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Fossil genetics

Warren Gallin wgallin at gpu.srv.ualberta.ca
Mon May 9 10:27:56 EST 1994

In Article <tdlong-070594105735 at pbmac-5.ucdavis.edu>, tdlong at ucdavis.edu
(Tony Long) wrote:
>In article <wgallin.1118714328C at NEWS.SRV.UALBERTA.CA>,
>wgallin at gpu.srv.ualberta.ca (Warren Gallin) wrote:
>> an early biological determinant of mating incompatibility is probably
>> changes in chromosome structure, which will interfere with functional
>> meiosis.  To see that, you would need a full karyotype analysis;  I think
>> that is extremely unlikely.  Most of the structural genes are not going to
>> confer mating incompatibility.
>        What is your evidence for this?  Sure there are a lot of karyotype
>differences between species, but such changes do not imply they are the
>cause of reproductive isolation.  I suggest looking at recent work out of
>the Wu lab at University of Chicago (or any of the other labs doing work in
>this area) on reproductive isolation between species in the Drosophila
>group.  This work suggests that a number of 'factors' are associated with
>reproductive isolation in these groups.  As these factors are not cloned
>(not yet anyway) it is not clear if they represent structural or regulatory
>genes.  Eitherway, at least in Drosphila, there is evidence that
>reproductive isolation can evolve before karyotype changes.  Mileage may
>vary in other taxa.

>> you're suggesting, but I think it is in the realm of science fiction right

>        Even if one had the sequence of the entire genome of an extinct
>the task of associating this variation with ANY phenotypic differences (be
>it isolation or nose size) between species would remain in the realm of
>science fiction.  In order to map such loci one needs to associate DNA
>variation with phenotypic variation in the F2 (or a subsequent backcross)
>generation of a cross between the species.  Of course in order to do this
>we would have to recreate the species in a Jurassic Park fashion ... or
>make transgenic animals that differ only in a candidate gene (this is
>possible in Drosophila).  

  Actually I was thinking of the case of horses and donkeys.  I agree that
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
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.
  When I said that most of the strucrual genes are not going to confer
compatibility, I think I was on firm ground.  Note, I was not saying that
structural genes won't do it, I said most of them have nothing to do with
it.  A small sample of genomic sequence (which is all you are likely to get
from a fossil sample, is unlikely to have the genes that are associated with
reproductive isolation, even if you knew what you were looking for, which we
both agree we don't.

Warren Gallin,
Department of Zoology, University of Alberta
wgallin at gpu.srv.ualberta.ca

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