Detangling the mechanisms and timing of MHC-dependent sexual selection using Soay sheep

Immune defence is a key component of fitness, and individuals are expected to have evolved preferences for mates that ensure immunocompetent offspring. Potential preferences include those for mates with specific heritable immune gene profiles ("good genes") or for immunogenetically dissimilar mates to increase offspring immune gene diversity. The major histocompatibility complex (MHC) is by far the most investigated immune gene in mate choice studies, but we still know very little about its role in sexual selection for genetic benefits. In this issue of Molecular Ecology, Huang et al. capitalize on the extraordinary wealth of behavioural, life history and genetic/genomic data from the free-living Soay sheep population on the Island of Hirta to address this problem. While the authors find evidence of both pre- and postcopulatory MHC-based sexual selection, postcopulatory MHC-dissimilar mate choice is indistinguishable from genome-wide effects, suggesting it is a byproduct of inbreeding avoidance in Soay sheep. The study's comprehensive sampling ensures that inferences are generalizable to the entire population and provides a gold standard for studies investigating immune gene-based sexual selection.