EVOLUTION OF THE CIRCADIAN CLOCK IN EXTREME ENVIRONMENT: LESSONS FROM CAVEFISH.

Evolution has been strongly influenced by the daily cycles of temperature and light imposed by the rotation of the Earth. Fascinating demonstrations of this are seen in extreme environments such as caves where some animals have remained completely isolated from the day-night cycle for millions of years. Most of these species show convergent evolution, sharing a range of striking physical properties such as eye loss. One fundamental issue is whether “hypogean” species retain a functional circadian clock. This highly conserved, physiological timing mechanism allows organisms to anticipate daily environmental changes and is synchronized primarily by light. The Somalian cavefish, Phreatichthys andruzzii does possess a circadian clock that is entrained by a daily regular feeding time but strikingly, not by light. Under constant conditions the P. andruzzii clock oscillates with an extremely long period and also lacks normal temperature compensation. We document multiple mutations affecting a light-induced clock gene, Period2 as well as the genes encoding the extra-retinal photoreceptors Melanopsin (Opn4m2) and TMT-opsin. Remarkably, we show that ectopic expression of zebrafish homologs of these opsins rescues light induced clock gene expression in P. andruzzii cells. Thus, by studying this natural mutant we provide direct evidence for a peripheral light-sensing function of extra-retinal opsins in vertebrates. Furthermore, the properties of this cavefish illustrate that evolution in constant darkness leads not only to anatomical changes but also to loss of gene function linked with the detection and anticipation of the day-night cycle.