Molecular mechanism and cellular distribution of insect circadian clocks

Circadian clocks are endogenous timing mechanisms that control molecular, c ellular, physiological, and behavioral rhythms in all organisms from unicel lulars to humans. Circadian rhythms influence many aspects of insect biolog y, fine-tuning life functions to the light and temperature cycles associate d with the solar day. Genetic studies in the fruit fly Drosophila melanogas ter have led to the cloning and characterization of several genes involved in the mechanism of the circadian clock. Periodic transcription and transla tion of these clock genes form the basis of a molecular feedback loop that has a "circa" 24-hour period. Rhythmic expression of clock genes in specifi c brain neurons appears to control behavioral rhythms in adult flies. Howev er, clock genes are also expressed in other tissues, both within and outsid e of the nervous system. These observations prompted chronobiologists to in vestigate whether nonneural tissues possess intrinsic circadian clocks, wha t role they may be playing, and what the relationships are between clocks i n the nervous system and those in peripheral tissues. Answers to those ques tions are providing important insights into the overall organization of the circadian system in insects.