Theory of Evolution of Eukaryotes, Crossing Over, and Sex

Andrew J. Roger roger at
Sun Sep 21 09:36:15 EST 1997

This is a long message. To offer criticism for the idea, I have chopped
bits out (so that someone might actually read it)

James Howard wrote:
>An increase in T-A bases would increase the effects of DHEA in
> the host genome. I suggest this is the reason that E. coli has
> approximately 25% of each of the nitrogenous bases, while humans and
> other mammals have about 21% cytosine and quanine and 29% thymine and
> adenine. 

So humans and other mammals account for how much eukaryotic diversity?
Eukaryotes probably evolved between 1-2 billion years ago. Mammals
appeared ~200 mya.  If you want to cite evidence for a theory of
the origin of eukaryotes, you should be concerned with features
by all eukaryotes (for instance PROTISTS).  Base composition has
changed many times in many different eukaryotic a bias in
probably reflects nothing about the ancestral state of eukaryotes
as a whole. Diplomonad flagellates, possibly one of the deepest
branches, in general exhibit elevated G+C and lowered A+T.....this
could equally be ancestral to eukaryotes!

I am not sure if what you have come up with is a theory of the ORIGIN of
eukaryotes...(perhaps this is not your intention).  Your theory is
more correctly viewed as a theory of 

a) selective forces that led to the establishment of mitochondria
b) origin of crossing-over
c) origin and evolution of sex

A theory about the ORIGIN of eukaryotes must at least explain
the following:

a) the origin of the cytoskeleton (actin, myosin, tubulins, dynein,
kinesin etc etc.)
b) the origin of the endomembrane system
c) the origin of the nucleus
d) the origin of mitosis
e) the origin of multiple linear chromosomes withe telomeres and
   origins of replication

It should probably also involve explanations of:

f) timing and origin of mitochondria
g) timing and origin of karyogamy and meiosis
h) timing and origin of eukaryotic flagella

So far your theory only deals with only 2 of these things....and is
therefore not
explaining the hardest things to explain about the origin of eukaryotes.

Incidentally, theories for the "chimaeric" origin of eukaryotes also
suffer from ignoring most of the above problems.  And therefore they
shouldn't be considered theories of this at all....rather theories
for explaining the conflict between gene phylogenies.

Andrew J. Roger
Marine Biological Laboratory
7 MBL St.
Woods Hole, MA

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