Splicing of a core clock gene regulates seasonal adaptations by a winter gating mechanism

Seasons bring changes to the environment. Many organisms adjust their physiology and behavior in response to seasonal changes in order to survive. Although the molecular mechanisms mediating the integration of seasonal cues are still unclear, the working model indicates the involvement of the circadian clock. Notably, the circadian neuropeptide Pigment Dispersing Factor (PDF), an output of the circadian clock, has been shown to alter its expression and activity in response to seasonal changes to facilitate seasonal adaptations in insects. Here, we show that the alternative splicing (AS) of a circadian clock gene, timeless (tim), regulates the seasonal responses of PDF signaling in Drosophila melanogaster. We first showed that tim-sc, the predominant isoform in winter, is regulated by photoperiod in addition to temperature, while the expression of the canonical tim-l isoform is primarily sensitive to temperature. We then demonstrated that tim-sc maintains physiology and behavior in a "winter lock" state by modulating PDF. At the cellular and molecular level, TIM-SC behavior differs from the canonical TIM-L. Interestingly, flies expressing tim-sc did not fully phenocopy wild-type flies reared in winter conditions, suggesting that other mechanisms are at play in regulating seasonal adaptations, despite the importance of tim AS.