Discussion

One of the central questions in evolutionary biology is to explain the mechanisms that maintain genetic diversity within and among populations. Parasites are ubiquitous, and are arguably one of the most important selection pressures which organisms face. In response to the costs of parasitism, hosts have evolved a variety of genetically encoded and complex immune defence mechanisms. It is often claimed that as a response to parasite-mediated balancing selection, the immune genes are among the most variable genes in jawed vertebrates. Because the current understanding of the maintenance of polymorphism mostly comes from empirical evidence on a limited number of candidate loci, the claim is mostly unsubstantiated. Here, we followed a genome-wide approach to detect genomic regions of increased polymorphism in populations of Gasterosteus aculeatus fish from diverse European and North American water bodies covering a large part of the species distribution. Based on >60 high quality genomes, we will report about genomic regions under balancing selection, and the diverse rate of polymorphism based on gene functions. Overall, out study highlights evidence for the maintenance of polymorphism of diverse functions evolving under different selection pressures, including parasitism.