Fossil genetics

Eric Cabot ecec at quads.uchicago.edu
Tue May 10 14:08:47 EST 1994

In article <wgallin.1118884916F at NEWS.SRV.UALBERTA.CA> wgallin at gpu.srv.ualberta.c
a (Warren Gallin) writes:
[in reference to hybrid sterility being due to kayrotypic changes]  
>  Actually I was thinking of the case of horses and donkeys.  I agree that

It is unfortunate that this impression persists in biological lore.  I recall
that we of the Wu lab were quite horrified to read in the new GRE test booklet
that the correct answer to the question "Why are the offspring of closely
related species sterile?" was "chromosomal incompatibility".

>this isn't the only cause, but neither is Drosophila necessarily
>representative of other species.  As I recall some of that is based on the

I think that this discussion serves as a useful reminder that the basis  of
hybrid sterility may be either genic or chromosomal.  Geneotypic  effects
leading to reproductive isolation need not be structural or even have anything 
to do with meiosis, for that matter. Consider, for example, cases of
geneic differences  with behavioral or morphological manifestations that cause
reproductive isolation.

>presence of an intracellular bacterium.  In general, karyotypic changes that
>lead to non-functional meiosis will necessarily lead to reproductive
>isolation.  The point is that karyotype changes are common; I don't know if
>some of the other factors occur as widely.  Karyotype differences (in
>particular translocations that lead to acentric or dicentric chromosomes on
>recombination) are effective blocks to interbreeding.  Whether they are the
>first cause of isolation is open to study for any species.

It doesn't necessarily follow that chromosomal changes are going to cause
isolation, though.  (For elaboration see Steve Schaeffer's post elsewhere
in this thread).

In fact some  rearrangements or aberations are so severe
that it is hard to envision how the first individuals with the new arrangements
found any compatible mates.

You raise an interesting point about the first causes of isolation. We found
lots of genetic factors causing hybrid sterility between D. simulans and its
relatives but still have no idea which, if any, of them were responsible for the
original isolation and which, if any, "merely" represent the accumulation of
mutations (or the rarefication of ancestral polymorphisms for that matter)
after the species diverged.

Eric Cabot
elmo at helix.nih.gov

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