Juvenile hormone biosynthesis in larval and adult stick insects, Carausius morosus

Corpora allata (CA) from adult egg-carrying Indian stick insects, Carausius morosus, synthesise and release juvenile hormone (JH) III in vitro. No JH biosynthesis was observed in larvae, young adults, and old adult females that do not carry sclerotised eggs. In females, which bear sclerotised eggs, a consistent JH biosynthesis was observed. Supplementation of precursors of JH biosynthesis (farnesol, mevalonic acid lactone) greatly enhanced JH biosynthesis in a stage-, age-, and dose-dependent manner, but CA from the last larval instar retained the biosynthesised JH within the gland. Elevated calcium concentration in the incubation medium stimulated JH biosynthesis by CA from older adults but had either no or a poor effect on CA from young adults and larvae. The results obtained with farnesol, mevalonic acid lactone, and calcium indicate that the rate-limiting steps of JH biosynthesis very likely occur before the formation of mevalonic acid and that these early steps cannot be stimulated by elevated calcium concentrations in larvae and young adults. In older adults, in which spontaneous JH biosynthesis occurs, elevated calcium concentration can markedly stimulate JH biosynthesis. A pre-purified extract from brains of adult females had a stimulating effect on JH biosynthesis by CA from adult females. The results indicate that JH biosynthesis in C. morosus may require food-derived farnesol and may be regulated by allatotropic signals from the brain, possibly triggered by sclerotised oocytes in the ovary.