Functional genomics of the evolution of increased resistance to parasitism in Drosophila

Individual hosts normally respond to parasite attack by launching an acute immune response (a phenotypic plastic response), while host populations can respond in the longer term by evolving higher level of defence against parasites. Little is known about the genetics of the evolved response: the identity and number of genes involved and whether it involves a pre-activation of the regulatory systems governing the plastic response. We explored these questions by surveying transcriptional changes in a Drosophila melanogaster strain artificially selected for resistance against the hymenopteran endoparasitoid Asobara tabida. Using micro-arrays, we profiled gene expression at seven time points during development (from the egg to the second instar larva) and found a large number of genes (almost 900) with altered expression levels. Bioinformatic analysis showed that some were involved in immunity or defence-associated functions but many were not. Previously, we had defined a set of genes whose level of expression changed after parasitoid attack and a comparison with the present set showed a significant though comparatively small overlap. This suggests that the evolutionary response to parasitism is not a simple pre-activation of the plastic, acute response. We also found overlap in the genes involved in the evolutionary response to parasitism and to other biotic and abiotic stressors, perhaps suggesting a 'module' of genes involved in a generalized stress response as has been found in other organisms.