Summary:Circadian clocks are endogenous timers adjusting behaviour and physiology with the solar day 1 .Synchronized circadian clocks improve fitness 2 and are crucial for our physical and mental wellbeing 3 . Visual and non-visual photoreceptors are responsible for synchronizing circadian clocks to light 4,5 , but clock-resetting is also achieved by alternating day and night temperatures with only 2°-4°C difference [6][7][8] . This temperature sensitivity is remarkable considering that the circadian clock period (~24 h) is largely independent of surrounding ambient temperatures 1,8 .Here we show that Drosophila Ionotropic Receptor 25a (IR25a) is required for behavioural synchronization to low-amplitude temperature cycles. This channel is expressed in sensory neurons of internal stretch receptors previously implicated in temperature synchronization of the circadian clock 9 . IR25a is required for temperature-synchronized clock protein oscillations in subsets of central clock neurons. Extracellular leg nerve recordings reveal temperature -and IR25a-dependent sensory responses, and IR25a mis-expression confers temperature-dependent firing of heterologous neurons. We propose that IR25a is part of an input pathway to the circadian clock that detects small temperature differences. This pathway operates in the absence of known 'hot' and 'cold' sensors in the Drosophila antenna 10,11 , revealing the existence of novel periphery-to-brain temperature signalling channels. Main Text:In Drosophila, daily activity rhythms are controlled by a network of ~150 clock neurons expressing the clock genes period (per) and timeless (tim). These encode repressor proteins that 3 negatively feedback on their own promoters resulting in 24 h oscillations of clock molecules.Temperature cycles (TC) synchronise molecular clocks present in peripheral appendages in a tissue-autonomous manner 9,12 , while synchronization of clock neurons in the brain largely depends on peripheral temperature receptors located in the chordotonal organs (ChO) and the ChO-expressed gene nocte 9,12,13 .To discover novel factors involved in temperature entrainment, we identified NOCTEinteracting proteins by co-immunoprecipitation and mass-spectrometry (Extended Data Tab. 1) 14 . We focused on IR25a, a member of a divergent subfamily of ionotropic glutamate receptors and verified the interaction by co-immunoprecipitation after overexpressing IR25a and NOCTE in all clock cells using tim-gal4 (Extended Data Fig. 1a). IR25a is expressed in different populations of sensory neurons, including those in the antenna and labellum [15][16][17] . In the olfactory system IR25a acts as a co-receptor with different odour-sensing IRs 15 .To investigate if IR25a is co-expressed with nocte in ChO we analysed IR25a expression in femur and antennal ChO using an IR25a-gal4 line 15 (Extended Data Fig. 2a). IR25a-gal4 driven mCD8-GFP labelled subsets of ChO neurons in the femur, overlapping substantially with nompC-QF driven QUAS-Tomato signals (Fig. 1 a-c). nompC-QF is expressed in larv...

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We show that different subsets of clock neurons operate at high and low temperatures to mediate clock synchronization to temperature cycles, suggesting that temperature entrainment is not restricted to measuring the amplitude of such cycles. CRY dampens temperature input to the clock and thereby contributes to the integration of different Zeitgebers.

Circadian clocks organize biological processes to occur at optimized times of day and thereby contribute to overall fitness. While the regular daily changes of environmental light and temperature synchronize circadian clocks, extreme external conditions can bypass the temporal constraints dictated by the clock. Despite advanced knowledge about how the daily light-dark changes synchronize the clock, relatively little is known with regard to how the daily temperature changes influence daily timing and how temperature and light signals are integrated. In Drosophila, a network of ∼150 brain clock neurons exhibit 24-hr oscillations of clock gene expression to regulate daily activity and sleep. We show here that a temperature input pathway from peripheral sensory organs, which depends on the gene nocte, targets specific subsets of these clock neurons to synchronize molecular and behavioral rhythms to temperature cycles. Strikingly, while nocte mutant flies synchronize normally to light-dark cycles at constant temperatures, the combined presence of light-dark and temperature cycles inhibits synchronization. nocte flies exhibit altered siesta sleep, suggesting that the sleep-regulating clock neurons are an important target for nocte-dependent temperature input, which dominates a parallel light input into these cells. In conclusion, we reveal a nocte-dependent temperature input pathway to central clock neurons and show that this pathway and its target neurons are important for the integration of sensory light and temperature information in order to temporally regulate activity and sleep during daily light and temperature cycles.