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Re: New in Nature: Birds Galore!



> To me, this is surprising: dinosaurs are famous for
> reducing serial homology
> between hands and feet, as seen by the symmetrical
> digit reduction of the
> feet and the asymmetric one of the hands.

True but these have definitely occured through the
evolutionary intercalation of terminal modifying
pathways that hijack the default plan. It is unlikely
that when flight feathers are invented they come
directly dowstream of such specialized pathways.
  
> I forgot which Bmp is involved in limb formation,
> but Shh appears to be
> involved in all outgrowths of the body wall. Without
> it (vertebrate) limbs
> form (sort of) but stay in the body.

Yes BMP2. See this paper:
http://www.ncbi.nlm.nih.gov:80/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=12210117&dopt=Abstract

This Bmp2-Shh pathway is an ancient module used
repeatedly in many organisms starting from at least
the base of the coelomate clade. Nematodes have no Hh
homologs though they have Bmp2 homologs. Even insect
wings are formed under their action and they function
early development in the Spemann organizer or its
eQUIVALENTS. In the case of limb development or that
of the tongue this Hh-Bmp module seems to function as
a fractal recursively used with a different scaling.
{of course biologists may disagree with this
visualization of it, but then I am just programmer and
tend to see these things my way.}

> Subsequent? Somehow contradicts the above, doesn't
> it?

As I stated in an earlier reply to Mr. Hecht, it is
common that primitive characters are retained in forms
coeval with derived forms. The selection depends on
the niche a form fits into.
-E

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