TRADE-OFF BETWEEN PARASITOID RESISTANCE AND LARVAL COMPETITIVE ABILITY IN DROSOPHILA-MELANOGASTER

The extent to which an organism is selected to invest in defences agai nst pathogens and parasites depends on the advantages that ensue shoul d infection occur, but also on the, costs of maintaining defences in t he absence of infection. the presence of heritable variation in resist ance suggests that costs exist, but we know very little about the natu re or magnitude of these costs in natural populations of animals(1). A powerful technique for identifying trade-offs between fitness compone nts is the study of correlated responses to artificial selection(2,3). We have selected Drosophila melanogaster for improved resistance agai nst an endoparasitoid, Asobara tabida. Endoparasitoids are insects who se larvae develop internally within the body of other insects, eventua lly killing them, although their hosts can sometimes survive attack by mounting a cellular immune response(4-6). We found that reduced larva l competitive ability in unparasitized D. melanogaster is a correlated response to artificial selection for improved resistance against A. t abida. The strength of selection for competitive ability and parasitoi d resistance is likely to vary temporally and spatially, which may exp lain the observed heritable variation in resistance.