@article{2248,
  abstract     = {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.},
  author       = {Capek, Daniel and Metscher, Brian and Müller, Gerd},
  issn         = {15525007},
  journal      = {Journal of Experimental Zoology Part B: Molecular and Developmental Evolution},
  number       = {1},
  pages        = {1 -- 12},
  publisher    = {Wiley-Blackwell},
  title        = {{Thumbs down: A molecular-morphogenetic approach to avian digit homology}},
  doi          = {10.1002/jez.b.22545},
  volume       = {322},
  year         = {2014},
}