Most animals and plants possess endogenous circadian clocks. We propose that altered expression of light-inducible genes provides a selective advantage to cavefish at the expense of a damped circadian oscillator. However, data from Chica Cave shows complete repression of clock function, while expression of several light-responsive genes is raised, including DNA repair genes. Micos River samples show similar per1 oscillations to those in the lab. From a molecular standpoint, cavefish appear as if they experience ‘constant light’ rather than perpetual darkness. These changes may be due to increased levels of light-inducible genes in cavefish, including clock repressor per2. In the lab, adult surface fish show robust circadian rhythms in per1, which are retained in cave populations, but with substantial alterations. Here we examine circadian clock function in Mexican blind cavefish Astyanax mexicanus and its surface counterpart. Biological clocks have evolved as an adaptation to life on a rhythmic planet, synchronising physiological processes to the environmental light–dark cycle.
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