The cycle gene is essential for both daily responses and seasonal reproduction in the Northern house mosquito, Culex pipiens.

The insect circadian clock synchronizes daily activities, such as feeding, eclosion, and mating to diurnal light:dark cycles, while the photoperiodic clock synchronizes development and reproduction to the appropriate season. Despite mounting evidence that the circadian and photoperiodic timekeeping systems are linked, it is unclear whether the circadian clock as a whole or individual genes within the circadian clock are responsible for measuring daylength, releasing hormones and/or inducing diapause phenotypes. To interrogate the role of a core circadian transcription factor, cycle, in mediating both seasonal and daily behaviors we used CRISPR/Cas-9 genome editing to introduce a deletion mutation into cycle in the Northern house mosquito, Culex pipiens. Females homozygous for this deletion exhibited high rates of reproductive arrest characteristic of an overwintering diapause even when reared in summer-like conditions, while a minority remained reproductive even when reared in winter-like conditions. Nearly all (79%) homozygotes lacked robust behavioral rhythms in constant darkness, while 52% of heterozygotes carrying one wild-type copy of the cycle gene lacked robust rhythmicity in constant darkness. Heterozygotes and homozygotes were more active in constant darkness than WT mosquitoes, but this difference was abolished under light:dark conditions. Lastly, the daily transcription profiles of various circadian genes, especially timeless and vrille, were disrupted in homozygous females even when they were reared under long and short days with light:dark cycles. These results demonstrate that cycle is essential for coordinating both daily activity and transcript abundance and seasonal diapause in Cx. pipiens.