Circadian rhythms of behavioral and physiological processes are driven by endogenous oscillators synchronized to environmental cycles. They represent an adaptive advantage that allows organisms to predict and anticipate cyclic environmental changes. The daily cycles of light-dark and the food availability are the most prevailing signals for the entrainment of circadian oscillators. However, the physiological mechanisms whereby light and food regulate the clock remain incompletely understood. To shed light on the evolution of the biological clock in vertebrates we have investigated at molecular and behavioural levels the circadian clock of cavefish. Cavefish have evolved in extreme hypogean environments characterized to constant darkness, stable temperature, and low nutrient levels. They show a convergent evolution, sharing a range of striking troglomorphic phenotypes such as anophthalmia, depigmentation, and reduced metabolic rate. We have performed a deep analysis of the circadian clock of cavefish investigating the effect of different feeding and lighting conditions on the behavioural rhythmicity and the clock gene expression. To do this we studied the Somalian cavefish Phreatichthys andruzzii in comparison with the zebrafish Danio rerio, that is normally exposed a cyclic environment. Previous investigation showed a strong feeding entrainment of locomotor activity for both species. Here, using different paradigms of periodic food availability, we show that food-entrainable oscillators are more strong in cavefish respect to zebrafish. Furthermore, in parallel, we investigated photic entrainment of locomotor activity in adult and larvae of fish exposed to different monochromatic light. Interestingly, while zebrafish showed a normal entrainment under all light-dark cycles tested, P.andruzzii changed the behavioral pattern (e.g. arrhythmicity, negative phototaxis) in response to the light stimulus. In order to explain at the molecular level these results, we examined the expression pattern of a set of opsin and clock gene homologs. Both behavioral and molecular characterizations can be the basis to understand these complex biological processes and also demonstrate the great utility of cavefish to study the evolution of the circadian clock.
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