microRNA-309 targets the Homeobox gene <i>SIX4</i> and controls ovarian development in the mosquito <i>Aedes aegypti</i>

Significance We report here that microRNA-309 (miR-309) plays a critical role in ovarian development of female Aedes aegypti mosquitoes. The genetic disruption of miR-309 by CRISPR/Cas9 system displays a failure of ovarian primary follicle formation, and several pathways associated with ovarian development are down-regulated after miR-309 depletion. Comprehensive screening and functional identification have revealed that SIX homeobox 4 protein (SIX4) is a direct target of miR-309. miR-309–targeted degradation of SIX4 mRNA is required for appropriate commencement of the preparatory phase and initiation of the blood feeding-triggered phase of ovarian development. Thus, miR-309 serves as a regulatory switch permitting a stage-specific degradation of the ovarian SIX4 mRNA, allowing a shift from previtellogenic to postvitellogenic phases of ovarian development.