Developmental state and the circadian clock interact to influence the timing of eclosion in Drosophila melanogaster.

In Drosophila melanogaster, the emergence of adults from their pupal cases (eclosion) is gated by the circadian clock such that it occurs during a window of approximately 8-10 h starting 1-2 h before lights-on in 12-h light:12-h dark cycles (LD). This gate is shifted several hours earlier by the clock mutant per(s), indicating that the clock controls the phase of eclosion under these conditions. Both the day and the time of eclosion are determined by the interplay between developmental state and the circadian clock. At a certain phase of the circadian cycle, the circadian clock, either directly or through some circadian clock-controlled mechanism, measures development state, and those pharate adults that have reached a certain developmental state by this phase eclose during the first available gate, while those that have not wait until a subsequent gate. Using wing pigmentation as a late developmental state marker, an early boundary for when the circadian clock assesses developmental state occurs roughly at the time when lights go out during LD cycles. This event is shifted several hours earlier in per(s), showing that it is under circadian control. A fly's developmental state at the time of developmental assessment also influences when eclosion will occur (during the gate) in that flies whose wings have become pigmented early (12-24 h before assessment) will eclose earlier in the gate than those whose wings become pigmented late (0-12 h before assessment). These data suggest that the circadian clock (or some clock-controlled mechanism) measures developmental state (wing pigmentation) in wild-type flies between lights-off and expression of the first clock-regulated marker approximately 4-5 h before eclosion and that the developmental state of the fly determines both which gate is chosen for eclosion and when eclosion occurs during that gate.