Conserved molecular mechanism for the stage specificity of the mosquito vitellogenic response to ecdysone

In the mosquito Aedes aegypti, the adult female becomes competent for a vitellogenic response to ecdysone after previtellogenic development. Here, we show that betaFTZ-F1, the nuclear receptor implicated as a competence factor for stage-specific responses to ecdysone during Drosophila metamorphosis, serves a similar function during mosquito vitellogenesis. AaFTZ-F1 is expressed highly in the mosquito fat body during pre- and postvitellogenic periods when ecdysteroid titers are low. The mosquito AaFTZ-F1 transcript nearly disappears in mid-vitellogenesis when ecdysteroid titers are high. An expression peak of HR3, a nuclear receptor implicated in the activation of betaFTZ-F1 in Drosophila, precedes each rise in mosquito FTZ-F1 expression. In in vitro fat body culture, AaFTZ-F1 expression is inhibited by 20-hydroxyecdysone (20E) and superactivated by its withdrawal. Following in vitro AaFTZ-F1 superactivation, a secondary 20E challenge results in superinduction of the early AaE75 gene and the late target VCP gene. Electrophoretic mobility-shift assays show that the onset of ecdysone-response competence in the mosquito fat body is correlated with the appearance of the functional AaFTZ-F1 protein at the end of the previtellogenic development. These findings suggest that a conserved molecular mechanism for controlling stage specificity is reiteratively used during metamorphic and reproductive responses to ecdysone.