Patterns of MHC-dependent sexual selection in a free-living population of sheep

Abstract

MHC genes are one of the most polymorphic gene clusters in vertebrates and play an essential role in adaptive immunity. There is evidence that sexual selection also plays a role in maintaining MHC diversity, but the specific mechanisms are controversial. In this study, we investigate evidence for non-random MHC-dependent mating patterns in a free-living population of Soay sheep. Using a large number of sheep diplotyped at the MHC class IIa region and genome-wide SNPs, together with field observations of consorts, we found sexual selection against one of eight haplotypes, C, in males at the pre-copulatory stage and sexual selection on female MHC heterozygosity acting in opposition directions at the pre- and post-copulatory stages. We also found disassortative mating at the post-copulatory stage, along with strong evidence of inbreeding avoidance at both stages. However, results from generalized linear mixed models suggest that the pattern of MHC-dependent disassortative mating could be a by-product of inbreeding avoidance. These results suggest that there is selection on the MHC at the pre- and post-copulatory stages, but that apparent disassortative mating with respect to the MHC may be driven by inbreeding avoidance.

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

“…Estimates with 95% confidence intervals that do not cross zero (dashed line) are significant. Data are from table 1 of Huang et al (2021). (d) The timing (pre‐ or postcopulatory) of MHC‐based mechanisms is determined by comparing the direction of MHC patterns between consort and parentage data.…”

“…Due to the notable amount of genetic, genomic and behavioural data amassed for an exceptional sample size, the study by Huang et al (2021) currently represents a gold standard for MHC‐based sexual selection studies. Given that genomic data are becoming increasingly available for nonmodel studies, one step for improvement would be making use of the genomics windows approach that is common in human MHC mate choice studies.…”

“…The study by Huang et al (2021) in this issue of Molecular Ecology illustrates the complexity of identifying MHC‐based sexual selection by using an exceptional study system: the St. Kilda Soay sheep on Hirta (Figure 1). The Soay sheep ( Ovis aries ) is a primitive breed that was brought to the Scottish St. Kilda archipelago around 4000 years ago and has survived on the island of Soay ever since.…”