Insect photoperiodic calendar and circadian clock: independence, cooperation, or unity?

The photoperiodic calendar is a seasonal time measurement system which allows insects to cope with annual cycles of environmental conditions. Seasonal timing of entry into diapause is the most often studied photoperiodic response of insects. Research on insect photoperiodism has an approximately 80-year-old tradition. Despite that long history, the physiological mechanisms underlying functionality of the photoperiodic calendar remain poorly understood. Thus far, a consensus has not been reached on the role of another time measurement system, the biological circadian clock, in the photoperiodic calendar. Are the two systems physically separated and functionally independent, or do they cooperate, or is it a single system with dual output? The relationship between calendar and clock functions are the focus of this review, with particular emphasis on the potential roles of circadian clock genes, and the circadian clock system as a whole, in the transduction pathway for photoperiodic token stimulus to the overt expression of facultative diapause.