Evolved Loss of Sleep and Circadian Rhythms in Cavefish

Sleep and locomotor rhythms are nearly universal behaviors that are critical for the function of diverse biological processes. While the timing and duration of sleep vary dramatically throughout the animal kingdom, little is known about the genetic or evolutionary basis underlying species-specific differences in these behaviors. Growing evidence suggests differences in sleep duration and timing are influenced by the ecological environment and evolutionary history. Cave environments insulated from the effects of the day-night cycle provide a unique opportunity to investigate the function and evolution of sleep and circadian clocks. Over 200 species of cavefish have been identified throughout the world, allowing for the investigation of trait evolution in a relatively constant environment. The Mexican cavefish and Somalian cavefish, in particular, have emerged as highly useful laboratory models to investigate the mechanistic and ecological basis for the evolution of circadian rhythms. Furthermore, the Mexican cavefish has provided new insight into the evolution and functional significance of sleep. These species display loss of both circadian processes and light detection under very different environmental conditions and timescales. Here, we review recent findings that identify roles for sensory processing, neuromodulation, and DNA repair pathways in the evolution of sleep and circadian rhythms. Furthermore, we highlight the implementation of gene-editing and transgenic tools that allow for mechanistic studies of the neural basis for the evolution of sleep and circadian rhythms. These studies have potential to shed light on the fundamental functions of sleep-wake regulation and the genetic basis for interspecies trait diversity throughout the animal kingdom.