Discovery of a new craniofacial gene influencing adaptive phenotypes in a non-model pupfish radiation.

Understanding the origins of new species with novel adaptations is a fundamental question in biology that also provides an opportunity to uncover new genes and regulatory networks with potential clinical relevance. Here we demonstrate a new role for galr2 in vertebrate craniofacial development using a non-model adaptive radiation of trophic specialist pupfishes endemic to San Salvador Island in the Bahamas. We confirmed the loss of a Sry transcription factor binding site in scale-eating pupfish and found significant spatial differences in galr2 expression among pupfish species in the Meckel's cartilage and premaxilla using in situ hybridization. We then experimentally confirmed a novel function for Galr2 in craniofacial development and jaw elongation by exposing embryos to Galr2. Galr2-inhibition reduced Meckel's cartilage length and increased chondrocyte density in both trophic specialists but not in the generalist genetic background. We propose a mechanism for jaw elongation in scale-eaters based on the reduced expression of galr2 putatively due to the loss of the Sry binding site that results in reduced opportunities for binding with their endogenous ligand during craniofacial chondrogenesis. Fewer Galr2 receptors in the scale-eater Meckel's cartilage may result in their enlarged jaw lengths as adults by limiting opportunities for Galr2 antagonists to bind to these receptors during development. Our findings illustrate the growing utility of linking candidate adaptive SNPs in non-model vertebrate systems with highly divergent phenotypes to novel gene functions with potential implications for human biology.