Performance of generalist hemiparasitic Euphrasia across a phylogenetically diverse host spectrum.

Generalist hemiparasites may attach to many different host species and experience complex parasite-host interactions. How these parasite-host interactions impact on the fitness of hemiparasitic plants remain largely unknown. We used experimentally tractable eyebrights (Euphrasia, Orobanchaceae) to understand parasite-host species interactions affecting the performance of a generalist hemiparasitic plant. Common garden experiments were carried out measuring Euphrasia performance across 45 diverse hosts and in different parasite-host combinations. We show that variation in hemiparasite performance can be attributed mainly to host species and host phylogenetic relationships (λ = 0.82; 0.17-1.00 CI). When variation in performance is considered temporally, annual host species cause earlier flowering, and lead to poorer performance late in the season. While Euphrasia species typically perform similarly on a given host species, some eyebrights show more specialized parasite-host species interactions. Our results show that generalist hemiparasites only benefit from attaching to a limited, but phylogenetically divergent, subset of hosts. The conserved responses of divergent Euphrasia species suggest hemiparasite performance is affected by common host attributes. However, evidence for more complex parasite-host species interactions show that a generalist hemiparasite can potentially respond to individual host selection pressures and may adapt to local host communities.