Evolutionary interactions between Drosophila melanogaster and its parasitoid Asobara tabida

Drosophila melanogaster is attacked by parasitoids that develop internally in the larva. They can defend themselves by a cellular immune response (hos t resistance), although this can be disabled by parasitoid countermeasures (parasitoid virulence). D. melanogaster and its parasitoids are an excellen t system in which to study coevolution experimentally. We designed an exper iment to compare changes in resistance and virulence in replicate populatio ns of flies and parasitoids maintained together for approximately 10 fly (f ive parasitoid) generations. The experiment had three treatments each with three replicates: (A) no parasitoids (B) outbred parasitoids (C) partially inbred parasitoids. Host resistance increased in treatments B and C but the re was no difference between these treatments. Parasitoid virulence appeare d not to change during the experiment. Host larvae in treatments B and C fe d at lower rates than those in A, evidence of a trade-off between resistanc e and larval competitive ability. We found no evidence for local adaptation , as hosts from the different replicates of treatment C performed no differ ently against parasitoids from the same and other replicates. Also, we foun d no evidence for the evolution of behavioural traits in the host that coul d lead to lower probabilities of being attacked. Comparing the evolution of host resistance in these seminatural settings with that in artificial sele ction experiments provides insight into how the conflicting selection press ures on host resistance interact.