ORIGIN OF CELLULARITY AND CELLULAR DIVERSITY

ORIGIN OF CELLULARITY AND CELLULAR DIVERSITY

Geological stratigraphy, together with radioactive dating, show the sequence of events in the history of the Earth. Note the entry for cyanobacteria and stromatolites only one billion years after the formation of the Earth. But what came before them? And how did cells become more complicated 1.5 B years later? Last impact heating ~3500

Life starts with chemistry Chemical experiments, initiated by Stanley Miller and Harold Urey, Showed that some biochemicals could be formed naturally (but not all--where did the rest come from?)

Protobionts vs. surface layers What is important about a cell? How would you start, if you wanted to make a model of a cell? Hypothesis: Metabolism (energy, reproduction) needs a way of concentrating intermediates. Cells Adsorptive surfaces

Protobionts: a step toward living cells? In 1957, Sidney Fox demonstrated that dry mixtures of amino acids could be encouraged to polymerize upon exposure to moderate heat. When the resulting polypeptides, or proteinoids, were dissolved in hot water and the solution allowed to cool, they formed small spherical shells about 2 μm in diameter microspheres. Under appropriate conditions, microspheres will bud new spheres at their surfaces.

Tiny compartments in mineral structures can shelter simple molecules, while mineral surfaces can provide the scaffolding on which those molecules assemble and grow. Beyond these sheltering and supportive functions, crystal faces of certain minerals can actively select particular molecules resembling those that were destined to become biologically Important [eg. L-amino acids, D-sugars]. The metallic ions in other minerals can jumpstart meaningful reactions like those that must have converted simple molecules into self-replicating entities. --R.M. Hazen, Life s Rocky Start Scientific American, April, 2001

Some have taken the term protobiont a step too far. But this does bring up the question, how did complex cells arise?

Origin of Eukarya! Named the "greatest single evolutionary discontinuity"! Most important in terms of evolutionary innovation, leading to wide range of new adaptations! What was (were) the ancestor(s) of the first eukaryotes! Was it a single event, or many?! If there was a single key effect, what was it?! What is basic (might help understand origin)?! What is derived (after origin, even if facilitated by basic changes)?! Double-membrane-bounded organelles have been focus of attention: nucleus, mitochondrion, plastids

Plasma membrane hypothesis! Nucleus from infolding of plasma membrane! A similar mechanism has been proposed for the origin of mitochondria and plastids

Endosymbiotic hypothesis--margulis! Nucleus from symbiosis of archaean in bacterium (or vice versa)! Mitochondria from symbiosis of alphaproteobacterium (includes E. coli, non-sulfur purple photsynthetic bacteria, Kreb cycle?) in nucleated host! Plastid from symbiosis of photosynthetic bacterium in nucleated host

! Endosymbiosis stabilized by loss of genetic material from symbiote (organelle) to nucleus, and the import of certain nuclear enzymes into symbiote needed for function

Evidence: Structural similarities between plastids (and mitochondria) and bacteria:! Circular DNA! Bacterial type ribosomes! Plasma membrane of (some) bacteria and the inner membrane of mitochondria have similar electron transport systems and ATP synthases! Other enzymes Present day endosymbioses show ease of symbiosis! Chlorella in Hydra and dinoflagellates in corals! Rhizobium in legume root nodule! Wolbachia in insects! Others more arcane:

Pelomyxa palustris! Single cell with nucleus but no Golgi, E.R., mitochondria, plastids, or spindle; instead, has 3 kinds of obligate endosymbiotic archaeans (2 methanogens)! Amoeboid, microaerophilic (pond mud), no mitosis (nuclear fission) Mixotricha paradoxa! Single cell in termite gut (symbiont: digests wood and excretes products)! No mitochondria; two kinds of spirochaetes and one rod bacterium on surface; internal bacteria symbiont (=> "beast with five genomes )

Summary DNA, prot synthes i s Energy metabolism Nucleus Archaean Bacterial Mitochondrion Bacterial Bacterial Plastid Bacterial Bacterial A+B1 => cell with nucleus, + B2 (a-proteobacteriumj) => cell with mitochondrion, + B3 (photosynthetic bacterium) => cell with plastid

Question: what was the key event in the evolution of eukaryotes?! Was there no key event, just a succession of symbioses, which could happen to any cells?! Was the key event a mutation that made the primeval eukaryote receptive to endosymbionts?! Was the key event the invention of a gene that allowed some prokaryotes to enter other cells? (Mycobacterium tuberculosis has a special invasion gene that allows it to enter animal cells.)! Was the key event the invention of ability to transfer genes from symbiote to host? (One Wolbachia has transferred 1% of genes to X chromosome of adzuki beetle.)

Question: one endosymbiotic event or many? Prokaryote divergence Endosymbiosis Eukaryotic evolution Two models: how can we distinguish between them?! Are plastid genes of different algae more similar to each other than to cyanobacteria?

One primary endosymbiosis Initial divergence Secondary endosymbioses Further divergence One model for endosymbiosis in algal evolution Tertiary endosymbioses P. J. Keeling et al., Science 306, 2191b (2004)

Summary Methods to arrange and concentrate metabolites must have arisen early. Ideas include protobionts (vesicles) and adsorptive mineral surfaces The first cells were prokaryotic. Endosymbiosis of bacteria in archaea produced eukaryotic cells. Although endosymbiosis can occur in the present, algae and plants seem to have evolved through a single primordial endosymbiosis of a cyanobacteria, followed in some cases by secondary endosymbiotic events.