radford at cs.toronto.edu (Radford Neal) wrote:
>Perhaps a professional might be able to argue that interbreeding is
>impossible due to chromosonal incompatibility, but no one has
>attempted this, and I doubt that present knowledge can conclusively
>rule out interbreeding on this basis. (Note that it is possible for
>organisms with different chromosome numbers to produce viable, fertile
>hybrids, as illustrated by the domesticated horse and the original
>wild horse of Mongolia.)
The following is intended as a clarification, not a flame.
I never meant to predict that if someone attempted to cross humans
and apes, that inviable embryos would be produced. I only meant that
if Mr. Alawi's statement was true that this had been tried several
times with those results, that genomic incompatability was a more
likely explanation than the claimed superior "complexity" of our
species over apes.
I also never said that chromosome number had anything to do with that
incompatability. There is an excellent article by Yunis and Prakash
(1985, Science 215:1525-1530) called "The Origin of Man: a Chromosomal
Pictorial Legacy". This paper demonstrates that while humans and
African apes have different chromosome numbers (23 pairs for humans,
24 pairs for apes), apes have two pairs of chromosomes that are
homologous to different halves of the large human chromosome number 2.
The implication is that at some point in the lineage between
human/chimp ancestor and modern humans, two chromosomes fused.
Therefore there must have been a population of hominoids that was
polymorphic for chromosome number, and we are descendents of that
population. I suspect that the example of the Mongolian/wild horses
represent such a case of recent fusion or split in which two
chromosomes of one species are directly homologous to one chromosome
of the other species, although I don't know the specifics of that
example (anybody?).
Yunis and Prakash further demonstrate that while banding patterns are
very similar between human and ape chromosomes, suggesting that many
genes physically map to the same locations, there are a considerable
number of inversions and translocations. This means that if a hybrid
were produced, it would likely be sterile due to mismatch during
meiosis. Once again, this has nothing to do with my original post
because Messers. Campton and Alawi were talking about unsuccessful
embryonic development, not hybrid sterility.
This leaves us with developmental genetics, a subject I know next to
nothing about. All I can say is that intergeneric hybrids among
vertebrates are rare, and that I am aware of studies in which attempts
to produce intergeneric hybrids in vertebrates resulted in successful
fertilization but abnormal development and death of the embryos, just
the phenomenon described in the original post of this thread.
I agree with Mr. Neal's points about prezygotic barriers and the
ethical issues of doing this experiment.
Ed Heist
