Ovarian ecdysone secretion is controlled by a brain hormone in an adult mosquito

Abstract

In the mosquito Aedes aegypti, a blood meal triggers egg development. Early workers used the simple technique of decapitation, or neck ligation, to show that a factor from the head was required for normal egg development^1–4. Lea subsequently demonstrated that the source of this factor was the medial neurosecretory cells of the brain⁵. The hormone from these cells, which he termed the egg development neurosecretory hormone (EDNH), is stored in the corpus cardiacum (CC) and released shortly after a blood meal⁶. Previous work had demonstrated that the ovary begins to produce ecdysone after a blood meal⁸. Considerable evidence suggests that the ecdysone, after being converted to 20-hydroxyecdysone, stimulates the synthesis and secretion of vitellogenin by the fat body^8–12. The vitellogenin is taken up by the growing oocytes and becomes part of the yolk. Analysis of the amount of 20-hydroxyecdysone present in females after a blood meal showed that significant quantities were first seen 4–6 h after a blood meal⁸. Vitellogenin synthesis begins at about the same time¹³. The early decapitation experiments of Gillett demonstrated that the head was necessary for 4–8 h after a blood meal for egg development to proceed normally³. A similar critical time was found for the control of vitellogenin synthesis by the brain hormone⁷. The coincidence of this timing suggested to us that the ovary was the target of the brain hormone. The experiments reported here were designed to test the hypothesis that the brain hormone released after a blood meal acted on the ovary, causing the synthesis of ecdysone. Our results confirm this.