Appendage expression driven by the Hoxd Global Control Region is an ancient gnathostome feature

Abstract

The evolutionary transition of the fins of fish into tetrapod limbs involved genetic changes to developmental systems that resulted in novel skeletal patterns and functions. Approaches to understanding this issue have entailed the search for antecedents of limb structure in fossils, genes, and embryos. Comparative genetic analyses have produced ambiguous results: although studies of posterior Hox genes from homology group 13 (Hoxa-13 and Hoxd-13) reveal similarities in gene expression between the distal segments of fins and limbs, this functional homology has not been supported by genomic comparisons of the activity of their cis-regulatory elements, namely the Hoxd Global Control Region. Here, we show that cis-regulatory elements driving Hoxd gene expression in distal limbs are present in fish. Using an interspecies transgenesis approach, we find functional conservation between gnathostome Hoxd enhancers, demonstrating that orthologous sequences from tetrapods, zebrafish and skate can drive reporter gene expression in mouse limbs and zebrafish fins. Our results support the notion that some of the novelties associated with tetrapod limbs arose by modification of deeply conserved cis- and trans-acting mechanisms of Hox regulation in gnathostomes.