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Institute
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 fxed 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.
Cryptochromes are conserved flavoprotein receptors found throughout the biological kingdom with diversified roles in plant development and entrainment of the circadian clock in animals. Light perception is proposed to occur through flavin radical formation that correlates with biological activity in vivo in both plants and Drosophila. By contrast, mammalian (Type II) cryptochromes regulate the circadian clock independently of light, raising the fundamental question of whether mammalian cryptochromes have evolved entirely distinct signaling mechanisms. Here we show by developmental and transcriptome analysis that Homo sapiens cryptochrome - 1 (HsCRY1) confers biological activity in transgenic expressing Drosophila in darkness, that can in some cases be further stimulated by light. In contrast to all other cryptochromes, purified recombinant HsCRY1 protein was stably isolated in the anionic radical flavin state, containing only a small proportion of oxidized flavin which could be reduced by illumination. We conclude that animal Type I and Type II cryptochromes may both have signaling mechanisms involving formation of a flavin radical signaling state, and that light independent activity of Type II cryptochromes is a consequence of dark accumulation of this redox form in vivo rather than of a fundamental difference in signaling mechanism.