Evidence for Co-Evolution of Masking With Circadian Phase in Drosophila Melanogaster

Heritable variation in the timing of rhythmic events with respect to daily time cues gives rise to chronotypes. Despite its importance, the mechanisms (clock or non-clock) regulating chronotypes remain elusive. Using artificial laboratory selection for divergent phasing of emergence of adults from pupae, our group has derived populations of Drosophila melanogaster which are early and late chronotypes for eclosion rhythm. Several circadian rhythm characteristics of these populations have since been described. We hypothesized that our selection protocol has inadvertently resulted in selection for masking, a non-clock phenomenon, in the early chronotype due to the placement of our selection window (which includes the lights-ON transition). We designed experiments to discriminate between enhanced masking to light versus circadian clock mediated changes in determining enhanced emergence in the morning window in our early chronotypes. Using a series of phase-shift protocols, LD-DD transition, and T-cycle experiments, we find that our early chronotypes have evolved positive masking, and their apparent entrained phases are largely contributed by masking. Through skeleton T-cycle experiments, we find that in addition to the evolution of greater masking, our early chronotypes have also evolved advanced phase of entrainment. Furthermore, our study systematically outlines experimental approaches to examine relative contributions of clock versus non-clock control of an entrained behavior. Although it has previously been suggested that masking may confer an adaptive advantage to organisms, here we provide experimental evidence for the evolution of masking as a means of phasing that can complement clock control of an entrained behavior.

Evidence for co-evolution of masking and circadian phase in Drosophila melanogaster

Heritable variation in the timing or circadian phases of rhythmic events with respect to daily time cues gives rise to chronotypes. Despite its importance, the mechanisms (clock or non-clock) regulating chronotypes remain elusive. Using artificial laboratory selection for divergent phasing of emergence of adults from pupae, our group has derived populations of Drosophila melanogaster which are early and late chronotypes for eclosion rhythm. Several circadian rhythm characteristics of these populations have since been described. We hypothesized that our selection protocol has inadvertently resulted in selection for masking, a non-clock phenomenon, in the early chronotype due to the placement of our selection window (which includes the lights-ON transition). Based on theoretical predictions and previous studies on our populations, we designed experiments to discriminate between enhanced masking to light versus circadian clock mediated changes in determining enhanced emergence in the morning window in our early chronotypes. Using a series of phase-shift protocols, LD-DD transition, and T-cycle experiments, we find that our early chronotypes have evolved positive masking, and their apparent entrained phases are largely contributed by masking. Through skeleton T-cycle experiments, we find that in addition to the evolution of greater masking, our early chronotypes have also evolved advanced phase of entrainment. Furthermore, our study systematically outlines experimental approaches to examine relative contributions of clock versus non-clock control of an entrained behavior. Although it has previously been suggested that masking may confer an adaptive advantage to organisms, here we provide experimental evidence for the evolution of masking as a mean of phasing of an entrained rhythm that can complement clock control of an entrained behavior.

Selection for Timing of Eclosion Results in Co-evolution of Temperature Responsiveness in Drosophila melanogaster

Circadian rhythms in adult eclosion of Drosophila are postulated to be regulated by a pair of coupled oscillators: one is the master clock that is light sensitive and temperature compensated and the other that is a slave oscillator whose period is temperature sensitive and whose phase is reflected in the overt behavior. Within this framework, we reasoned that in populations of Drosophila melanogaster that have been artificially selected for highly divergent phases of eclosion rhythm, there may be changes in this network of the master-slave oscillator system, via changes in the temperature-sensitive oscillator and/or the coupling of the light- and temperature-sensitive oscillators. We used light/dark cycles in conjunction with different constant ambient temperatures and 2 different amplitudes of temperature cycles in an overall cool or warm temperature and analyzed phases, gate width, and normalized amplitude of the rhythms in each of these conditions. We found that the populations selected for eclosion in the morning ( early flies) do not vary their phases with change in temperature regimes, whereas the populations selected for eclosion in the evening ( late flies) show phase lability of up to ~5 h. Our results imply a genetic correlation between timing of behavior and temperature sensitivity of the circadian clock.