The effects of ultra-short photoperiods on the seasonal clock inSarcophaga argyrostoma

Abstract

The ‘external coincidence’ model for the photoperiodic clock, in its original form (e.g. Pittendrigh 1966), fails to account for the entireshape of the photoperiodic response curve in an insect such asSarcophaga argyrostoma (see Fig. 1) because it predicts as much diapause in ‘ultra-short’ daylengths (and in continuous darkness) as it does in ‘strong’ short daylengths closer to the critical photoperiod. This paper suggests, however, that a multioscillator version of external coincidence can explain the lowered incidence of pupal diapause in photoperiods of less than about 9 h, and therefore implies that external coincidence should not be rejected on this account alone. 1. Using trains of 4 h light pulses (LD 4∶20, 240 uW cm−2) with the first pulse commencing at all phases or circadian times, the incidence of diapause was shown to be negatively correlated with the number of non-steady-state or transient cycles before entrainment is achieved. Furthermore, trains of ‘weak’ pulses (1 to 8 h) gave a higher incidence of diapause when commencing at Ct 12 (the end of the subjective day) than at Ct 24 (the beginning of the subjective day), although trains of ‘stronger’ pulses (10 to 21 h) showed no such difference. 2. Increasing the irradiance of light pulses from 240 μW cm−2 to 16,000 μW cm−2 significantly increased diapause induction with 2 and 4 h pulses, but less so with pulses of 8 and 12 h duration. 3. These results are consistent with a ‘multioscillator’ photoperiodic clock inS. argyrostoma (see also Saunders 1978). Night-length measurement by such a clock is assumed to be more effective when the circadian system isat steady state with the driving light cycle. Thus, ‘weak’ light pulses starting initially far from the steady state phase relationship cause the circadian system to pass through a series of transients before entrainment, and result in a lowered diapause incidence because an insufficient number of effective (steady state) long nights are accumulated before the end of the larval sensitive period. Raising the intensity of these shorter pulses increases their ‘strength’, probably lowers the number of transients, and increases the incidence of pupal diapause. The reduced effectiveness of night-length measurement during transient cycles might be caused by an incoherence among the oscillators comprising the clock. These results, however, do nothing to distinguish ‘ external coincidence ’ from the various forms of ‘internal coincidence’, but a multioscillator version of the former remains a valid type of model forS. argyrostoma. 4. It is probable that insect species with a high sensitivity to light (i.e. a transition from Winfree's Type 1 to Type 0 resetting with short pulse durations) show a photoperiodic response curve (PPRC) in which ultra-short photoperiods (∼1 h) are strongly inductive, whereas those (likeS. argyrostoma) in which Type 1-Type 0 transitions occur with longer pulse durations show a ‘bell-shaped’ PPRC.