Regulation of locust vitellogenesis

Abstract

The reproduction of insects is frequently synchronized to the availability of specific host plants or to seasonal climatic condi- tions. Synchronization is achieved in most cases either by migration to favourable habitats or by diapause, a genetically determined state of suppressed develop- mental and metabolic rates, frequently induced by photoperiod, which enables the insect to over-winter (hibernate) or aesti- vate. In most cases, diapause involves the cessation of neuroendocrine activity. Some of the species of migratory locusts known from different parts of the world also diapause as adults and thus make use of both mechanisms for synchronization 1. Adult (or reproductive) diapause is charac- terized by the absence of vitellogenesis - the synthesis of female-specific egg-yolk protein which is produced in the fat body of locusts (a tissue analogous to the liver of vertebrates) and transported in the haemolymph to the developing oocyte. In conjunction, migration often commences during the previtellogenic phase of locust development. The vitellogenin molecule is a complex glycolipophosphoprotein that appears to have been highly conserved during evolu- tion. Perhaps this is related to the numerous constraints imposed on it subsequent to its primary translation and post-translational modification. These include: secretion and transport (and resistance at that stage to hydrolytic degradation), discriminatory recognition by the plasma membrane of the developing oocyte, specific uptake and packaging as yolk platelets. Some features of locust vitellogenesis and vitellogenin are similar to vertebrate systems with which we are more familiar. Other features, in particular, hormonal con- trol, but also structural features, such as low phosphate content and degree of proteolytic processing, differ remarkably from verte- brate systems. The extent to which such dif- ferences can be exploited to design agents for control of insect reproduction bears on the relevance of this approach to modern agricultural practice. Hormonal control of vitellogenesis Vitellogenin is induced in the vertebrate liver by estrogens 2. In locusts, no steroid hormones are known to participate in vitel- logenin induction. Instead, a class of hor-