Brain-mediated 20-hydroxyecdysone regulation of juvenile hormone synthesis by the corpora allata of the tobacco hornworm, Manduca sexta

Abstract

In the latter part of the last larval instar (Days 6–7) of Manduca sexta an increase in the hemolymph titer of juvenile hormone (JH) occurs which is involved in directing the pharate pupal development elicited by the large increase in the ecdysteroid titer on Days 7–8 of the instar. A time course analysis on Days 3–4 of the instar of both the first hemolymph ecdysteroid titer peak, which is responsible for wandering behavior and pupal commitment, and of the capacity of the corpora allata (CA) to synthesize JH I and III in vitro revealed a temporal correlation between the increasing ecdysteroid titer and a following increase in CA activity. The temporal arrangement of these increases suggested that ecdysteroids, i.e., 20-hydroxyecdysone, may elicit the increase in CA activity which contributes to the second increase in the JH titer during the latter part of the instar. Regulation of the CA by 20-hydroxyecdysone was investigated in vitro by assessing the effect of this ecdysteroid on JH I and III synthesis by CA incubated either as a complex with the brain-corpora cardiaca or as isolated glands. Dose-response analyses revealed that 20-hydroxyecdysone indeed had a stimulatory effect on JH synthesis by the CA, but only when the glands were complexed with the brain-corpora cardiaca. The effective concentration of 20-hydroxyecdysone was the same as the hemolymph concentration of the hormone at this time. Thus it appears that the increase in CA activity between Days 5 and 7 of the last larval instar is elicited in response to the commitment peak of 20-hydroxyecdysone, acting indirectly via the braincorpora cardiaca. These results constitute the first direct demonstration of interhormonal regulation between the ecdysteroids and juvenile hormones in an insect, and suggest that these hormones act to regulate postembryonic development not only at the level of the target tissue but by regulating the synthesis of the hormones by the endocrine glands themselves.