Global anteroposterior polarity of the tetrapod limb skeleton is controlled by Sonic hedgehog signaling, but downstream molecular and developmental mechanisms controlling digit identity and digit number, and morphological variation, remain poorly understood. Five members of the T-box family of transcription factors have been identified that exhibit spatial and temporal expression patterns consistent with a role in specifying digit identity in the developing chick limb. This proposal is designed to test the functional roles of T-box genes in regulating digit patterning through gain-of-function misexpression studies in the chick model system, and to assess the conservation of T-box function through comparative expression and function analyses in basal tetrapod (Ambystoma) and actinopterygian (Lepisosteus) taxa. To gain insight into the evolutionary mechanisms of digit reduction, a phylogenetic tree based on DNA sequence homology will be generated for the scincid lizard genus Lerista, which exhibits the most diverse spectrum of tetrapod limb reduction. Comparison of digital patterns within the context of this DNA sequence phylogeny will provide the first example of an actual evolutionary progression of limb reduction in a closely related, natural population.