REGULATION OF JUVENILE-HORMONE BIOSYNTHESIS BY ECDYSTEROID LEVELS DURING THE EARLY STAGES OF THE LAST 2 LARVAL INSTARS OF BOMBYX-MORI

Juvenile hormone (JH), produced by the corpora allata (CA), is critica l for the regulation of insect postembryonic development, In the prese nt study, a radiochemical assay was used to examine developmental chan ges in the JH biosynthetic rate in vitro in Bombyx mori; its regulatio n by the ecdysteroid levels in the haemolymph during the early stages of the last two larval instars was clarified, During the early stages of the penultimate larval instar, the ecdysteroid titres were maintain ed at levels ranging from 42 to 57 ng/ml, the JH biosynthetic rate flu ctuated but was maintained at detectable levels, During the early stag es of the last larval instar, the ecdysteroid titres decreased to very low levels (3-12 ng/ml), CA ceased to produce JH midway through the i nstar, The ecdysteroid levels during the early stages of these two lar val instars were artificially changed by various experimental manipula tions, then, changes in the JH biosynthetic rate were assessed and the subsequent development of larvae was carefully examined, We concluded that whether CA continued to produce JH or became inactive during sub sequent development is controlled by the ecdysteroid levels present in the haemolymph, Low ecdysteroid levels are a prerequisite for CA to b ecome inactive and thus an important developmental signal preceding la rval-pupal transformation, Incubation of isolated CA from newly-ecdyse d last instar larvae with 20-hydroxyecdysone revealed that 20-hydroxy- ecdysone directly stimulates CA activity. Copyright (C) 1996 Elsevier Science Ltd