title: The evolution of HoxD-11 expression in the bird wing: insights from Alligator mississippiensis.
abstract: BACKGROUND
  Comparative morphology identifies the digits of the wing of birds as 1,2 and 3, but they develop at embryological positions that become digits 2, 3 and 4 in other amniotes. A hypothesis to explain this is that a homeotic frame shift of digital identity occurred in the evolution of the bird wing, such that digits 1,2 and 3 are developing from embryological positions 2, 3 and 4. Digit 1 of the mouse is the only digit that shows no late expression of HoxD-11. This is also true for the anterior digit of the bird wing, suggesting this digit is actually a digit 1. If this is the case, we can expect closer relatives of birds to show no HoxD-11 expression only in digit 1. To test this prediction we investigate HoxD-11 expression in crocodilians, the closest living relatives of birds.
  
  
  RESULTS
  Using degenerate primers we cloned a 606 nucleotide fragment of exon 1 of the alligator HoxD-11 gene and used it for whole-mount in-situ detection in alligator embryos. We found that in the pentadactyl forelimbs of alligator, as in the mouse, late expression of HoxD-11 is absent only in digit 1.
  
  
  CONCLUSIONS
  The ancestral condition for amniotes is that late-phase HoxD-11 expression is absent only in digit 1. The biphalangeal morphology and lack of HoxD-11 expression of the anterior digit of the wing is like digit 1 of alligator and mouse, but its embryological position as digit 2 is derived. HoxD-11 expression in alligator is consistent with the hypothesis that both digit morphology as well as HoxD-11 expression are shifted towards posterior in the bird wing.
authors.affiliation: Department of Ecology and Evolutionary Biology, Yale University, New Haven, Connecticut, USA. thearchosaur@gmail.com
authors.forename: Alexander O
authors.initials: AO
authors.lastname: Vargas
authors.forename: Tiana
authors.initials: T
authors.lastname: Kohlsdorf
authors.forename: John F
authors.initials: JF
authors.lastname: Fallon
authors.forename: John
authors.initials: J
authors.lastname: Vandenbrooks
authors.forename: Günter P
authors.initials: GP
authors.lastname: Wagner
mesh_terms: D000489:Alligators and Crocodiles / Q000196:embryology / Q000235:genetics*; D000595:Amino Acid Sequence; D000818:Animals; D001717:Birds / Q000235:genetics*; D003001:Cloning, Molecular; D019143:Evolution, Molecular; D005091:Exons; D005801:Genes, Homeobox; D017403:In Situ Hybridization; D008969:Molecular Sequence Data; D017386:Sequence Homology, Amino Acid; D014921:Wings, Animal / Q000033:anatomy & histology / Q000378:metabolism*