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Abstract

Although crucial for the understanding of adaptive evolution, genetically resolved examples of local adaptation
are rare. To maximize survival and reproduction in their local environment, hosts should resist their
local parasites and pathogens. The major histocompatibility complex (MHC) with its key function in parasite
resistance represents an ideal candidate to investigate parasite-mediated local adaptation. Using replicated
field mesocosms, stocked with second-generation lab-bred three-spined stickleback hybrids of a lake
and a river population, we show local adaptation of MHC genotypes to population-specific parasites, independently
of the genetic background. Increased allele divergence of lake MHC genotypes allows lake fish
to fight the broad range of lake parasites, whereas more specific river genotypes confer selective advantages
against the less diverse river parasites. Hybrids with local MHC genotype gained more body weight and
thus higher fitness than those with foreign MHC in either habitat, suggesting the evolutionary significance
of locally adapted MHC genotypes.