A single parasitoid segregating factor controls immune suppression in Drosophila

Encapsulation has evolved as an efficient mechanism whereby an insect host can survive infection by parasitoids This ability is controlled by a major gene in Drosophila melanogaster hosts. The parasitoid Leptopilina boulardi (Hymenoptera Eucoilidae) can suppress the Drosophila immune reaction by injecting viruslike particles. Analysis of Mendelian crosses between strains of L. boulardi of opposite immune suppressive abilities indicated that the trait is controlled by a single chromosomal factor with semidominant effect. We developed a method to test the monogenic hypothesis. The range of possible genotypic values in back-crosses was studied using various progeny that were genotypically homogenous. These could be obtained because of the arrhenotokous mode of reproduction. The progeny groups were divided into two clusters according to the major gene classification and the hypothesis of another unlinked genetic factor was rejected. Lastly, there was a residual progeny effect within the major groups, indicating that minor genes are also present. This study rules out the polygenic effect for a trait governing the interaction between the insect and parasitoid. It demonstrates that the gene-for-gene model commonly found in plant-parasite interactions may also explain natural variations in insect-parasitoid traits.