An experimental and theoretical analysis of photoperiodic induction in the flesh-Fly,Sarcophaga argyrostoma
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The circadian rhythm of pupal eclosion inSarcophaga argyrostoma was used as a “measure” of the phase of the covert photoperiodic oscillation in an experimental and formal analysis of photoperiodic induction, particularly in terms of entrainment and phase coherence within the multioscillator circadian system.
Phase response curves for the eclosion pacemaker were obtained in single pulse resetting experiments for a series of pulse-lengths (1 to 20 h). These data were also analysed for “coherence” of the eclosion peaks in terms of Winfree's (1970) arrhythmicity or R-values.
One and 3 h pulses of white light (240 μW cm−2) gave rise to “weak” or Type 1 resetting curves, whereas pulses of 5 h or more gave rise to “strong” or Type 0 curves. Pulses close to 4 h in duration starting at points close to 4 h after the transition fromLL to DD gave rise to highly arrhythmic eclosion patterns, suggesting that this combination of pulse duration and phase moved the system on to its “singularity.”
Arrhythmicity was also observed in cultures in which the number of hours of darkness between theLL/DD transition and the beginning of the resetting pulse (D hours), and the duration of the pulse (L hours) added up to a value (D+L) close to 12, 36, 60, 84 or 108 h (modulo τ+1/2τ). Highly rhythmic cultures, however, were obtained when D+L added up to values close to 24, 48, 72, 96 or 120 h (modulo τ).
The phase response curves for single 12 h pulses of white light were followed through the first 5 days of larval life. In the first two cycles following the transition fromLL to DD these pulses gave rise to strong (Type 0) curves; in subsequent cycles these curves “decayed” from Type 0 to Type 1. This change is thought to be associated with a developmental change, perhaps in the photoreceptor or its coupling to the “clock” or merely to a change in behaviour.
The data obtained from phase response curves were used in a computer program to calculate steady-state entrainment to a variety of “complete” and “skeleton” photoperiods. These computed data were also compared with those experimentally determined for cultures exposed to identical regimes, and to diapause induction data obtained from earlier experiments.
In “complete” photoperiods (T=24 h) the median of eclosion (φr) was shown to phase-lead dawn in cycles containing less than 14 h of light, but to phase-lag dawn in longer photoperiods. The point at whichφr crossed the “dawn threshold” closely matched the value of the critical photoperiod for diapause induction. In symmetrical “skeletons” (T= 24 h) and asymmetrical “skeletons” (T=24 and 72 h) computed phases ofφr, observed phases ofφr, and diapause induction data were all in close agreement.
The results are interpreted in terms of Pittendrigh's (1966) “external coincidence” model for the photoperiodic clock, the model which appears to offer the most plausible explanation for photoperiodic induction in this species. The model is modified, however, to incorporate the degree of internal organisation or disorganisation within the multioscillator circadian system. In particular, this accounts for the fall in diapause incidence in ultra-short daylengths, and for the results of “resonance” experiments.
The “external coincidence” model, as adopted forS. argyrostoma, is compared with the formal properties of the photoperiodic clock in other insect species which have been adequately investigated. In particular, the strong similarities betweenS. argyrostoma and the aphid,Megoura viciae (Lees, 1973), are stressed.
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