THE POTENTIAL FOR COEVOLUTION IN A HOST-PARASITOID SYSTEM .1. GENETIC-VARIATION WITHIN AN APHID POPULATION IN SUSCEPTIBILITY TO A PARASITICWASP

For coevolution to occur, there must be genetic variation in each spec ies for traits relevant to their interaction. Here, statistically sign ificant variation in susceptibility to a parasitic wasp was found amon g pea-aphid clones collected from a single population. In a subset of clones that was tested further, wasps were found to oviposit in aphids from both resistant and susceptible lines, but eggs failed to develop in resistant hosts. Significant genetic variance in susceptibility pr ovides evidence that this aphid population has the potential to evolve resistance in response to selection by one of its major natural enemi es. Predictions of an expected response to selection based on the expe rimental measures of variation and field parasitism rates suggested th at there should be a detectable change in susceptibility over the cour se of a season. However, an experimental comparison of mean susceptibi lity of clones collected early and late in the summer, a period of sev eral generations, revealed no response to selection by the wasps. Aphi ds collected late in the season were as susceptible, on the average, a s those collected early in the summer. Possible constraints on the res ponse of the aphids to selection by this natural enemy are considered.