Regulation of juvenile hormone biosynthesis by 20-hydroxyecdysone during the fourth larval instar of the tobacco hornworm, Manduca sexta

Abstract

A previous study of the role of 20-hydroxyecdysone in the regulation of juvenile hormone (JH) biosynthesis during pupal commitment in the last (fifth, V) larval instar of Manduca sexta revealed that the increase in the hemolymph edysteroid titer that elicits commitment also stimulates the corpora allata (CA) to synthesize JHs I and III and/or their acids. This stimulation is exerted indirectly via the brain-corpora cardiaca and results in the postcommitment increase in the JH titer that is important for the molt to a pupa. The possibility that a similar form of interendocrine regulation of JH biosynthesis by 20-hydroxyecdysone is involved in the control of larval molting in Manduca was investigated for the molt from the fourth (IV) to the V larval instar. During the IV instar, the increase in the hemolymph ecdysteroid titer and the capacity of the CA to synthesize JHs I and III in vitro correlated temporally in a manner suggestive of CA regulation by 20-hydroxyecdysone. With an in vitro approach, physiological concentrations of 20-hydroxyecdysone (⩾ 1 μg ml −1 hemolymph) were shown to have a dose-dependent trophic effect on CA activity, specifically stimulating JH I synthesis. Since neither basally active nor stimulated CA contained significant amounts of JH I, it appears that 20-hydroxyecdysone acts at the level of biosynthesis rather than at the level of release. The specificity of 20-hydroxyecdysone stimulation of JH I synthesis was demonstrated by the inability of the biologically inactive ecdysteroids 22-isoecdysone and 5α-ecdysone to activate the glands. 20-Hydroxyecdysone's regulation of IV instar larval CA compared with its regulation of the CA during pupal commitment is discussed.