Abstract
Pathogen-mediated selection (PMS) is thought to maintain the high level of allelic diversity observed in the major histocompatibility complex (MHC) class II genes. A comprehensive way to demonstrate contemporary selection is to examine associations between MHC variation and individual fitness. As individual fitness is hard to measure, many studies examine associations between MHC variation and phenotypic traits, including direct or indirect measures of adaptive immunity thought to contribute to fitness. Here, we tested associations between MHC class II variation and five phenotypic traits measured in free-living sheep captured in August: weight, strongyle faecal egg count, and plasma IgA, IgE and IgG immunoglobulin titres against the gastrointestinal nematode parasite Teladorsagia circumcincta. We found no association between MHC class II variation and weight or strongyle faecal egg count. We did, however, find associations between MHC class II variation and immunoglobulin levels which varied with isotype, age and sex. Our results suggest associations between MHC and phenotypic traits are more likely to be found for traits more closely associated with pathogen defence than integrative traits such as bodyweight and highlight the association between MHC variation and antibodies in wild populations.
Highlights
The immune system provides a variety of mechanisms to protect the host from infection by rapidly evolving and highly variable pathogens
With larger sample sizes and improved genetic resolution of haplotypes compared with the previous study (Paterson et al, 1998), we examine the associations between major histocompatibility complex (MHC) variation and five phenotypic traits in Soay sheep
When the Wald test was significant (p < .05) we examined the significance of specific MHC haplotypes by conducting an additional analysis comparing the estimated effect of each haplotype against the mean of the effect estimates of all the other haplotypes
Summary
The immune system provides a variety of mechanisms to protect the host from infection by rapidly evolving and highly variable pathogens. Consistent with the male total fitness result, the frequency of haplotype D has increased more than expected by drift over the study period (Huang et al, 2020) These results indicate that there is contemporary selection on MHC class II variation in Soay sheep. With larger sample sizes and improved genetic resolution of haplotypes compared with the previous study (Paterson et al, 1998), we examine the associations between MHC variation and five phenotypic traits in Soay sheep These traits were all measured in an annual August sheep catch, and include weight, a fitness-related nonimmune trait, strongyle faecal egg count, FEC, a fitness-related trait with a strong link to the immune system, and three immune traits, Teladorsagia circumcincta-specific immunoglobulin isotypes IgA, IgE and IgG (“anti-T. circ antibodies”). TA B L E 2 Number of records of phenotypic traits used in our study class II variation? (2) Does the association between MHC variation and phenotypic traits vary with age, sex or population density? (3) Are there consistent patterns between MHC-phenotypic trait associations and MHC-fitness associations?
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