Drosophila ezoana uses an hour‐glass or highly damped circadian clock for measuring night length and inducing diapause

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.

The characterization of the circadian clock in the olive fly Bactrocera oleae (Diptera: Tephritidae) reveals a Drosophila-like organization

“…The latter has been shown to be essential for robust activity rhythms under constant darkness 53 . Fly species lacking PDF in the s-LN v show arrhythmic activity under DD conditions 52 , 53 , 57 , 58 . In contrast to DD conditions, B. oleae flies were arrhythmic under constant light.…”

Peptidergic signaling from clock neurons regulates reproductive dormancy in Drosophila melanogaster

“…For example, D . ezoana enters diapause when day-length falls below 16 hours [96]. Although we do not know whether the s-LN v s of the high-latitude species still express sNPF, we speculate that the lack of PDF-signaling to the IPCs of these species might facilitate the termination of the reproductive state under short-day condition, and induce ovarian arrest [97].…”

Model and Non-model Insects in Chronobiology

“…These species have a high incidence of reproductive arrest already under long-day lengths, which is an adaptation to the low temperatures even under summer photoperiods at these clines. For example, D. ezoana enters diapause when day-length falls below 16 h (Vaze and Helfrich-Förster, 2016). The lack of PDF-signaling to the IPCs of these species might facilitate the termination of the reproductive state already at these relatively long days.…”