Meeting Abstract
How paired fins gave rise to limbs during the invasion of land is one of the compelling questions in vertebrate evolution. This morphological transition involved several key changes in appendage anatomy, including the loss of the fin-fold and dermal skeleton, and an elaboration of the distal endoskeleton to form an autopod with digits. HoxD cluster genes are active during both fin and limb development, and over the last two decades, have been the focus of much work aimed at gaining insight into 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 hypothesized to be isolated from global Hox regulation. Additionally, we show that in both Polyodon and the catshark Scyliorhinus canicula (a representative chondrichthyan) late phase HoxD transcripts are present throughout the fin-fold mesenchyme and co-localize with And1, a component of the fin-fold actinotrichia and dermal skeleton. These new data support an ancestral role for HoxD genes in patterning the fin-fold compartment of jawed vertebrates, and call for a reassessment of current models of fin/limb evolution. Furthermore, these data fuel new hypotheses about the evolution of cluster regulation and the potential downstream differentiation outcomes of distinct HoxD-regulated compartments.
