Introduction to the symposium--keeping time during evolution: conservation and innovation of the circadian clock.

Diurnal and seasonal cues play critical and conserved roles in behavior, physiology, and reproduction in diverse animals. The circadian clock is a transcription-translation feedback loop that represents the molecular mechanism underlying many of these periodic processes, frequently through responses to light. Although much of the core regulatory machinery is deeply conserved among diverse animal lineages, there are also many examples of innovation in the way the clock either is constructed at the molecular-level or deployed in coordinating behavior and physiology. The nine articles contained within this issue address aspects of circadian signaling in diverse taxa, utilize wide-ranging approaches, and collectively provide thought-provoking discussion of future directions in circadian research.