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Re: Embryological evidence that bird fingers are I, II, III
> That's debatable, but in any case the paper, which I'm reading
> right now, _destroys_ the the frameshift hypothesis.
> -- Frameshift: digits I and V disappear, digits II to IV assume the
> anatomies of digits I to III.
That isn't quite what the Frameshift proposes. You are still applying
Owenian models of homology to the situation, but the Frameshift model
interprets the assignment of homology of condensation and of
chondrification and ossification to be separate phenomena, under
separate genetic control.
I just worded it poorly. By "position", I mean homology of condensation;
by "anatomy", I basically mean homology of chondrification and
While in most organisms the two sets of homologies match up, the
Frameshift model proposes that chondrification/ossification phase of
identity assignment has moved over.
And indeed, the paper shows it moves several times! When you truncate
development of a chicken foot by stopping Sonic hedgehog expression at a
specific time, you get a foot with two first toes (the lateral one an
outgrowth of the polarizing region); if you truncate later, you get a
foot with I-II-II (the lateralmost toe coming from the polarizing
region); later, I-II-III-III; later still, it's too late, and you get
I-II-III-IV. Digit identities are assigned by Shh expression; the longer
a developing condensation is close to the polarizing region, the closer
to identity IV it comes.
If you additionally stop the growth of the zone where the condensations
I to III would form, you get only one digit in that region, and which
identity that digit assumes depends on when Shh expression stops. When
you stop it early, you get I-I as above; later, II-II; later, III-III;
later still, it's too late, and you get III-IV.
If you repeat _that_ trick with the hand, you get a single digit in
total, because the polarizing region doesn't grow into a digit. If you
stop Shh expression early, you get I only; later, II only; later, III only.
"Outgrowth" is literal. The experiments were all done with
Gfp-expressing polarizing regions transplanted from Gfp-expressing
chicken embryos. In the foot, the entire region of metatarsal IV and toe
IV expressed Gfp, both the cartilages and the soft tissue, and no other
digital ray did; in the hand, no digital ray did, only the postaxial
_margin_ of the soft tissue of ray III did.
Thus the ossified homologs for
the avian digits ARE digits I-III of non-birds, and the genes and
gene products that specify them are the same. However, the "starter
material" used are the condensations that in other organisms become
And that's the hypothesis that the paper disproves. The condensation
that elsewhere becomes manual digital ray IV does not form in chickens.
It's not there to begin with, so it can't assume any anatomical identity.
So the frameshift specifies that it isn't simply that "digits II to
IV assumes the anatomies of digits I to III"; it is "condensation
positions II to IV develop into anatomical digits I to III". They ARE
digits I-III from all gene-product standpoints.
Late enough in development, yes.