External coincidence and the photoinducible phase in theSarcophaga photoperiodic clock

Abstract

1. Data from single light pulse resetting experiments on the circadian rhythm of pupal eclosion inSarcophaga argyrostoma were used to design and predict the outcome of one- and two-pulse cycles in terms of the induction of pupal diapause. Experiments were interpreted in terms of the “External Coincidence” model of Pittendrigh (1966). 2. Two-pulse asymmetrical “skeleton” photoperiods (night-interruption experiments), in which a short supplementary pulse scans the “night” of a diapause-inductive cycle, show two points (A and B) of short-night effect (the non-diapause or “summer” pathway). The External Coincidence model suggests that the photoinducible phase (φi) lies at point B. 3. Single short pulses of light (1 h) were used in cycles of different length (so-called T-experiments) in such a way that the light pulses fell on particular circadian phases in each cycle. “Short-night” or non-diapause effects were only produced when the light pulse fell, in each cycle, on that phase (Circadian time, Ct, 21.5) equivalent to point B. 4. In T-experiments comprising l h pulses of light in a cycle of LD 1∶20.5 (T 21.5), with the first pulse in the train starting at different circadian phases, the incidence of diapause was shown to be a function of the number of transient cycles before the circadian pacemaker achieved steady-state entrainment to the light cycle. 5. In two-pulse asymmetric “skeletons” in which the scanning pulse was arranged to fall on point A, it was shown that the diapause-averting effects of the scanning pulse could be “reversed” by a terminal dark period greater than the critical night length. In asymmetrical skeletons in which the pulse fell on point B, however, the diapause-averting effects were irreversible. These experiments demonstrate that the photoinducible phase lies late in the subjective night at a circadian phase (Ct 21.5) marked by point B in night interruption experiments. 6. Although the results do not exclude the possibility of an “Internal Coincidence” type of photoperiodic clock (Pittendrigh, 1972) they remain consistent with the External Coincidence model as adopted in earlier papers forS. argyrostoma. The results also underscore the essential similarities between the photoperiodic clocks inSarcophaga and the aphidMegoura viciae (Lees, 1973).