Insulin signaling and FOXO regulate the overwintering diapause of the mosquito Culex pipiens

The short day lengths of late summer program the mosquito Culex pipiens to enter a reproductive diapause characterized by an arrest in ovarian development and the sequestration of huge fat reserves. We suggest that insulin signaling and FOXO (forkhead transcription factor), a downstream molecule in the insulin signaling pathway, mediate the diapause response. When we used RNAi to knock down expression of the insulin receptor in nondiapausing mosquitoes (those reared under long day lengths) the primary follicles were arrested in a stage comparable to diapause. The mosquitoes could be rescued from this developmental arrest with an application of juvenile hormone, an endocrine trigger known to terminate diapause in this species. When dsRNA directed against FOXO was injected into mosquitoes programmed for diapause (reared under short day lengths) fat storage was dramatically reduced and the mosquito's lifespan was shortened, results suggesting that a shutdown of insulin signaling prompts activation of the downstream gene FOXO, leading to the diapause phenotype. Thus, the results are consistent with a role for insulin signaling in the short-day response that ultimately leads to a cessation of juvenile hormone production. The similarity of this response to that observed in the diapause of Drosophila melanogaster and in dauer formation of Caenorhabditis elegans suggests a conserved mechanism regulating dormancy in insects and nematodes.