Within-trio tests provide little support for post-copulatory selection on major histocompatibility complex haplotypes in a free-living population

Sexual selection has been proposed as a force that could help maintain the diversity of major histocompatibility complex (MHC) genes in vertebrates. Potential selective mechanisms can be divided into pre-copulatory and post-copulatory, and in both cases, the evidence for occurrence is mixed, especially in natural populations. In this study, we used a large number of parent-offspring trios that were diplotyped for MHC class II genes in a wild population of Soay sheep ( Ovis aries ) to examine whether there was within-trio post-copulatory selection on MHC class II genes at both the haplotype and diplotype levels. We found there was transmission ratio distortion of one of the eight MHC class II haplotypes (E) which was transmitted less than expected by fathers, and transmission ratio distortion of another haplotype (A) which was transmitted more than expected by chance to male offspring. However, in both cases, these deviations were not significant after correction for multiple tests. In addition, we did not find any evidence of post-copulatory selection at the diplotype level. These results imply that, given known parents, there is no strong post-copulatory selection on MHC class II genes in this population.

A rare MHC haplotype confers selective advantage in a free-living ruminant

The major histocompatibility complex (MHC) is the most variable gene family known in vertebrates. Parasite-mediated selection (PMS) is believed to be the main force maintaining diversity at MHC genes, but it has proven hard to demonstrate the exact PMS regime that is acting in natural populations. Demonstrating contemporary selection on MHC alleles is not trivial, and previous work has been constrained by limited genetic tools, low sample sizes, short time scales and anticonservative statistical approaches. Here, we use a conservative statistical approach to examine associations between MHC genes and fitness components, using haplotypes of expressed MHC class II genes in a large sample of Soay sheep (Ovis aries) monitored over their lifetimes between 1989 and 2012. Of the eight MHC haplotypes (A-H) identified, we found that haplotype C was associated with decreased male breeding success, haplotype D was associated with increased female life span, and haplotype F was associated with decreased female life span. Consistent with the increased lifespan in females, haplotype D has increased in frequency throughout the study period. Our results suggest the existence of contemporary balancing selection on MHC class II genes in Soay sheep as might be expected under the hypotheses of rare allele advantage or fluctuating selection and do not support an effect of heterozygous advantage.