Ecdysteroid titres during pupal-adult development of Aedes aegypti: Basis for a sexual dimorphism in the rate of development

The ecdysteroids are steroid hormones primarily responsible for moulting and metamorphosis in insects. In mosquitoes, much is known about their role in the control of ovarian development in adults, however, essentially nothing is known about their control of this dipteran's postembryonic development. In this study, quantitative and qualitative changes in the ecdysteroid titre during the pupal-adult development of the yellow fever mosquito, Aedes aegypti, were correlated with a sexual dimorphism in the time of adult eclosion. During development there was a single large peak in the ecdysteroid titre of male and female Aedes. The peak in males (∼220 pg ecdysone equivalents mg −1), was ∼1.5-fold greater than that in females (∼150 pg ecdysone equivalents mg −1), and occurred ∼4 h sooner. Subsequently, males eclosed ∼12 h sooner than females. The ecdysteroids present in developing males and females included, ecdysone, 20-hydroxyecdysone, 26-hydroxyecdysone, 20, 26-dihydroxyecdysone and polar metabolites. The relative amounts of these ecdysteroids and their conjugates fluctuated during development, and were essentially the same in males and females. When total ecdysteroid levels were calculated relative to the radioimmunassay cross-reactivities of the ecdysteroids present, a quantitative sexual difference was not evident. Therefore, it appears that the sexual dimorphism in the rate of Aedes development is probably a result of temporal differences in the respective ecdysteroid titres. A biological explanation for the earlier eclosion of male Aedes is presented.