Entrainment of eclosion rhythm in Drosophila melanogaster populations reared for more than 700 generations in constant light environment

In this paper, we report the results of our extensive study on eclosion rhythm of four independent populations of Drosophila melanogaster that were reared in constant light (LL) environment of the laboratory for more than 700 generations. The eclosion rhythm of these flies was assayed under LL, constant darkness (DD) and three periodic light-dark (LD) cycles (T20, T24, and T28). The percentage of vials from each population that exhibited circadian rhythm of eclosion in DD and in LL (intensity of approximately 100 lux) was about 90% and 18%, respectively. The mean free-running period (τ) of eclosion rhythm in DD was 22.85 ± 0.87 h (mean ± SD). Eclosion rhythm of these flies entrained to all the three periodic LD cycles, and the phase relationship (ψ) of the peak of eclosion with respect to “lights-on” of the LD cycle was significantly different in the three periodic light regimes (T20, T24, and T28). The results thus clearly demonstrate that these flies have preserved the ability to exhibit circadian rhythm of eclosion and the ability to entrain to a wide range of periodic LD cycles even after being in an aperiodic environment for several hundred generations. This suggests that circadian clocks may have intrinsic adaptive value accrued perhaps from coordinating internal metabolic cycles in constant conditions, and that the entrainment mechanisms of circadian clocks are possibly an integral part of the clockwork.