A SINGLE PARASITOID SEGREGATING FACTOR CONTROLS IMMUNE SUPPRESSION INDROSOPHILA

Encapsulation has evolved as an efficient mechanism whereby an insect host can survive infection by parasitoids This ability is controlled b y a major gene in Drosophila melanogaster hosts. The parasitoid Leptop ilina boulardi (Hymenoptera Eucoilidae) can suppress the Drosophila im mune 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 chromosom al factor with semidominant effect. We developed a method to test the monogenic hypothesis. The range of possible genotypic values In back-c rosses was studied using various progeny that were genotypically homog enous. These could be obtained because of the arrhenotokous mode of re production, The progeny groups were divided into two clusters accordin g to the major gene classification and the hypothesis of another unlin ked genetic factor was rejected. Lastly, there was a residual progeny effect within the major groups, indicating that minor genes are also p resent, This study rules out the polygenic effect for a trait governin g the interaction between the insect and parasitoid, It demonstrates t hat the for-gene model commonly found in plant-parasite interactions m ay also explain natural variations in insect-parasitoid traits.