A new paper in open access:
In crown group tetrapods, individual digits are homologized in relation to a pentadactyl ground plan. However, testing hypotheses of digit homology is challenging because it is unclear whether digits represent distinct and conserved gene regulatory states. Here we show dramatic evolutionary dynamism in the gene _expression_ profiles of digits, challenging the notion that five digits have conserved developmental identities across amniotes. Transcriptomics shows diversity in the patterns of gene _expression_ differentiation of digits, although the anterior-most digit of the pentadactyl limb has a unique, conserved _expression_ profile. Further, we identify a core set of transcription factors that are differentially expressed among the digits of amniote limbs; their spatial _expression_ domains, however, vary between species. In light of these results, we reevaluate the frame shift hypothesis of avian wing evolution and conclude only the identity of the anterior-most digit has shifted position, suggesting a 1,3,4 digit identity in the bird wing.