Associations between MHC class II variation and phenotypic traits in a
free-living sheep population
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 diversity and phenotypic traits
which include direct or indirect measures of adaptive immunity thought
to contribute to fitness. Here, we tested associations between MHC class
II variation and five representative phenotypic traits measured in
August: weight, strongyle faecal egg count, and plasma IgA, IgE and IgG
immunoglobulin titres against the gastrointestinal nematode parasite
Teladorsagia circumcincta in a free-living population of Soay sheep. We
found no association between MHC class II variation and August weight or
strongyle faecal egg count. We did however find associations between MHC
class II variation and immunoglobulin levels which varied with age,
isotype 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 body
weight and highlight a useful role of MHC-antibody associations in
examining selection on MHC genes.