Local adaptation of a generalist hemiparasitic plant to one of its potential host plants

Coevolution is often found in parasite–host interactions, but has not yet been described for hemiparasitic plants and their hosts. Root hemiparasites like Rhinanthus alectorolophus perform photosynthesis but also parasitize other plant species, some of which (e.g. Plantago lanceolata) may defend themselves against parasite attack by blocking the haustoria of the parasites. We grew seedlings of the hemiparasite R. alectorolophus and the potential host P. lanceolata from seven grassland sites in a factorial design. To detect differences in host defence, we also included hosts from two ‘naïve' populations from regions where the parasite does not occur. R. alectorolophus grew consistently larger and had higher fitness with sympatric than with allopatric hosts, suggesting parasite adaptation to local host populations. Moreover, R. alectorolophus remained smallest with allopatric hosts from the same region and reached intermediate sizes with allopatric hosts from other regions or naïve hosts, suggesting host adaptation to parasites at the regional scale. Parasite presence reduced the size of the host plants already after four weeks, but only that of hosts with ‘experience' of the parasite, suggesting an early host response. Follow‐up experiments confirmed that parasites attach to hosts already after four weeks and hosts respond by changing belowground allocation patterns. However, parasite roots did not preferentially grow towards sympatric hosts. Our results suggest that local adaptation to hosts can occur even in generalist parasites and does not require specialization on individual hosts. We discuss the role of potential mechanisms, including variation in chemical signalling (early) and in host defence (late effects).