Meeting Abstract
Digits are an excellent model for studying the evolution and development of individuality in serial homologs. How digit identity is regulated or, indeed, whether digits have stable transcriptomic profiles that indicate discrete regulatory states, is unclear. While a number of genes have been identified as markers of digit identities, for example low expression of Hoxd11 seems to mark digit I of amniotes, comprehensive and comparative analyses of gene expression in digits are lacking. To discover the genetic basis of digit identity, we analyzed transcriptomes of developing limbs of American alligator (Alligator mississipiensis), green anole (Anolis carolinensis), and mouse (Mus musculus), chicken (Gallus gallus), and human (Homo sapiens). Limbs were sampled at comparable developmental stages, after digital condensation and once inter-digital webbing has begun reducing. RNA-Seq was performed on dissected digits and their associated, posterior inter-digital webbing, a known source of signals for digital identity. In situ hybridization was used to validate patterns of expression of identified candidate digit-identity genes. These analyses inform the variational independence and modularity of amniote digits, mechanistic hypotheses of limb morphogenesis, and human developmental malformations. Further, we identify stable transcriptomic profiles in chicken between the fore- and hindlimb. These analyses suggest a scenario for the evolution of the avian wing wherein the digits of chicken forelimb correspond to digits I, III, IV of other species.
