The cycle gene is essential for both circadian rhythmicity and seasonal responses in Culex pipiens

In addition to measuring daily time, the circadian clock in insects has long been thought to be involved in measuring daylength and appropriately coordinating seasonal responses, but this has never been definitively demonstrated. To explore this century-old question, we used CRISPR/Cas9 genome editing to generate a line of Culex pipiens mosquitoes that lack a functional cycle gene, a core circadian oscillator that encodes a transcription factor that regulates several downstream targets that are implicated in the diapause response. After being exposed to short days, females of Cx. pipiens enter diapause that is characterized by lower levels of daily activity, arrested egg follicle development and high levels of fat. We hypothesized that cyc-null mutants would exhibit circadian arrhythmicity as indicated by locomotor activity analysis, and that these mosquitoes perceive all daylengths as short and constitutively enter diapause. Our preliminary results partially support our hypotheses: cyc-null mutant mosquitoes exhibit disrupted and, in many cases, completely arhythmic activity patterns. Mutant mosquitoes that are reared under long-day, summer-like conditions can only be induced to reproduce by applying a juvenile hormone analog; and cyc­-null mutants are unable to appropriately respond to season cues, with some entering and averting diapause under both long and short day conditions. This study demonstrates that a core component of the mosquito circadian clock plays an integral role in regulating both daily and seasonal phenotypes.