Is there local adaptation in Drosophila-parasitoid interactions?

Local adaptation has received little attention in host-parasitoid associations. Here we combine data on the outcome of parasitism in 20 sympatric populations of Drosophila melanogaster and its parasitoid Asobara tabida. We present data on resistance and virulence when each host is tested against a single allopatric strain of parasitoid, and when each parasitoid is tested against a single allopatric strain of host. We argue that the extent to which these allopatric interactions can be used to predict sympatric interactions sets an upper bound to the importance of local adaptation. In a statistical model, we found that 56% of the variance in the outcome of sympatric interactions could be explained by parasitoid virulence and host resistance measured using the allopatric reference strains, with the former being the much more important of the two. The geographical distance between the provenances of the sympatric and reference parasitoid (but not host) populations was also statistically significant and increased the variance explained to 69%; but against expectation, parasitoid success was negatively correlated with distance. We also explore the factors determining the frequency with which neither host nor parasitoid survive. We conclude that, although the critical tests have yet to be performed, the available evidence points towards local adaptation not being of major importance in this system