The regulation of circadian clocks by light in fruitflies and mice

A circadian clock has no survival value unless biological time is adjusted (entrained) to local time and, for most organisms, the profound changes in the light environment provide the local time signal (zeitgeber). Over 24 h, the amount of light, its spectral composition and its direction change in a systematic way. In theory, all of these features could be used for entrai nment, but each,should be subject to considerable variation or 'noise'. Des pite this high degree of environmental noise, entrained organisms show rema rkable precision in their daily activities. Thus, the photosensory task of entrainment is likely to be very complex, but fundamentally similar for all organisms. To test this hypothesis we compare the photoreceptors that medi ate entrainment in both flies and mice, and assess their degree of converge nce. Although superficially different, both organisms use specialized (empl oying novel photopigments) and complex (using multiple photopigments) photo receptor mechanisms. We conclude that this multiplicity of photic inputs, i n highly divergent organisms, must relate to the complex sensory task of us ing light as a zeitgeber.