Reorganisation of circadian activity and the pacemaker circuit under novel light regimes

Many environmental features are cyclic, with predictable changes across the day, seasons and latitudes. Additionally, anthropogenic, artificial light-induced changes in photoperiod or shiftwork-driven novel light/dark cycles also occur. Endogenous timekeepers or circadian clocks help organisms cope with such changes. The remarkable plasticity of clocks is evident in the waveforms of behavioural and molecular rhythms they govern. Despite detailed mechanistic insights into the functioning of the circadian clock, practical means to manipulate activity waveform are lacking. Previous studies using a nocturnal rodent model showed that novel light regimes caused locomotor activity to bifurcate such that mice showed two bouts of activity restricted to the dimly lit phases. Here, we explore the generalizability of these findings and leverage the genetic toolkit of Drosophila melanogaster to obtain mechanistic insights into this unique phenomenon. We find that dim scotopic illumination of specific durations induces circadian photoreceptor CRYPTOCHROME-dependent activity bifurcation in male flies. We show circadian re-organisation of the pacemaker circuit wherein the ‘evening’ neurons regulate the timing of both bouts of activity under novel light regimes. Our findings indicate that environmental regimes can be exploited to design light regimes to ease the circadian waveform into synchronising with challenging conditions. 

Methods
Data was collected using Drosophila activity monitors. The activity counts have been normalised by the total amount of activity in a day and the percentage values have been plotted in activity profiles. The same activity profiles have been provided in the dataset. Bifurcation index has been calculated using raw activity counts. The details of which can be found in the associated preprint. Values of bifurcation index plotted in each of figures have also been included in the dataset.