Pleiotropic function of the oca2 gene underlies the evolution of sleep loss and albinism in cavefish

Abstract

Adaptation to novel environments often involves the evolution of multiple morphological, physiological and behavioral traits. One striking example of multi-trait evolution is the suite of traits that has evolved repeatedly in cave animals, including regression of eyes, loss of pigmentation, and enhancement of non-visual sensory systems [1,3]. The Mexican tetra, Astyanax mexicanus, consists of fish that inhabit at least 30 caves in Northeast Mexico and ancestral-like surface fish which inhabit the rivers of Mexico and Southern Texas [6]. Cave A. mexicanus are interfertile with surface fish and have evolved a number of traits that are common to cave animals throughout the world, including albinism, eye loss, and alterations to behavior [8-10]. To define relationships between different cave-evolved traits, we phenotyped 208 surface-cave F2 hybrid fish for numerous morphological and behavioral traits. We found significant differences in sleep between pigmented and albino hybrid fish, raising the possibility that these traits share a genetic basis. In cavefish and many other species, mutations in oculocutaneous albinism 2 (oca2) cause albinism [11-15]. Surface fish with CRISPR-induced mutations in oca2 displayed both albinism and reduced sleep. Further, this mutation in oca2 fails to complement sleep loss when surface fish harboring this engineered mutation are crossed to different, independently evolved populations of albino cavefish with naturally occurring mutations in oca2, confirming that oca2 contributes to sleep loss. Finally, analysis of the oca2 locus in wild caught cave and surface fish suggests that oca2 is under positive selection in at least three cave populations. Taken together, these findings identify oca2 as a novel regulator of sleep and suggest that a pleiotropic function of oca2 underlies the adaptive evolution of both of albinism and sleep loss.