Re: CPN, Z or S, + CP evolution

Liane Cochran-Stafira (lcochran@midway.uchicago.edu)
Fri, 23 Jun 1995 14:26:49 -0500

Jan, you
>mentioned that there is no selection of genotype, but I remember studying
>that this does exist. Like when a genotype is hybrid between similar
>taxons, the chromosomes may not separate well in meiosis, resulting in
>inviable gametes with different numbers of chromosomes. Depending on the
>situation, 1/2, or 1/3, or 2/3, etc., of the gametes will be inviable. I
>think there were a few other similar cases where certain genotypes could
>be more advantageous.
>
>
>
> FERNANDO RIVADAVIA
> Sao Paulo, Bras(z)il

Fernando,

I'm not really sure I would call this genotype selection. It sounds more
like a form of post-zygotic reproductive isolation mechanism that helps
prevent species from interbreeding by making the resultant hybrids sterile
or at least reducing their reproductive success. For example - although
horses can breed with donkeys to produce viable offspring, mules, the mules
are sterile because they cannot form gametes for exactly the reason you
mention. Horses and donkeys have different numbers of chromosomes and the
mules end up with an odd number of chromosomes. When it comes time for
meiosis in the mule, synapsis cannot occur, and no viable gametes are
produced. If I've missed something in your argument, let me know.

Liane Cochran-Stafira