Lom OMP, a putative ecdysiotropic factor for the ovary in Locusta migratoria

The primary target of the gonadotropic neurohormone of Locusta migratoria (Lom OMP) was investigated in locust fifth instar larvae (last larval instar) and adult females by analysing the effect of Lom OMP and 20-hydroxyecdysone (20-HE) on the appearance of vitellogenin (Vg) in the haemolymph and on ovarian growth, Detection of Vg was carried out using 7.5% polyacrylamide gel electrophoresis of haemolymph while ovarian growth was estimated by measuring the mean length of basal oocytes. Injections of 20-HE had no effect in larvae but induced, in adults, a precocious occurrence of Vg and precocious ovarian maturation similar to injections of Lom OMP, Moreover, injections of 20-HE counterbalanced the endogenous Lom OMP inactivated by its immune serum, Finally, Lom GRIP did not induce the appearance of Vg when it was injected in ovariectomized fifth instar larvae, These results demonstrate, in contrast to the prevailing concept, that ecdysone is a gonadotropic hormone in Locusta and strongly suggest that Lom OMP induces ovarian ecdysone production, The possibility that not only the ovary but also the fat body could be a target organ of Lom OMP is discussed.