Fig. 1.

Schematic of the experiment. (A) Breeding design. Wild fish from two populations (P-generation) were crossed to produce F₁s that were heterozygous across the genome with respect to population of origin. These were allowed to mate at random to produce F₂s that segregated into heterozygotes and two types of homozygotes at the focal MHC class II locus, while having, on average, “mixed” genetic backgrounds [23 chromosome pairs (40), plus crossing-over when F₁s reproduce]. (B) Controlled experimental infections. Two gyrodactylid worms from one of the P-generation source streams were inoculated on to the caudal fin of each F₂ fish. Each infected fish was then kept in isolation and its infection monitored every other day for 17 d (22).