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Lecture 98

BIOL 3600 Lecture 98: Origin of Complex Life

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BIOL 3600

Origin of Complex Life: Eukaryogenesis 20 April 2017 G. Ozan Bozdag • Why are we focusing on the evolution of complex cell? o Microbial life starts 3.7 bya, with bacteria and archaea o Can evolve to become multicellular but not complex • Eukaryotic cell(s): Cell within a cell o Mitochondria can reproduce independent of the cell ▪ They have their own genomes o Eukaryotic cell is very different from prokaryotic cell • Evolution of eukaryotes: Not a regular evolutionary change • Cells within cells: Modern examples of phagocytosis o Endosymbionts inside larger cells which can reproduce • Endosymbiotic “Hypothesis” Theory o 1905: Mereschkowski thought larger cells engulf smaller cells o 1967: Margulis reintroduced Mereschkowski’s idea after people forgot about it; extrapolated to animal cells, not just plant cells • Eukaryogenesis—A major transition in evolution o Not a simple branching into 2 from 1 lineage o “1+1=1” o Over 15 hypothesis for the origin of the host o 17 different hypothesis for origin of the endosymbiont • Part 1: Origin of the mitochondria o Mitochondria form a clade o They share similar genomes with alpha-proteobacteria o The endosymbiont was an alpha-proteobacteria o Genome reduction in organelles ▪ When alpha-proteobacteria entered cell, caused huge genome reduction o Genome reduction in mitochondria ▪ Protist mtDNA share some genes with yeast mtDNA, but they share with alpha-proteobacteria o How does the transfer happen: RNA-intermediate or bulk- transfers? o Origin of the mitochondrion (the organelle): Not that simple! ▪ Not all of the 1,100 proteins come from alpha- proteobacteria to mitochondria o “” Ongoing debate ▪ Alpha-proteobacteria form with premitochondrion to form mitochondria • Part II: Origin of the host o Nuclear genome is more identical to the archaeal lineages o Other than self-replicating organelles: Intracellular membranes/trafficking ▪ Carries nutrients to cell; waste away from cell ▪ Composed of different extensions of membranes, different proteins ▪ Not found in bacteria o 2 competing hypotheses 2 ▪ Archezoa Hypothesis: Mitochondria late • The origin of the host (rooted RNA phylogeny): More sophisticate archaea-like ancestor? • Simple cells start to evolve a complex, genetic structure • Some features derived earlier than mitochondria etc. • Archaea feeds on bacteria  evolves to become complex life  forms nuclear envelope that surrounds its genome  evolving cytoskeleton  all this to eat bacterial food (alpha-proteobacteria)  this interaction of eating causes cell to enter that is not digested and starts to reproduce within complex archaea  its genome is transferred to host (nuclear DNA)  protein transport system evolves  communication between both evolve  highly energy efficient, complex cell evolves (mitochondria LATE theory) • The “first” nail in the “Archezoan” coffin: ‘Long branch attraction’ artifact o Mitochondria entered, and all complex life evolved from it o All the groups were located within eukaryotic group (they are not a separate group) o The “first” nail in the “Ar
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