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Re: Digit Identity WAS (Re: Yay! Cladobabble! :-))

Nick Pharris writes:
"Well, yes, but it also appears that any digit has the genetic potential to
assume any identity, until the Sonic hedgehog signal determines which genes
will be activated. (Then again, the cells in your finger have the *genetic*
potential to grow a spleen, or a brain, or a bone, or any other tissue, don't

Yes, but that genetic potential or fate becomes entrenched as organogenesis proceeds. In many amphibians, amputation of a limb causes a series of "reversals" to an omnipotent cell state and a new limb, complete with bones, muscles, and nerves will form. For some reason, this is not possible in reptiles, birds, and mammals -- there are many reasons, of course, but I'm not going into detail on them here.

"Now suppose a mutant arises that produces only three digits (II-IV). If
nothing else changes, the concentration of the "thumb-determining" protein
will now be highest in ancestral digit II (since I isn't there at all), and so
the digit homologous to ancestral digit II (being the third digit formed) will
develop just like ancestral digit I. Voila`: frame shift. Am I missing
something? Why would something like this not be *expected* to happen?"

Sure -- not a problem -- but I wasn't saying anything about the frameshift hypothesis. And I wasn't saying digit frameshifts don't or can't occur. All I was pointing out, in relation to a question Dave Marjanovic had about digits, is that there appear to be five digit identities in all extant tetrapods, and that cases of polydactyly, for example, never produce more than five digit identities. Instead, you get repeated digits III or V or whatever. The point was that digit identity is not encoded simply in genes or a morphogen gradient, but is a result of a combination of factors, as you explain above.

Again, the other point of interest to me is that digit identity in birds is being based, in part, on supposed wrist identity and homology. Also, not very many bird embryos have been examined in detail to see how the manus and carpus develop -- there could very well be differences in different birds. Whereas I think the frameshift hypothesis offers an interesting explanation for the possible difference in digit identity in birds and dinosaurs, another possibility is that birds do indeed have I, II, III (without a frameshift) and that these have been mis-identified because of problems with determining wrist homology. "Mis-identified" is not a comment on the very difficult work of embryologists trying to piece these things together -- but with the difficulty there, this could be another morphological explanation for what's going on.


Matthew F. Bonnan, Ph.D.
Department of Biological Sciences
Western Illinois University
Macomb, IL 61455
(309) 298-2155

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