Attempting genetic inference from directional asymmetry during convergent hindlimb reduction in squamates.

Loss and reduction in paired appendages are common in vertebrate evolution. How often does such convergent evolution depend on similar developmental and genetic pathways? For example, many populations of the threespine stickleback and ninespine stickleback (Gasterosteidae) have independently evolved pelvic reduction, usually based on independent mutations that caused reduced expression. Reduced expression has also been implicated in pelvic reduction in manatees. Thus, hindlimb reduction stemming from reduced expression has arisen independently in groups that diverged tens to hundreds of millions of years ago, suggesting a potential for repeated use of across vertebrates. Notably, hindlimb reduction based on the reduction in expression produces left-larger directional asymmetry in the vestiges. We used this phenotypic signature as a genetic proxy, testing for hindlimb directional asymmetry in six genera of squamate reptiles that independently evolved hindlimb reduction and for which genetic and developmental tools are not yet developed: , , , , , and . Significant asymmetry occurred in one taxon, , whose left-side pelvis and femur vestiges were 18% and 64% larger than right-side vestiges, respectively, suggesting modification in expression in that species However, there was either right-larger asymmetry or no directional asymmetry in the other five taxa, suggesting multiple developmental genetic pathways to hindlimb reduction in squamates and the vertebrates more generally.