RESEARCH ARTICLE A Stochastic Burst Follows the Periodic Morning Peak in Individual Drosophila Locomotion

Coupling between cyclically varying external light and an endogenous biochemical oscilla-tor known as the circadian clock, modulates a rhythmic pattern with two prominent peaks in the locomotion of Drosophila melanogaster. A morning peak appears around the time lights turn on and an evening peak appears just before lights turn off. The close association between the peaks and the external 12:12 hour light/dark photoperiod means that respec-tive morning and evening peaks of individual flies are well-synchronized in time and, conse-quently, feature prominently in population-averaged data. Here, we report on a brief but strong stochastic burst in fly activity that, in contrast to morning and evening peaks, is detectable only in single fly recordings. This burst was observed across 3 wild-type strains of Drosophila melanogaster. In a single fly recording, the burst is likely to appear once ran-domly within 0.5–5 hours after lights turn on, last for only 2–3 minutes and yet show 5 times greater activity compared to the maximum of morning peak with data binned in 3 minutes. Owing to its variable timing and short duration, the burst is virtually undetectable in popula-tion-averaged data. We use a locally-built illumination system to study the burst and find that its incidence in a population correlates with light intensity, with ~85 % of control flies showing the behavior at 8000 lux (1942 μW/cm2). Consistent with that finding, several mutant flies with impaired vision show substantially reduced frequency of the burst. Addi-tionally, we find that genetic ablation of the clock has insignificant effect on burst frequency. Together, these data suggest that the pronounced burst is likely generated by a light-acti-vated circuit that is independent of the circadian clock