Abstract
Daily temporal organisation of behavioural and physiological functions offers a fitness advantage for most animals. Optimized temporal niches are determined by an interplay between external environmental rhythms and internal circadian clocks. While daily light:dark cycles serve as a robust time cue (Zeitgeber) to synchronise circadian clocks, it is not clear how animals experiencing only weak environmental cues deal with this problem. Like humans, flies of the genus Drosophila originate in sub-Saharan Africa and spread North in Europe up to the polar circle where they experience extremely long days in the summer or even constant light (LL). LL is known to disrupt clock function, due to constant activation of the deep brain photoreceptor CRYPTOCHROME (CRY), which induces constant degradation of the clock protein TIMELESS (TIM). Temperature cycles are able to overcome these arrhythmia inducing effects of LL, reinstating clock protein oscillations and rhythmic behaviour. We show here that for this to occur a recently evolved natural allele (ls-tim) of the timeless gene is required, whereby the presence of this allele within the central clock neurons is sufficient. The ls-tim allele encodes a longer, less-light sensitive form of TIM (L-TIM) in addition to the shorter (S-TIM) form, the only form encoded by the ancient s-tim allele. Only after blocking light-input by removing functional CRY, s-tim flies are able to synchronise molecular and behavioural rhythms to temperature cycles in LL. Additional removal of light input from the visual system results in a phase advance of molecular and behavioural rhythms, showing that the visual system contributes to temperature synchronization in LL. We show that ls-tim, but not s-tim flies can synchronise their behavioural activity to semi-natural LL and temperature cycle conditions reflecting long Northern Europe summer days, the season when Drosophila populations massively expand. Our observations suggest that this functional gain associated with ls-tim is driving the Northern spread of this allele by directional selection.
Competing Interest Statement
The authors have declared no competing interest.
Footnotes
1)Using different methods we were not able to confirm a role for norpA in destabilizing CRY within clock neurons. Because our original and revision experiments reveal only minor differences between norpA; s-tim mutants and wild type s-tim flies with regard to molecular and behavioral synchronization to LLTC, we decided to focus the new version of the manuscript on the striking differences between the natural s-tim and ls-tim polymorphisms in the clock gene timeless. 2)Careful comparison of TIM and PER expression within clock neurons of s-tim and ls-tim flies revealed the absence of a functional molecular clock in s-tim flies under constant light and temperature cycles (LLTC). This could be confirmed on a behavioral level, by essentially identical behavior of s-tim and tim loss-of-function mutants in LLTC. 3)Removal of CRY largely restores s-tim molecular and behavioral synchronization to LLTC, implicating that the reduced interaction of L-TIM with CRY as the underlying molecular mechanism for the ability of ls-tim fly to synchronize to LLTC. We show that ls-tim expression restricted to the clock neurons only, is sufficient for robust synchronization to LLTC. 4)ls-tim, but not s-tim flies are able to synchronize their behavior to semi-natural conditions mimicking so called white nights summer conditions (3-4 hours of civil twilight, rest of the day LL, temperature cycle with 10C amplitude) in Scandinavia. We therefore propose that the ability to synchronize to LLTC is the major driving force for the ongoing seasonal directional selection of ls-tim towards Northern latitudes.
