Fig. 1

Theoretical framework of the study. Reference hosts came from two contrasting populations, indicated by violet (DE) and orange (NO) dots and lines. For the sake of simplicity, we exemplify possible outcomes with a subset of hypothetical parasites A to E. We asked whether main effects of the host, the parasite, and/or host-parasite interaction effects shaped epidemiological traits (life history traits of the host and/or the parasite). (a, b, c, d) Partitioning of host, parasite, and interaction effects on an epidemiological trait. (a, b) Host genotype and parasite genotype main effects. The host effect (vertical spacing between the two lines) indicates the genetic difference between the two host types. Parallel horizontal lines in (a) indicate absence of a plastic response towards infection. Differences among hosts that are infected with the same parasite (vertical spacing between the dots) indicate a phenotypic plastic response of the parasite. The positive slope in (b) indicates different effects of the two parasite types (parasite effect) and thus a phenotypic plastic response of the host and the parasite. (c) and (d) demonstrate host genotype-parasite genotype interaction effects, because the host effect depends on the parasite type. Crossing reaction norms in (d) clearly show the interaction effect; but note in (c) that the main-effect components can cumulate, causing non-crossing reaction norms. We tested the predictions with data from contrast 1. (e, f) To further understand the parasite effect on a larger geographic scale, each of the two host types was exposed to parasites from different geographic clusters across the Northern Hemisphere. We tested these predictions with data from contrast 2 and contrast 3