HoxD expression in the fin-fold compartment of basal gnathostomes and implications for paired appendage evolution.

The role of Homeobox transcription factors during fin and limb development have been the focus of recent work investigating the evolutionary origin of limb-specific morphologies. Here we characterize the expression of HoxD genes, as well as the cluster-associated genes Evx2 and LNP, in the paddlefish Polyodon spathula, a basal ray-finned fish. Our results demonstrate a collinear pattern of nesting in early fin buds that includes HoxD14, a gene previously thought to be isolated from global Hox regulation. We also show that in both Polyodon and the catshark Scyliorhinus canicula (a representative chondrichthyan) late phase HoxD transcripts are present in cells of the fin-fold and co-localize with And1, a component of the dermal skeleton. These new data support an ancestral role for HoxD genes in patterning the fin-folds of jawed vertebrates, and fuel new hypotheses about the evolution of cluster regulation and the potential downstream differentiation outcomes of distinct HoxD-regulated compartments.