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new chicken digit homology?

A new article on digit homology of chicken suggests a hexadactyle archetype for 
archosaurs in general; thus removing the 1-2-3 vs 2-3-4 homology problem.

Monique C. M. Welten, Fons J. Verbeek, Annemarie H. Meijer, and Michael K. 
Richardson (2005): Gene expression and digit homology in the chicken embryo 
wing. ? Evolution & Development, 7 (1): 18?28.

The bird wing is of special interest to students of
homology and avian evolution. Fossil and developmental data
give conflicting indications of digit homology if a pentadactyl
??archetype?? is assumed. Morphological signs of a vestigial
digit I are seen in bird embryos, but no digit-like structure
develops in wild-type embryos. To examine the developmental
mechanisms of digit loss, we studied the expression of
the high-mobility group box containing Sox9 gene, and bone
morphogenetic protein receptor 1b (bmpR-1b)Fmarkers for
precondensation and prechondrogenic cells, respectively. We
find an elongated domain of Sox9 expression, but no bmpR-1b
expression, anterior to digit II. We interpret this as a digit I
domain that reaches precondensation, but not condensation or
precartilage stages. It develops late, when the tissue in which it
is lodged is being remodeled.We consider these findings in the
light of previous Hoxd-11 misexpression studies. Together,
they suggest that there is a digit I vestige in the wing that can
be rescued and undergo development if posterior patterning
cues are enhanced. We observed Sox9 expression in the
elusive ??element X?? that is sometimes stated to represent a
sixth digit. Indeed, incongruity between digit domains and
identities in theropods disappears if birds and other archosaurs
are considered primitively polydactyl. Our study provides the
first gene expression evidence for at least five digital domains
in the chick wing. The failure of the first to develop may be
plausibly linked to attenuation of posterior signals.

Markus Moser

Dr. Markus Moser
Staatliches Museum fuer Naturkunde Stuttgart
Museum am Loewentor (= Rosenstein 1)
D-70191 Stuttgart

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