The circadian eclosion rhythm inSarcophaga argyrostoma: Some comparisons with the photoperiodic “clock”

Summary

• 1. The pupal eclosion rhythm imSarcophaga argyrostoma was compared with the photoperiodic induction of diapause in the same species. The eclosion rhythm is controlled by a circadian system which free-runs in continuous darkness (DD), after exposure to light/dark cycles (LD) or to continuous light (LL), with an endogenous period, τ, close to 24 h. InLD cycles photoperiod controls the rate of larval development, and which eclosion gates are used by the emerging flies.

• 2. Sequential transfer of cultures fromLL to DD, or exposure to a 7 h delay in the light cycle at different ages, showed that the response (in terms of initiation and phase control of the eclosion rhythm) was maximal with young larvae, less with older larvae, and only marginal with the early intra-puparial stages. Older intrapuparial stages (developing adults) appear to show a renewed responsiveness to light pulses.

• 3. Phase response curves for the eclosion rhythm were “weak” or Type 1 with 1 h pulses; those for 8 h and 12 h pulses were “strong” or Type 0. After light periods in excess of about 12 h the eclosion peaks occurred at characteristic time (11.3+modulo τ h) after the light-off or “dusk” signal, provided that theLD cycles were experimentally discontinued before the end of larval development.

• 4. Cultures transferred fromLL into a dark (D) and a final light (L) period which together added up to a value (D+L) close to 24 or 48 h gave rise to highly rhythmic eclosion patterns. Those in whichD + L values were close to 36 or 60 h, however, were far less coherent. This difference is attributed to the multi-oscillator construction of the circadian system, and its internal temporal organisation.

• 5. A phase response curve for the photoperiodic oscillator was determined for 15 min resetting pulses by the use of 3-point “skeleton” photoperiods. Like the 1 h response curve for pupal eclosion, this was of the “weak” type with small “amplitude” phase changes.

• 6. The similarities between the eclosion rhythm and the photoperiodic oscillator inS. argyrostoma (same responsive period, same phase response curve, same phase relationships to driving light cycles) are discussed. The two systems are considered to be sufficiently alike for the overt system (eclosion) to be used as a measure of phase in the analysis of the photoperiodic clock, as recommended by Pittendrigh and Minis (1964).