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Digit Identity WAS (Re: Yay! Cladobabble! :-))
Throwing my hat in here for a minute:
David Marjanovic asks:
"- How can we integrate Feduccia's finding of fingers II-III-IV in birds
the hypothesis of their theropod ancestry? (Mind you, I think this is _not_
a finding, but a _misinterpretation_, that the fingers Feduccia found are
not I -- V, but the prepollex and I -- IV [plus a very small V that he
overlooked, proximal to IV]; but I'm currently discussing this with Feduccia
himself, so we'll see, perhaps.)"
From what I recall, part of the problem here is that the designation of the
digits depends, in part, on how one interprets the homology and identity of
the carpal bones in the bird wrist -- not an easy endeavor! So far as I
know, part of the digit identity (II, III, IV) is being established on how
these digits arise in connection with wrist development -- in other words,
digit x is identified based on its position relative to wrist element y.
Thus, depending in part on one's interpretation of the anatomy and homology
of the bird wrist, you either get I, II, and III or II, III, and IV.
"- Are frameshifts, as suggested by Wagner & Gauthier, going to happen? (I'm
somewhat confused by this question ... Secondly, it assumes _a priori_ that
digit identity is fixed by genes. _This may not be the case!_ Instead it may
well be produced by, say, the amount of a signal molecule that's produced at
one side of the limb bud [cranial or caudal] and diffuses to the other --
producing a gradient --, or who knows. If a gradient, threshold values etc.
instead of direct genes are involved, then of course digit "identity" will
change on all occasions.)"
Well, sort of. The digit identity, so far as embryologists know, is already
encoded in Hox genes that are activated by different concentrations of a
morphogen, a diffusing protein that activates or inactivates various
combinations of genes. Sonic hedgehog is the name given to the protein
morphogen gradient that regulates limb and finger development and its
gradient across the developing hand determines digit identity by turning on
and off different combinations of Hox genes (see Zimmer's nice explanation
in his 1998 book AT THE WATER'S EDGE). So, it's not just a matter of a
chemical gradient, but a gradient that turns on latent genes in the
developing limb bud. It appears, based on numerous experiments, that there
are a total of 5 encoded digit identities in the genes for all vertebrates
-- you can't get any more than that. In fact, in humans where variations of
polydactyly occur, it is always a replication of one of the five digits, and
not the creation of a new digit.
Apparently, then, digit identity is gene encoded PRIOR to the Sonic hedgehog
signal. I suspect the debate over the digit identity in dinosaurs will
still continue until better information on wrist bone homologies is
attained. I'm telling you, don't knock the functional morphology and
evolution of the manus and carpus until you've tried it. =) Phylogeny needs
functional morphology, functional morphology needs phylogeny, and all is
Matthew F. Bonnan, Ph.D.
Department of Biological Sciences
Western Illinois University
Macomb, IL 61455
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