Geographic Patterns in the Evolution of Resistance and Virulence in<i>Drosophila</i>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, especially those in the genera Asobara and Leptopilina. Geographic patterns have been found in parasitoid virulence and host resistance, the clearest of which is a cline in Asobara tabida virulence from the north (low) to the south (high) of Europe. Drosophila melanogaster resistance is highest in central-southern Europe and lower elsewhere. We review and interpret these patterns in the light of recent experimental and theoretical studies of the evolution and coevolution of these traits. We find no evidence for genotype-specific virulence and defense, which makes "Red Queen"-type coevolution unlikely. The 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 limited evidence that D. melanogaster resistance is influenced by the virulence of its parasitoids. We critically appraise whether the evidence available so far supports a coevolutionary explanation for the levels of these traits.