The evolution of HoxD-11 expression in the bird wing: insights from Alligator mississippiensis.
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.
Vargas
Alexander O
AO
Department of Ecology and Evolutionary Biology, Yale University, New Haven, Connecticut, USA. thearchosaur@gmail.com
Kohlsdorf
Tiana
T
Fallon
John F
JF
Vandenbrooks
John
J
Wagner
Günter P
GP
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*