Local adaptation of a holoparasitic plant, Cuscuta europaea: variation among populations

Locally adapted parasites have higher infectivity and/or fitness on sympatr ic than on allopatric hosts. We tested local adaptation of a holoparasitic plant, Cuscuta europaea, to its host plant, Urtica dioica. We infected host s from five sites with holoparasites from the same five sites and measured local adaptation in terms of infectivity and parasite performance (biomass) in a reciprocal cross-infection experiment. The virulence of the parasite did not differ between sympatric and allopatric hosts. Overall, parasites h ad higher infectivity on sympatric hosts but infectivity and parasite perfo rmance varied among populations. Parasites from one of the populations show ed local adaptation in terms of performance, whereas parasites from one of the populations had higher infectivity on allopatric hosts compared with sy mpatric hosts. This among-population variation may be explained by random v ariation in parasite adaptation to host populations or by time-lagged co-ev olutionary oscillations that lead to fluctuations in the level of local ada ptation.