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Re: Horse common ancestor
David Froehlich (eohippus@curly.cc.utexas.edu)
Mon, 16 Oct 1995 16:19:59 -0500
On 16 Oct 1995, Alex Duncan wrote:
> In article <45opna$oi9@ednet1.osl.or.gov> donald e. tyler,
> dtyler@ednet1.osl.or.gov writes:
>
> >With the donkey with 62 chromosomes,
> >the Mongolian wild horse with 66,
> >modern horse with 64, andn 3 species
> >of zebras with 32, 44, and 46, are
> >there any ideas as to the number
> >of chromosomes the common ancestor
> >had? Or are we allowed to question
> >basic premises here?
>
> Given the data you cite, I see no way we can reconstruct the number of
> chromosomes of the LCA of extant equids.
>
> What does Froelich have to say about it?
Why did you have to put me on the hot seat. I work with 50 m.y. old horses
not these big things. To get any guess at all on the chromosomal # you
would probably have to look at the distribution among the outgroups. I
have no idea where to look for karyotypes for the rhinos, the tapirs, or
if you wanted to expand into the tethytheres, the hyracoids, elephants,
and sirenians. Given the numbers in those organisms (and a lot of
assumptions since divergence times are on the order of 50-60 million
years) you might (emphasize might) get some handle on it. However, since
all of the modern horses evolved from an Equus ancestor no more than 5-10
million years ago, and they are all a monophyletic clade I find this
unlikely (the last common ancestor most probably had a single chromosomal #
and all the variation seen today is subsequent to that). A phylogeny of
the equid clade might help as well, since you might be able to determine
the sequence of cladogenesis and thus possibly the sequence of karyotype
divergence.
Hope that muddied the waters sufficiently. Upshot is "good luck, you are
going to need it".
David J. Froehlich Phone: 512-471-6088
Vertebrate Paleontology Laboratory Fax: 512-471-5973
J.J. Pickle Research Campus
The University of Texas, Austin, Texas 78712
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