Abstract
Photoperiodic control of diapause termination was systematically investigated in Pseudopidorus fasciata. In 24 h light-dark cycles, the rate of diapause termination in this species depended on photoperiod. The critical night length (CNL) for diapause termination was 10 h, 0.5 h shorter than that for diapause induction. Night-interruption experiments with T = 24 showed that diapause was effectively terminated when the scotophases separated by light pulse were shorter than the critical night length (10 h); no developing individuals were found if the duration of the pre-interruption scotophase or the post-interruption scotophase exceeded the CNL. A 15-min light pulse was sufficient to reverse the effect of long night when it was placed 8 h after lights-off. Resonance experiments with a constant photophase of 12 h or 16 h and various scotophases of 4-80 h showed an hourglass-type photoperiodic response, where no rhythmicity was found. In another resonance experiment with constant scotophase of 8 h and various photophases of 4-72 h, all individuals developed into cocoons. In the BĂźnsow experiment, the response curve showed two apparent peaks for diapause termination, one being 8 h after lights-off, and another 8 h before lights-on. However, there was no periodic rhythmicity, which again indicates an hourglass principle. The results lead to the conclusion that the same photoperiodic clock mechanism (a long-night measuring hourglass) is involved in both diapause induction and termination.
Highlights
Of all the physical factors that change seasonally, pho toperiod is the most regular and reliable cue to seasons
If an insect relies upon photoperiodic information for terminating its diapause, one might logically assume that the clock system underlying termination of diapause is the same mechanism as the one involved in induction of dia pause
Diapausing larvae were maintained at a daily mean temperature of 26.7°C in an insectarium and a constant temperature of 26 ± 1°C in the incubators (LRH-250-G) with the photoperiods ranging between 2L : 22D and 22L : 2D as well as continuous light (LL) and continuous darkness (DD)
Summary
Of all the physical factors that change seasonally, pho toperiod is the most regular and reliable cue to seasons. Photoperiodic control of diapause induction has been examined in a large number of insect species. Termination of diapause by photoperiod was reported only in about forty species (see Beck, 1980; Saunders, 1982; Koveos et al, 1993; Claret & Arpagaus, 1994). Brunnarius and Dumortier (1984) were the first to apply the same technique in an experiment on diapause termination in Pieris brassicae. They found that in both diapause induction and termina tion, it was the scotophase length that was being measured. The same conclusion was found in Tetranychus urticae (Koveos et al, 1993)
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