Local adaptation, resistance, and virulence in a hemiparasitic plant-host plant interaction

Coevolution may lead to local adaptation of parasites to their sympatric ho sts. Locally adapted parasites are, on average, more infectious to sympatri c hosts than to allopatric hosts of the same species or their fitness on th e sympatric hosts is superior to that on allopatric hosts. We tested local adaptation of a hemiparasitic plant, Rhinanthus serotinus (Scrophulariaceae ), to its host plant, the grass Agrostis capillaris. Using a reciprocal cro ss-infection experiment, we exposed host plants from four sites to hemipara sites originating from the same four sites in a common environment. The par asites were equally able to establish haustorial connections to sympatric a nd allopatric hosts, and their performance was similar on both host types. Therefore, these results do not indicate local adaptation of the parasites to their sympatric hosts. However, the parasite populations differed in ave rage biomass and number of flowers per plant and in their effect on host bi omass. These results indicate that the virulence of the parasite varied amo ng populations, suggesting genetic variation. Theoretical models suggest th at local adaptation is likely to be detected if the host and the parasite h ave different evolutionary potentials, different migration rates, and the p arasite is highly virulent. In the interaction between R. serotinus and A. capillaris all the theoretical prerequisites for local adaptation may not b e fulfilled.