Therx3gene contributes to the evolution of eye loss in the cavefishAstyanax mexicanus

Abstract

Uncovering mechanisms by which sensory systems evolve is critical for understanding how organisms adapt to a novel environment.Astyanax mexicanusis a species of fish with populations of surface fish that inhabit rivers and streams and cavefish that have adapted to life within caves. Cavefish have evolved sensory system changes relative to their surface fish counterparts, providing an opportunity to investigate mechanisms underlying sensory system evolution. Here, we report the role of the generetinal homeobox 3(rx3) in cavefish eye evolution. We generated surface fish with putative loss-of-function mutations in therx3gene using CRISPR-Cas9 to determine the role of this gene in eye development in this species. Theserx3mutant surface fish fail to develop eyes, demonstrating thatrx3is required for surface fish eye development. Further,rx3mutant surface fish exhibit altered behaviors relative to wild-type surface fish, suggesting that the loss of eyes impacts sensory-dependent behaviors. Finally, eye development is altered in cave-surface hybrid fish that inherit the mutant allele ofrx3from surface fish relative to siblings that inherit a wild-type surface fishrx3allele, suggesting that cis-regulatory variation at therx3locus contributes to eye size evolution in cavefish. Together, these findings demonstrate that, as in other species,rx3is required for eye development inA. mexicanus. Moreover, they suggest that variation at therx3locus plays a role in the evolved reduction of eye size in cavefish, shedding light on the genetic mechanisms underlying sensory system evolution in response to extreme environmental changes.