Interview with the authors: Associations between MHC class II variation and phenotypic traits in a free-living sheep population

In a recent paper in Molecular Ecology, Huang, Dicks and colleagues analysed variation in the major histocompatibility complex (MHC) and phenotypic traits in an unmanaged population of sheep living on an island off the coast of Scotland. This population of sheep has been studied closely for more around 70 years, providing a very rare level of insight and statistical power to evolutionary genetic studies. The MHC is among the most variable parts of mammalian genomes and has long been known to be encode proteins central to the adaptive immune system. Through their analyses, Huang, Dicks and colleagues found associations with levels of circulating antibodies and variation at MHC loci.

We sent some questions to the corresponding author of this work, Wei Huang, to get more detail on this new study.

Rams in St Kilda. Photo credit, Martin Adam Stoffel.

Can you describe the significance of this research for the general scientific community in one sentence?

This study demonstrated the direct link between immune genes and antibody levels in wild populations.

What led to your interest in this topic / what was the motivation for this study? 

The major histocompatibility complex (MHC) contains a number of genes linked with immune defence in vertebrates. Associations between MHC variation and phenotypic traits or pathogens have been identified in many species. Also, selection on MHC genes has also been demonstrated in some studies. However, many previous studies only examined associations between MHC variation and a limited number of phenotypic traits or pathogens. Few of them have examined both MHC-fitness associations and MHC-trait associations. The longitudinal study of Soay sheep in St Kilda is a great system to study the associations between MHC variation and phenotypic traits and how the associations are linked with selection on MHC genes. Using three representative phenotypic traits monitored in thousands of sheep over decades, we are able to provide a full picture of MHC-trait associations in wild populations.

Can you describe the significance of this research for your scientific community in one sentence?

This study suggests associations between MHC and phenotypic traits are more likely to be found for traits more closely associated with pathogen defence than integrative traits and highlights the association between MHC variation and antibodies in wild populations.

What difficulties did you run into along the way? 

It is extremely hard to monitor populations and collect longitudinal data over decades. Thanks to our great field assistants and volunteers, the Soay sheep data has provided a good foundation. In terms of the specific study, the first difficulty is to genotype MHC in a large number of sheep. We used two steps to genotype the MHC genes. We first used genotype-by-sequencing to genotype hundreds of sheep. Then, benefiting from the high-density sheep SNP chip, we were able to use 13 SNPs to genotype MHC in the other thousands of sheep successfully.

Additionally, it is hard to choose the appropriate model. Some of our traits are not normally distributed and are also not closed to other common error structures. We instead used Bayesian statistical methods to run the analysis.

What is the biggest or most surprising innovation highlighted in this study? 

We used three representative traits to examine the associations between MHC variation and phenotypic traits. The traits included a fitness-related integrative trait, body weight, a measure of gastrointestinal parasites, faecal egg count, and level of three antibodies. All of the three traits are related to fitness. We only found associations between MHC variation and antibodies. Such results reflect the important role of MHC in immune defence in wild populations. Our study is one of the first studies to examine associations between MHC variation and multiple phenotypic traits. 

How do you think your results generalize to other systems?
Our study is based on the longitudinal study of Soay sheep. The large sample size provides great statistical power. Therefore, our results are reliable and solid. Also, we investigated phenotypic traits that have different links with immune defence. Therefore, our results can reflect the general pattern of MHC-trait associations.  

You conclude from your study that MHC variation is more likely to be associated with immune traits. How would you validate your findings for species with less rich data?

First, it is possible to use experiments to test the associations. In terms of wild populations, future studies can investigate multiple populations or multiple traits in a single population if they are restricted by the study length.

Moving forward, what are the next steps in this area of research?

Our study demonstrates that it is important to study MHC-antibody associations. Future studies should focus on immune traits rather than only examine MHC-pathogen associations. Also, previous studies are often restrained by small sample size. It would be nice if future studies could increase their sample size to strength the statistical power.

Huang, W.*, Dicks, K. L.*, Ballingall, K. T., Johnston, S. E., Sparks, A. M., Watt, K., Pilkington, J. G., & Pemberton, J. M. (2022). Associations between MHC class II variation and phenotypic traits in a free-living sheep population. Molecular Ecology, 31, 902– 915. 

*These authors contributed equally to this work