Mechanisms of photic entrainment of activity/rest rhythms in populations of Drosophila selected for divergent timing of eclosion

It is a common notion that phases-of-entrainment of circadian rhythms are adaptive, in that they enable organisms to time their behavior to specific times of the day to enhance their fitness. Therefore, understanding mechanisms that bring about such phases-of-entrainment is crucial to chronobiologists. Our previous studies have shown that selection for morning and evening phasing of adult emergence in Drosophila melanogaster populations leads to divergent coevolution of free-running periods of both adult emergence and activity/rest rhythms, such that early (morning) and late (evening) adult emergence chronotypes have shorter and longer circadian periods, respectively. However, there is little evidence to support the notion that phases-of-entrainment in these fly stocks is indeed driven by non-parametric mechanisms. Extending from a previous hypothesis based on anecdotal evidence for parametric mechanisms being in play, we explore the extent of non-parametric and parametric effects of light on circadian clocks of early and late chronotypes. We systematically tested predictions of the non-parametric model of entrainment, sketched the Circadian Integrated Response Characteristic (CIRC) of our stocks, assessed the effect of light pulses on amplitude of the behavior and the effect of duration of light pulse on phase-shifts of the clock. We demonstrate that, in addition to the differences in clock period, divergent CIRCs contribute to entrainment of the activity/rest rhythm. The differences in CIRC could be explained by differential transient amplitude responses and duration responses of the clock’s phase between the early and late chronotypes. Our study thus highlights the role of amplitude responses and phase-shifts due to long durations of light in entrainment of circadian rhythms of D. melanogaster.