The deep homology of the autopod: insights from hox gene regulation

Synopsis 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 morpho-logical 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