Meeting Abstract

99.4  Thursday, Jan. 7  Towards the mechanistic basis of digit identity frame shift in birds WAGNER, G. P. *; YOUNG-BRIM, R. ; Yale University; Yale University gunter.wagner@yale.edu

One of the most enduring conflicts in homology assessment is the evidence concerning the identity of anvian digits in the wing. Anatomical and paleontological evidence shows that the three definite digits of the avian hand are digits 1, 2, and 3, while the embryological evidence shows that these digits arise from digit condensations 2, 3, and 4. To solve this conflict it has been proposed that, in the stem lineage of birds, digit identity has been transferred from their ancestral positions 1,2, and 3 to the positions 2, 3, and 4 as found in the bird hand. In support of this hypothesis it has been shown that in five digited anmniotes, mouse and alligator, digit 1 is distinguished by the absence of HoxD11 and HoxD12 expression. In the bird wing it was found that this gene expression signature is found in the most anterior digit, i.e. the digit that develops in position 2. Digit identity frame shift can also be induced experimentally and are also associated with a posterior shift in HoxD expression. Here we present evidence that chick wing development is associated with a two day reduction of HoxD gene expression relative to the hind limb. No corresponding decrease in forelimb gene expression is found in mouse limb development. In contrast, no difference could be found in Shh expression. We thus conclude that the genetic difference, which leads to the digit identity frame shift in the bird wing, acts downstream of Shh signaling and upstream of HoxD expression. We hypothesize that the cis-regulatory elements of HoxD12 and HoxD11 have a derived responsiveness to a transcription factor specifically expressed in the anterior autopod.