Geographic variation and evolution of immunosuppressive genes in a Drosophila parasitoid

Parasitoids that must kill the host to complete their development are expected to evolve towards increased virulence. In some conditions however Leptopilina boulardi loses its ability to counteract the host immune reaction. This trait is determined by major genes. For each host species there is a specific gene for immune suppression by the parasite. Here the geographic variations of the immunosuppressive gene frequencies are investigated in relation to the distribution of the host species. The necessity to deal with host immunity is a major constraint on the host range of L. boulardi. Against Drosophila simulans and D. yakuba, the presence of the immunosuppressive allele is correlated with the presence of the host species in the locality. Against D. melanogaster, the data suggest that this gene is counterselected when the parasite is exposed to numerous host species. This counterselection is explained by the existence of a cost of immunosuppressive genes. Against D. yakuba, this cost was evaluated in population cages as a selective coefficient of s = -0.20. The cost differs between the genes. Against D. melanogaster, it was not significant in population cage conditions. The parasitoid invests more in the suppression of the D. yakuba reaction than that of D. melanogaster. This variation of the investment in immunosuppression is discussed within the framework of the adaptive budget theory.