The Origin and Diversification of Eukaryotes

Eukaryotic Origins and the Integration of Archaea and Bacteria

  • Taxonomic Definitions and Phylogeny

    • Protists: This is a convenience term used for eukaryotes that are not plants, animals, or fungi. It is not a formal taxonomic group.

    • Monophyly: Eukaryotes form a monophyletic clade.

    • Evolutionary Relationships: Eukaryotes are more closely related to Archaea than to Bacteria.

    • Bacterial Origins: Mitochondria and chloroplasts are clearly derived from bacterial lineages.

  • Major Events in the Evolution of Eukaryotic Cells

    • Loss of the Cell Wall: The protective cell wall was lost, which was a prerequisite for other features.

    • Flexible Cell Surface:

      • Allows for cell infolding, which increases surface area-to-volume ratio.

      • Enables the cell to become larger.

      • Makes endocytosis possible, allowing cells to pinch off bits of the environment and bring them inside.

    • Origin of a Complex Cytoskeleton:

      • Evolved from simple cytoskeletons in prokaryotes.

      • Includes microfilaments and microtubules that provide structural support.

      • Allows for changes in cell shape and the movement of materials within the cell.

      • Enables the distribution of daughter chromosomes.

      • Microtubules specifically allowed for the development of the eukaryotic flagellum for propulsion.

    • Internal Membrane Systems:

      • Infolding of the cell membrane added surface area.

      • Ribosome-studded internal membranes (endoplasmic reticulum) formed.

      • Nuclear Envelope: Likely developed from DNA attached to the membrane of an infolded vesicle (in prokaryotes, DNA is attached to the inner plasma membrane).

    • Evolution of Vacuoles:

      • Digestive Vacuoles: Evolved into lysosomes using enzymes from the early endoplasmic reticulum.

      • Contractile Vacuoles: Used to excrete excess water taken in by osmosis, particularly in freshwater species like Paramecium.

  • Endosymbiosis and the Development of Organelles

    • Phagocytosis: The ability to engulf and digest other cells was a requirement for endosymbiosis.

    • Origin of Mitochondria:

      • Formed through endosymbiosis with a proteobacterium.

      • Initial Function: Might have been to detoxify O2O_2 produced by cyanobacteria by reducing it to water (H2OH_2O).

      • Later Function: Detoxification became coupled with the formation of ATPATP.

    • Origin of Chloroplasts: Developed via a series of endosymbiotic events.

      • Primary Endosymbiosis: A larger eukaryotic cell engulfed a cyanobacterium. This gave rise to chloroplasts in glaucophytes, red algae, green algae, and land plants. These chloroplasts possess two membranes. Glaucophytes are unique because they retain remnants of a peptidoglycan cell wall from the original cyanobacterium.

      • Secondary Endosymbiosis: A eukaryote engulfed a green alga cell, which then became a chloroplast. This is seen in euglenids, whose chloroplasts have $3$ membranes and the same pigments as land plants.

      • Tertiary Endosymbiosis: A dinoflagellate lost its original chloroplast and took up another protist that had already acquired its chloroplast through secondary endosymbiosis.

Major Lineages and Diversification of Eukaryotes

  • Evolutionary Timeline

    • Primary diversification of the eight major clades of eukaryotes began approximately 1.5 billion1.5 \text{ billion} years ago during the Precambrian.

  • Diversity of Form

    • Microbial Eukaryotes: Most protistan eukaryotes are unicellular and microscopic.

    • Multicellularity: Has arisen dozens of independent times in eukaryotes. Experimental studies show that artificial selection can cause normalement unicellular species to evolve multicellularity in only a few months, indicating it can evolve relatively easily.

    • Growth Forms: Range from unicellular to colonies to large multicellular organisms like giant kelp (60 meters60 \text{ meters} long).

  • The Alveolates

    • Characterized by sacs called alveoli just beneath the cell membrane; they are all unicellular and mostly photosynthetic.

    • Dinoflagellates:

      • Mostly marine primary producers.

      • Possess two grooves: an equatorial groove and a longitudinal groove, with a flagellum.

      • Some cause toxic red tides.

      • Some live as endosymbionts in invertebrates such as corals.

    • Apicomplexans:

      • Obligate parasites with an apical complex (organelles at the tip used to invade host tissues).

      • Plasmodium: The causative agent of malaria. It is an extracellular parasite in its alternate host, the Anopheles mosquito, and an intracellular parasite in human hosts.

      • Toxoplasma: Alternates between cats and rats; infected rats lose their fear of cats.

    • Ciliates:

      • Use numerous hairlike cilia for precise locomotion.

      • Examples include Paramecium, Didinium nasutum, and Trichodina.

      • Complex body includes a pellicle with defensive trichocysts (sharp darts).

      • Possess two types of nuclei: the macronucleus (controls cell activities) and micronuclei (functional in genetic recombination).

  • The Stramenopiles

    • Characterized by rows of tubular hairs on the longer of two flagella.

    • Diatoms:

      • Unicellular; yellowish/brown color from carotenoids.

      • Unique two-piece cell walls made of silica with intricate patterns.

      • Perform approximately 1/51/5 of all global carbon fixation.

      • Lack flagella except in male gametes.

    • Brown Algae:

      • All are multicellular and marine.

      • Brown color comes from the carotenoid fucoxanthin.

      • Use holdfasts with alginic acid (an emulsifier used in ice cream and cosmetics) to anchor to rocks.

    • Oomycetes (Water Molds and Downy Mildews):

      • Absorptive heterotrophs that secrete enzymes to digest food.

      • Saprobic (feed on dead organic matter).

  • The Rhizaria

    • Unicellular, mostly aquatic, and characterized by long, thin pseudopods.

    • Cercozoans: Soil and aquatic; one group has chloroplasts from secondary endosymbiosis containing a trace of the alga's nucleus.

    • Foraminiferans: Secrete external shells of calcium carbonate. Their branched pseudopods form a sticky net to catch plankton. Fossilized shells are used in stratigraphy and to estimate historical temperatures.

    • Radiolarians: Exhibit radial symmetry and possess thin, stiff pseudopods reinforced by microtubules. They secrete glassy endoskeletons.

  • The Excavates

    • Diplomonads and Parabasalids: Unicellular and lack mitochondria (a derived condition). Giardia lamblia (diplomonad) causes giardiasis; Trichomonas vaginalis (parabasalid) causes trichomoniasis.

    • Heteroloboseans: Have amoeboid body forms. Naegleria can cause fatal nervous system diseases.

    • Euglenids and Kinetoplastids: Feature mitochondria with disc-shaped cristae and flagella with crystalline rods.

      • Euglenids: Some are photosynthetic with a pigment shield and photoreceptor; others are heterotrophic.

      • Kinetoplastids: Parasites like Trypanosomes which have a kinetoplast containing multiple circular DNA molecules and can frequently change surface molecules to evade the immune system.

  • The Amoebozoans

    • Characterized by lobe-shaped pseudopods.

    • Loboseans: Feed by phagocytosis. Testate amoebas live in shells made of sand grains.

    • Plasmodial Slime Molds: Vegetative state is a plasmodium, a coenocyte (mass of cytoplasm with many diploid nuclei). They use cytoplasmic streaming for movement.

    • Cellular Slime Molds: Vegetative state consists of individual haploid amoeboid cells called myxamoebas. Under poor conditions, they aggregate into a slug or pseudoplasmodium to form a fruiting structure.

Protist Reproduction and Life Cycles

  • Asexual Reproduction

    • Mitosis and Cytokinesis: Equal splitting of one cell into two.

    • Multiple Fission: Splitting into more than two cells.

    • Budding: Outgrowth from the parent cell surface.

    • Sporulation: Formation of spores that develop into new individuals.

    • Clonal Lineages: Offspring are genetically identical to the parent.

  • Sexual Processes and Alternation of Generations

    • Conjugation in Paramecium: Two individuals fuse and exchange micronuclei. This is a sexual process but not reproductive because no new individuals are produced during the event.

    • Alternation of Generations:

      • A life cycle featuring a multicellular diploid spore-producing organism (sporophyte) and a multicellular haploid gamete-producing organism (gametophyte).

      • Sporocytes: Specialized diploid cells that divide meiotically to produce four haploid spores.

      • Gamete Production: In this cycle, gametes are produced by mitosis, not meiosis.

      • Isomorphic: The two generations look similar.

      • Heteromorphic: The two generations differ morphologically.

Ecological and Economic Importance of Protists

  • Primary Production

    • Phytoplankton are essential primary producers. Diatoms alone perform carbon fixation comparable to that of terrestrial rainforests.

  • Symbiosis and Disease

    • Coral Bleaching: Occurs when dinoflagellate endosymbionts die or are expelled due to environmental stressors like rising temperatures.

    • Malaria Pathogenesis: The Plasmodium life cycle involves:

      1. Mosquito ingests gametocytes.

      2. Gametes fuse into a zygote in the gut.

      3. Zygote forms a cyst, producing sporozoites.

      4. Sporozoites invade the mosquito's salivary gland and are injected into a human.

      5. Sporozoites infect liver cells and develop into merozoites.

      6. Merozoites invade and lyse red blood cells.

  • Geological and Industrial Applications

    • Petroleum and Gas: Formed from ancient diatoms sinking to the ocean floor.

    • Diatomaceous Earth: Sedimentary rock of silica cell walls used for insulation, filtration, and as an "Earth-friendly" insecticide that clogs insect breathing tubes.

    • Limestone: Formed from the accumulation of foraminiferan shells.

Questions and Discussion

  • Q: The many organisms designated as protists are…

    • A: Highly diverse and not all closely related to one another.

  • Q: What feature of eukaryotes makes endosymbiosis possible?

    • A: A flexible cell membrane.

  • Q: The increasing concentration of atmospheric oxygen in Earth’s early atmosphere…

    • A: Conferred a selective advantage to a eukaryote that possessed a mitochondrion.

  • Q: After millions of generations, the endosymbiotic events led to…

    • A: The chloroplast retaining prokaryotic DNA that codes for proteins in the chloroplast.

  • Q: Which statement most strongly supports the assertion that multicellularity can evolve relatively easily?

    • A: Experimental studies showing multicellularity can be artificially selected for in normally unicellular species in a matter of months.

  • Q: Characteristics of Plasmodium (mass of organelles at the tip) suggest it belongs to…

    • A: Apicomplexans.

  • Q: Symptoms of diarrhea and bloating after drinking stream water, with a find of unicellular organisms lacking mitochondria with multiple flagella, suggests infection by…

    • A: The diplomonad Giardia.

  • Q: Why is conjugation in Paramecium called "sex without reproduction"?

    • A: DNA is exchanged, but no new individuals are produced.

  • Q: Red tides that kill fish are caused by toxic species of…

    • A: Dinoflagellates.

  • Q: Dinoflagellates in corals are useful because they…

    • A: Photosynthesize.

  • Q: Which spiny remains are used in insecticides?

    • A: Diatoms.