The Deep Homology of the Autopod: Insights from Hox Gene Regulation

The evolution of tetrapod limbs from fish fins was a significant functional and morphological shift, but how significant was it in terms of the underlying genetic mechanisms? The fossil record provides insight into the morphological changes. However, to understand the underlying mechanisms, we must peer into the gene regulatory networks of living vertebrates. Analysis of HoxA/D expression in a basal actinopterygian, the North American paddlefish, Polyodon spathula, reveals patterns of expression long considered to be a unique developmental signature of the autopod (hands/feet, digits) and shown in tetrapods to be controlled by a "digit enhancer" regulatory landscape. These data, along with recent interspecific transgenic experiments, expression results from chondrichthyans, and data from fossils support the notion that the autopod shares a deep homology with the distal endoskeleton of the fin (distal radials) of other gnathostomes.