Abstract
Insects inhabiting the temperate zones measure seasonal changes in day or night length to enter the overwintering diapause. Diapause induction occurs after the duration of the night exceeds a critical night length (CNL). Our understanding of the time measurement mechanisms is continuously evolving subsequent to Bünning's proposal that circadian systems play the clock role in photoperiodic time measurement (Bünning, 1936). Initially, the photoperiodic clocks were considered to be either based on circadian oscillators or on simple hour‐glasses, depending on ‘positive’ or ‘negative’ responses in Nanda–Hamner and Bünsow experiments (Nanda & Hammer, 1958; Bünsow, 1960). However, there are also species whose responses can be regarded as neither ‘positive’, nor as ‘negative’, such as the Northern Drosophila species Drosophila ezoana, which is investigated in the present study. In addition, modelling efforts show that the ‘positive’ and ‘negative’ Nanda–Hamner responses can also be provoked by circadian oscillators that are damped to different degrees: animals with highly sustained circadian clocks will respond ‘positive’ and those with heavily damped circadian clocks will respond ‘negative’. In the present study, an experimental assay is proposed that characterizes the photoperiodic oscillators by determining the effects of non‐24‐h light/dark cycles (T‐cycles) on critical night length. It is predicted that there is (i) a change in the critical night length as a function of T‐cycle period in sustained‐oscillator‐based clocks and (ii) a fixed night‐length measurement (i.e. no change in critical night length) in damped‐oscillator‐based clocks. Drosophila ezoana flies show a critical night length of approximately 7 h irrespective of T‐cycle period, suggesting a damped‐oscillator‐based photoperiodic clock. The conclusion is strengthened by activity recordings revealing that the activity rhythm of D. ezoana flies also dampens in constant darkness.
Highlights
Photoperiodic time measurement is referred to as the ability to measure seasonal variation in day/night length and is an integral feature of the seasonal adaptations in organisms inhabiting temperate zones
The critical night length for diapause induction in D. ezoana is independent of T: the flies appear to measure night length and to use a strongly damped oscillator to do this showed the expected shape with a sharp reduction of diapause incidence when night length decreased below a certain percentage of T-cycle period (Fig. 4)
The T-cycle study shows that the photoperiodic clock of the northern fruit fly species D. ezoana measures an absolute night length of approximately 7 h to induce diapause, irrespective of the photoperiod and the period of T-cycles (Fig. 5)
Summary
Photoperiodic time measurement is referred to as the ability to measure seasonal variation in day/night length and is an integral feature of the seasonal adaptations in organisms inhabiting temperate zones. In 1936, Erwin Bünning proposed a role for the circadian clock in photoperiodic time measurement. Bünning’s model assumes two distinct phases of the circadian cycle, namely the photophil and scotophil phases, whereby the exposure of the scotophil phase to light is dependent on the photoperiod. He proposed that flowering is induced in the common bean Phaseolus vulgaris only when its scotophil phase is not exposed to light (Bünning, 1936).
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