenoids Store food as starch Walls of cellulose Lichens: green algae + fungi Rhodophyta (red algae) Unicellular to multicellular Chlorophyll a, phycobilins Food stored as floridian starch Phaeophyta (brown algae) All multicellular Chlorophylls a and c, fucoxanthin Store good as laminarin
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Figure Chlorophytes
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Figure Red Algae
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Figure Brown Algae
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Importance of Protists
Link to eukaryotic origins Impact on Human Health Plasmodium (malaria) Trypanosoma (African sleeping sickness; Chagas) Ecological Importance as Primary Producers Dinoflagellates Marine phytoplankton Endosymbiotic with corals Ride tides and algal blooms Diatoms Common in fresh water Diatomaceous earth Chlorophytes (green algae) Links to origins of animal kingdom - choanoflagellates
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Figure 27.8 Dinoflagellate Endosymbionts are Photosynthesizers
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Figure 27.10 Chromalveolates Can Bloom in the Oceans
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Figure 27.19 Diatom Diversity
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