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Lecture 8

BIO153 Lecture 8.pdf

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Christoph Richter

2009 BIO153: Lecture 8 The Origin of the Eukaryotes February 4, 2008 The eukaryotic cell is extremely complex relative to the prokaryotic cell: ▯ membrane bound organelles ▯ endoplasmic reticulum ▯ cytoskeleton ▯ often does not have a cell wall ▯ nucleus; DNA in ▯ chromosomes ▯ complex flagellum ▯ sex common How did eukaryotes arise? ▯ from prokaryotic cells ▯ loss of cell wall; infolding of cell membrane to increase SA ▯ vesicles; some studded with ribosomes ▯ infolding enclosed a nucleus; cytoskeleton developed ▯ some acquired a flagellum for locomotion The loss of the prokaryotic cell wall & infolding of cell membrane (enclosing nucleus ER, Golgi apparatus, lysosomes) was a critical step ▯ the infolding of the cell membrane creates a large area for biochemical reactions to occur The evolution of mitochondria: likely occurred through the endosymbiosis of a bacterium performing respiration by an anaerobic eukaryote The advantage: a higher yield of ATP (respiration yields more ATP than fermentation, the process occurring in anaerobic organisms) The evolution of chloroplasts likely occurred in the same fashion: ▯ engulfed cyanobacteria that were capable of oxygenic photosynthesis ▯ evidence: the lamellar structure (thylakoids) that is found in 1 ▯ free-living cyanobacteria is similar to that found in chloroplasts Evidence for the endosymbiosis hypothesis: It is thought then, that chloroplasts and mitochondria were once free-living bacteria engulfed by early eukaryotes: 1. mitochondria & chloroplasts are about the right size to be bacterial cells 2. both mitochondria and chloroplasts replicate by fission 3. mitochondria and chloroplasts have their own genomes (circular chromosome), ribosomes; and make their own proteins 4. mitochondria and chloroplasts have double membranes 5. mitochondria and chloroplasts are affected by antibiotics (which don’t affect eukaryotic cells) 2° endosymbiosis: In some lineages, a protist engulfed another protist! evidence: ▯ in some organisms, the chloroplasts have 4 membranes ▯ in some, 3 membranes (1 lost) The first eukaryotes were anaerobes Although the vast majority of contemporary eukaryotes are aerobes, it makes sense that the earliest eukaryotes were anaerobes until they acquired mitochondria. A few anaerobic eukaryotes exist today: ▯ contemporary anaerobic eukaryotes that are well known are mostly pathogens (e.g. Giardia, an intestinal parasite; Trichomonas, a pathogen that causes reproductive tract infections) ▯ contemporary anaerobic eukaryotes lack plastids; mitochondria* *however, there is some evidence that in Giardia, the mitochondrion was present and has been lost, considerable complicating the story! 2 The protists proto = first Protists are all eukaryotes that are not animals, plants or fungi; thus it is a group defined by exclusion ▯ there are no shared, derived characters unique to protists (protists are paraphyletic) ▯ all live in moist or aquatic environments (ancestral condition!) 1) Protists are functionally diverse: ▯ sessile or motile ▯ autotrophic or heterotrophic ▯ unicellular, colonial, multicellular ▯ parasitic or free-living ▯ sexual or asexual…… 2) Protists show tremendous diversity in size: Among the unicellular protists: ▯ the smallest is Micromonas pusilla (1-3 m -- really tiny! The size of some bacteria!) ▯ some large ones: Pelomyxa palustris (3mm); Acetabularia (mermaid’s wineglass): green alga; single cell 0.5 – 10 cm!! 3 9 ▯ Pelomyxa is 1,000 X as long; (1,000) = 10 as heavy as Micromonas: this is 10x greater than the weight difference between a blue whale and a shrew: (whale (190 tonnes) is only 10 X as heavy as a 2 g shrew!) Multicellular protists: some brown algae can be over 100m long! 3) Protists are innovators in reproduction: evolution of sex ▯ among the protists, we start to see the production of genetically unique offspring through sexual recombination ▯ there are many types of life cycle in protists 4) Protists are innovators in terms of morphology: evolution of multicellularity: ▯ true multicellularity: differentiation of function; complex cell-cell signaling 3 ▯ true multicellularity evolved several times in many lineages Systematics and taxonomy of protists: is a great big mess!! There are at least 60 – 70 different lineages distinct enough to be given Kingdom status. Current phylogenies are usually based on genes for ribosomal RNA (rRNA), because these genes are found in all organisms, they are good for determining some of the major branching patterns on the tree of life. Why is it so difficult to figure out the evolutionary relationships among the protists? Keep in mind that the vast majority are poorly studied (really, we only know much about protists that are medically important or economically important, and those that are easy to study in the lab.) This creates a huge sampling bias! It’s a bit like trying to understand all of anthropology by studying a few people from Red Deer, Alberta; another few from a village in Sweden; another few from Mumbai… fine places, all, but not representative of all of humanity! The major well- studied groups: (numbers represent several presumed adaptive radiations) 1.
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