Natural Zeitgebers cannot compensate for the loss of a functional circadian clock in timing of a vital behaviour in Drosophila

Abstract The adaptive significance of adjusting behavioural activities to the right time of the day is intuitive. Laboratory studies have implicated an important role of circadian clocks in behavioural timing and rhythmicity. Yet, recent studies on clock-mutant animals questioned this importance under more naturalistic settings, as various clock mutants showed nearly normal diel activity rhythms under semi-natural Zeitgeber conditions. We here report evidence that proper timing of eclosion, a vital behaviour of the fruit fly Drosophila melanogaster , requires a functional molecular clock even under quasi-natural conditions. In contrast to wildtype flies, period 01 mutants with a defective molecular clock eclose mostly arrhythmically in a temperate environment even in the presence of a full complement of abiotic Zeitgebers. Moreover, period 01 mutants eclose during a much larger portion of the day, and peak eclosion time becomes more susceptible to variable day-to-day changes of light and temperature. Under the same conditions, flies with impaired peptidergic inter-clock signalling ( pdf 01 and han 5304 mutants) stayed largely rhythmic with normal gate sizes. Our results suggest that the presence of natural Zeitgebers can mitigate a loss of peptide-mediated phasing between central clock neuron groups, but cannot substitute for the lack of a functional molecular clock under natural temperate conditions.