ORIGIN OF CELLULARITY AND CELLULAR DIVERSITY

Last impact heating~3500

Geological stratigraphy,together with radioactivedating, show the sequence of events inthe history of the Earth. Note the entry for “cyanobacteria” and “stromatolites” only onebillion years after the formation of the Earth.

But what came before them?And how did cells becomemore complicated 1.5 Byears later?

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?)

Life starts with chemistry

Protobionts vs. surface layersWhat is important about a cell?How would you start, if you wanted to make a model of a cell?

Hypothesis:Metabolism (energy, reproduction) needs away of concentrating intermediates.

CellsAdsorptive surfaces

Protobionts: a step toward living cells?

In 1957, Sidney Fox demonstrated that dry mixtures of amino acidscould be encouraged to polymerize upon exposure to moderate heat.When the resulting polypeptides, or proteinoids, were dissolved in hotwater and the solution allowed to cool, they formed small sphericalshells about 2 μm in diameter—microspheres. Under appropriateconditions, microspheres will bud new spheres at their surfaces.

Tiny compartments in mineral structures can shelter simplemolecules, while mineral surfaces can provide the scaffoldingon which those molecules assemble and grow. Beyondthese sheltering and supportive functions, crystal facesof certain minerals can actively select particular moleculesresembling those that were destined to become biologicallyImportant [eg. L-amino acids, D-sugars]. The metallic ions in otherminerals can jumpstart meaningful reactions like those that musthave 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 alpha-

proteobacterium (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 is

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?

Two models: how can we distinguish between them?

! Are plastid genes of different algae more similar to

each other than to cyanobacteria?

Prokaryote divergence

Endosymbiosis

Eukaryotic

evolution

P. J. Keeling et al., Science 306, 2191b (2004)

One primary endosymbiosis

Initial divergence

Secondary endosymbioses

Tertiary endosymbioses

Further divergence

One model for endosymbiosis inalgal evolution

Summary

Methods to arrange and concentrate metabolites musthave arisen early. Ideas include “protobionts” (vesicles)and adsorptive mineral surfaces

The first cells were prokaryotic. Endosymbiosis ofbacteria in archaea produced eukaryotic cells.

Although endosymbiosis can occur in the present, algaeand plants seem to have evolved through a singleprimordial endosymbiosis of a cyanobacteria, followed insome cases by secondary endosymbiotic events.