Circadian regulation of hemolymph and ovarian ecdysteroids during egg development in the insect Rhodnius prolixus (Hemiptera)

Abstract

Ecdysteroids, classically studied as the molting hormones of insects, occur at lower levels in the adult stage, but their source and significance in adult physiology is debated. In Rhodnius prolixus, a blood meal initiates a cycle of egg development and ecdysteroids appeared in the hemolymph within 1–2h of feeding. Systematic changes in hemolymph ecdysteroid titer then occurred during egg development. There was a clear circadian rhythm in the hemolymph ecdysteroid titer throughout egg development. This is the first report of an ecdysteroid rhythm in any adult female insect. Of various tissues examined in vitro, only ovaries released ecdysteroids. The amounts released were adequate to account for observed hemolymph ecdysteroid titers. Therefore, ovaries are the primary, perhaps sole, source of hemolymph ecdysteroids. Ovaries in vitro showed a circadian rhythm of changes in ecdysteroid content and release that was in synchrony with the hemolymph ecdysteroid rhythm, indicating that ovarian ecdysteroid release drives the rhythm seen in the hemolymph. Potential regulation of ovarian ecdysteroids is discussed in relation to the known rhythms in prothoracicotropic hormone and insulin-like peptides that occur during egg development. The prospect of circadian regulation of egg development itself by hormones and/or an ovarian clock is also discussed.