Parallel genetic origins of pelvic reduction in vertebrates

<jats:p>Despite longstanding interest in parallel evolution, little is known about the genes that control similar traits in different lineages of vertebrates. Pelvic reduction in stickleback fish (family Gasterosteidae) provides a striking example of parallel evolution in a genetically tractable system. Previous studies suggest that cis-acting regulatory changes at the<jats:italic>Pitx1</jats:italic>locus control pelvic reduction in a population of threespine sticklebacks (<jats:italic>Gasterosteus aculeatus</jats:italic>). In this study, progeny from intergeneric crosses between pelvic-reduced threespine and ninespine (<jats:italic>Pungitius pungitius</jats:italic>) sticklebacks also showed severe pelvic reduction, implicating a similar genetic origin for this trait in both genera. Comparative sequencing studies in complete and pelvic-reduced<jats:italic>Pungitius</jats:italic>revealed no differences in the<jats:italic>Pitx1</jats:italic>coding sequences, but<jats:italic>Pitx1</jats:italic>expression was absent from the prospective pelvic region of larvae from pelvic-reduced parents. A much more phylogenetically distant example of pelvic reduction, loss of hindlimbs in manatees, shows a similar left–right size bias that is a morphological signature of<jats:italic>Pitx1</jats:italic>-mediated pelvic reduction in both sticklebacks and mice. These multiple lines of evidence suggest that changes in<jats:italic>Pitx1</jats:italic>may represent a key mechanism of morphological evolution in multiple populations, species, and genera of sticklebacks, as well as in distantly related vertebrate lineages.</jats:p>