Static Expression of Major Histocompatibility Complex Genes Across Life-Stages, Generations and Tissues in a Songbird.

The extraordinarily polymorphic Major Histocompatibility Complex (MHC) is an iconic genomic region that has been associated with, for example, disease susceptibility, mate choice, and fitness. Both the MHC class I and II gene families have expanded considerably in songbirds (Passeriformes) compared to other bird orders, but our understanding of the functional relevance of the large number of gene copies in songbirds is hampered by lack of data on gene expression. We therefore took advantage of longitudinal data from our wild long-term monitored population of great reed warblers Acrocephalus arundinaceus as well as a long-term experiment to study patterns of MHC class I (MHC-I) gene expression during individuals' lifetimes and across generations, and between tissues. As could be expected within a species, the expressed diversity increased linearly with genomic MHC-I diversity. However, most MHC-I alleles were expressed at low levels, with only a handful of alleles showing moderate to high expression in each individual. The MHC-I expression patterns in blood were highly consistent (i) over an individual's lifetime, (ii) between parents and offspring, and (iii) among siblings, and (iv) were strongly correlated with expression in other tissues. The moderately to highly expressed MHC-I genes likely play a key role in antigen presentation in adaptive immunity and thus comprise primary targets for studies of host-pathogen coevolution.