Circadian rhythmicity participates in the photoperiodic determination of diapause in spider mites

Abstract

The role of circadian rhythmicity in photoperiodic processes in plants and animals is still poorly understood. Involvement of circadian rhythmicity in photoperiodism has been demonstrated for a number of plants1,2, two mammals3,4, a number of birds5,6, and some species of insects7–11. Three theoretical alternatives have been suggested with regard to the form this involvement may take, with emphasis on the possibility that the circadian system is concerned with photoperiodic time measurement12–14. Most of the evidence for the involvement of the circadian system in both plant and animal photoperiodism comes from experiments with nondiel photoperiods—light/dark cycles with period lengths other than 24 h. The experimental design which has been used most frequently is the so-called resonance technique, in which the light component of a light/dark cycle is held constant and the dark component is varied over a wide range in successive experiments, to provide cycles with period lengths up to 72 h or more. Rhythmic variations in the photoperiodic response with peaks about 24 h apart are evidence for the involvement of circadian rhythmicity in the photoperiodic process studied. However, in some plants1, a reptile15, and a number of insect species16–20, the resonance technique failed to demonstrate any influence of circadian rhythmicity on photoperiodism. Using the same technique we have now found a rhythmic response in the incidence of diapause in the common spider mite, Tetranychus urticae, thus showing that in mites also, circadian rhythmicity is involved in the photoperiodic reaction.