Data for: 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
Rhinanthus alectorolophus and the potential host Plantago 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).