Prolonged oviposition decreases the ability of the parasitoid Leptopilina boulardi to suppress the cellular immune response of its host Drosophila melanogaster

Abstract

The cellular immune response of Drosophila against metazoan parasites is characterized by the production of melanotic capsules comprised mostly of host blood cells (hemocytes). During the latter part of the ovipositional period of the cynipid wasp parasitoid Leptopilina boulardi, eggs are deposited into host larvae of Drosophila melanogaster that are more susceptible to destruction by melanotic encapsulation than are eggs laid earlier. The increase in parasitoid mortality is attributed to a decline in the wasp's ability to suppress the host immune response. The decrease in active immune suppression is dependent on the reproductive physiology of the wasp, and this correlates with the extent of her prior ovipositional experience and not on her chronological age nor on the number of eggs remaining in the ovarioles. Such females with prior ovipositional experience which lack the ability to immune suppress infect far fewer hosts than females with no prior ovipositional experience. The reluctance of experienced wasps to infect hosts is not due to egg depletion, but instead is attributed to a depletion in immune suppressive substances. Perhaps by ovipositional restraints, retaining eggs that would otherwise become encapsulated reduces selection pressure in host populations for specific immune reactivity.