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Avian digit homology based on molecular-morphogenetic model

From: Ben Creisler

A new online paper with another analysis of the underlying identity of
avian digits:

Daniel Capek, Brian D. Metscher & Gerd B. Müller (2013)
Thumbs down: A molecular-morphogenetic approach to avian digit homology.
Journal of Experimental Zoology Part B: Molecular and Developmental
Evolution (advance online publication)
DOI: 10.1002/jez.b.22545

Avian forelimb digit homology remains one of the standard themes in
comparative biology and EvoDevo research. In order to resolve the
apparent contradictions between embryological and paleontological
evidence a variety of hypotheses have been presented in recent years.
The proposals range from excluding birds from the dinosaur clade, to
assignments of homology by different criteria, or even assuming a
hexadactyl tetrapod limb ground state. At present two approaches
prevail: the frame shift hypothesis and the pyramid reduction
hypothesis. While the former postulates a homeotic shift of digit
identities, the latter argues for a gradual bilateral reduction of
phalanges and digits. Here we present a new model that integrates
elements from both hypotheses with the existing experimental and
fossil evidence. We start from the main feature common to both earlier
concepts, the initiating ontogenetic event: reduction and loss of the
anterior-most digit. It is proposed that a concerted mechanism of
molecular regulation and developmental mechanics is capable of
shifting the boundaries of hoxD expression in embryonic forelimb buds
as well as changing the digit phenotypes. Based on a distinction
between positional (topological) and compositional (phenotypic)
homology criteria, we argue that the identity of the avian digits is
II, III, IV, despite a partially altered phenotype. Finally, we
introduce an alternative digit reduction scheme that reconciles the
current fossil evidence with the presented molecular-morphogenetic
model. Our approach identifies specific experiments that allow to test
whether gene expression can be shifted and digit phenotypes can be
altered by induced digit loss or digit gain.