The locomotor activity rhythm of Drosophila melanogaster is controlled by a dual oscillator system

Wild-type fruit flies show a pronounced bimodal locomotor activity pattern under light–dark conditions. This pattern seems to be due to two oscillatory components — the morning and evening oscillators. Whereas the evening oscillator is controlled by the well studied clock gene period, the morning oscillator appears independent of the period gene. The present study shows that per0 mutants retain the morning oscillator. The latter is apparently not strong enough to drive rhythmic activity under conditions of constant darkness. However, under light–dark conditions the activity of per0 mutants is entrained whereby the onset of activity depends on the Zeitgeber period. This behaviour is characteristic of circadian oscillators. The nature of the morning oscillator and its putative relationship to the photoperiodic timing is discussed.