Re: Origin of sex - not why but who and when?

Nic wrote:

John Harshman wrote:

Nic wrote:


Perplexed in Peoria wrote:


"John Harshman" <jharshman.diespamdie@xxxxxxxxxxx> wrote in message news:vVoCg.4564$o27.2095@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx


Nic wrote:


In practice I think what happens (as with bdelloid rotifers) is that
chromosome number remains as is, but one gene in each homologous pair
is free to decay, until the information content of the genome has
fallen to the same quantity as our still sexual cousins are passing on
in only one gamete!

Not necessarily. They could instead attain permanent heterosis. And in
many cases dosage is important too. What I think would actually happen
is that the homologous chromosomes would be free to diverge until they
were no longer recognizable. Lack of recombination would produce lots of
gene families with two members. Some copies would degenerate into
pseudogenes, and others would differentiate in function.

Another process to be considered is intragenomic "gene conversion".
This can convert a gene pair from Aa (randomly) to either AA or aa.
Add selection to express a preference for AA over aa, say, and you
can avoid degeneration.



This is
actually a very interesting evolutionary question with implications far
beyond asexual diploids, and I wonder it anyone is doing research on
ancient asexual lineages (like bdelloids) to figure out what actually
happens. I don't actually know of anyone working in rotifer molecular
systematics, but I bet you could write a very strong proposal. How easy
are they to culture?

Quite a bit of work has been done, as you can find using Google or
PubMed. Or find links by searching some recent discussion of the
topic over on sci.bio.evolution.

One interesting thing about the bdelloids is how they evolved from
sexual ancestors. Apparently it involved a hybridization of two
related facultative sexual rotifers such that the ancestral bdelloid
found herself with an odd number of chromosomes (some homologous, but
some not) and no way to reproduce sexually any more.


I didn't know that. That would make it a similar case to cnemidophorus
lizards. So far, I only know of daphnia pulex, cnemidophorus, and
bdelloid rotifers as examples of things doing ok without sex. The
first two seem repeatedly to drop sex, get on ok asexually for a while,
but eventually go extinct (presumably getting overtaken by their sexual
cousins who can evolve faster). The bdelloids on the other hand, seem
to be here to stay, and have diversified since their loss of sex. I
say 'seem', because in this area, little-known facts keep cropping up
to change the picture completely.

It would be interesting to know if all asexual eukaryotes are
neo-asexual. I mean did the nucleus that got included by endosymbiosis
*already have* meiosis? What would lend probability to this is that
the machinery of meiosis seems not to have evolved multiple times
independently. I say 'seems' here because where something like a
flagellum evolves multiple times, the two varieties have very salient
differences: rotating vs. reciprocating.


Meiosis seems primitive for the eukaryote crown group. But you
understand that the last common ancestor of living eukaryotes doesn't
necessarily have anything to do with any of the events that produced the
current eukaryote cell structure. It could easily postdate all of them,
including the endosymbiosis of mitochondria. Of course meiosis and
(eukaryote) flagella are closely linked.


Point taken. The last free living ancestor of the eukaryote nucleus

I'm not sure what you mean by that. Are you supposing that the eukaryote
nucleus was produced by endosymbiosis? If so, what was the genome of the
organism that engulfed the nucleus, and where has it gone?

had something special which qualified it for being recruited as a
specialist librarian. That could be any or all of: the use of DNA
instead of RNA for greater stability,

This is hardly possible, since the ancestor of all extant life precedes
the eukaryote ancestor and is the ancestor of all eukaryote organellar
genomes too. Thus the use of DNA as genetic material must greatly
predate the evolution of eukaryotes. Unless there was some cryptic
organism that supplied the eukaryote cell body, left no trace in the
genome or anywhere else, was outside the tree of all living species, and
used something else for a genome.

an especially efficient means of
packing DNA neatly in to a small space, the machinery of mitosis as we
know it. Everything else, like meiosis, is just wild speculation.
However, I keep thinking of the fungi. There are examples there where
the endosymbiosis of the nucleus is a little bit more like ordinary
symbiosis - i.e. cooperation with the rest of the cell, but only up to
a point.

I have no idea what you mean by that. How can the rest of the cell wxist
at all without a genome?

See paragraph 3 in section CONCLUSIONS AND FUTURE PERSPECTIVES on:

http://mmbr.asm.org/cgi/content/full/64/2/316#SEC3_4_2

If this is a glimpse in to the ancestral relationship between the
nucleus and the rest of the cell, it would tend to support the above
mentioned wild speculation.

I think you have confused the relationship between two haploid nuclei
for the relationship between nucleus and cell.

The thing that gets me is they've known about this for decades, but the
news never seems to spread outside of mycology.

I really don't see the relevance to what we've been talking about.
Anyway, fungal sex must be highly derived. You could conceive of it as a
model for primitive sex, but it must be analogous, not homologous.

.

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