PARASITE MANIPULATION OF INSECT REPRODUCTION - WHO BENEFITS

Host fertility is often curtailed as a result of parasitic infection. The hypothesis that this may confer an adaptive advantage upon the sym bionts if nutrients are directed from reproduction and made available for host/parasite maintenance is explored. The suggestion is made that an understanding of the mechanisms underlying the pathophysiology of fecundity reduction may shed light upon the evolutionary implications of this strategy for both parasite and host. To illustrate this the do wn-regulation of egg production is explored with reference to a partic ular model system, the association between metacestodes of the rat tap eworm, Hymenolepis diminuta and the mealworm beetle, Tenebrio molitor. Several aspects of host reproductive behaviour and physiology are aff ected by infection in this association, including vitellogenesis. Meta cestodes directly inhibit the fat body synthesis of vitellogenin in a stage-specific, density-dependent manner. This inhibition is likely to be orchestrated by a modulator molecule, produced by the parasite. In the ovarian follicles, juvenile hormone III binding to a specific fol licular membrane-binding protein is inhibited in infected beetles, res ulting in the down-regulation of a cascade of events which enables vit ellogenin to pass into the developing oocyte. Data to support the prop osed existence of a parasite-induced antigonadotrophin, of host origin , are discussed. Evidence that similar mechanisms operate in Plasmodiu m-infected anopheline mosquitoes and Onchocerca-infected blackflies is presented in support of the possibility that a parasite-induced reduc tion in host reproductive fitness is an adaptive strategy and an asses sment of who is manipulating whom is made.