Involvement of the circadian system in photoperiodic termination of diapause in the spider mite Tetranychus urticae

Abstract

In the spider mite Tetranychus urticae, both diapause induction and diapause termination are under photoperiodic control. Resonance experiments with three strains from different localities in Europe revealed the involvement of the circadian system in the photoperiodic termination of diapause. In all strains the τ-value of the rhythm involved in diapause termination appeared to be shorter than that of the rhythm involved in diapause induction. This may be caused by a change in period length of a circadian oscillator involved in both induction and termination of diapause, or it may indicate that different circadian oscillators affect diapause induction and diapause termination in the spider mite. In previous experiments it has been demonstrated that the photoperiodic clock controlling diapause termination is most probably the same as the one controlling diapause induction. In eight strains of the spider mite, originating from widely different localities in Europe, the critical nightlengths for diapause induction and diapause termination appeared to be identical. Apparently some change occurs in the circadian system between diapause induction and diapause termination, without a concurrent change in photoperiodic time measurement. This makes it unlikely that the oscillator(s) involved in diapause induction and termination would constitute the photoperiodic clock. The results are interpreted according to the so-called non-clock or resonance hypothesis for the involvement of the circadian system in photoperiodic phenomena. Temperature appeared to affect the resonance rhythm of diapause termination differently in different strains of the spider mite: in two strains the rhythm appeared to be temperature compensated, in two other strains it was not. In two strains of the spider mite, originating from the same latitude, critical nightlength appeared to be the same, whereas the period of the rhythm involved in diapause termination differed from 1–3 h among the strains, depending on temperature. These results do not provide evidence in favour of a circadian-based photoperiodic clock in T. urticae. Another effect of temperature was found on the appearance of the different resonance peaks: temperature had little effect on the time course of appearance of the first peak (at nights shorter than the critical nightlength), but subsequent peaks (at nights longer than the critical nightlength) developed faster at higher temperature. This suggests that the mechanism of diapause termination in regimes with nights shorter than the critical nightlength is different from that with nights longer than the critical nightlength. The first represents the photoperiodic response proper, controlled by the photoperiodic clock at nightlengths around the critical nightlength, whereas the latter is most probably a ‘resonance’ effect, revealing the influence of the circadian system on the expression of the photoperiodic response in extended nights. The above effect of temperature therefore also appears to corroborate the non-clock hypothesis for photoperiodic time measurement in the spider mite.