Regulation of Juvenile hormone biosynthesis by ecdysteroid levels during the early stages of the last two larval instars of Bombyx mori

Abstract

Juvenile hormone (JH), produced by the corpora allata (CA), is critical for the regulation of insect postembryonic development. In the present study, a radiochemical assay was used to examine developmental changes in the JH biosynthetic rate in vitro in Bombyx mori; its regulation 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 maintained at levels ranging from 42 to 57 ng ml , the JH biosynthetic rate fluctuated but was maintained at detectable levels. During the early stages 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 instar. The ecdysteroid levels during the early stages of these two larval instars were artificially changed by various experimental manipulations, 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 subsequent development is controlled by the ecdysteroid levels present in the haemolymph. Low ecdysteroid levels are a prerequisite for CA to become inactive and thus an important developmental signal preceding larval-pupal transformation. Incubation of isolated CA from newly-ecdysed last instar larvae with 20-hydroxyecdysone revealed that 20-hydroxyecdysone directly stimulates CA activity.