Geographic patterns in the evolution of resistance and virulence in Drosophila and its parasitoids

Many insects are attacked by internal parasitoids against which they mount a largely cellular immunological defense. The resistance of a host and the virulence of a parasitoid determine which species survives after parasitism . Drosophila is parasitized by several hymenopterous parasitoids, especiall y those in the genera Asobara and Leptopilina. Geographic patterns have bee n found in parasitoid virulence and host resistance, the dearest of which i s a dine in Asobara tabida virulence from the north (low) to the south thig h) of Europe. Drosophila melanogaster resistance is highest in central-sout hern Europe and lower elsewhere. We review and interpret these patterns in the light of recent experimental and theoretical studies of the evolution a nd coevolution of these traits. We find no evidence for genotype-specific v irulence and defense, which makes "Red Queen"-type coevolution unlikely. Th e most important explanation for the patterns is geographic differences in host-parasitoid community structure. Asobara tabida virulence is positively correlated with the resistance of its main hosts, and there is more limite d evidence that D. melanogaster resistance is influenced by the virulence o f its parasitoids. We critically appraise whether the evidence available so far supports a coevolutionary explanation for the levels of these trails.