Understanding selection processes driving the pronounced allelic polymorphism of the major histocompatibility complex (MHC) genes and its functional associations to parasite load have been the focus of many recent wildlife studies. Two main selection scenarios are currently debated which explain the susceptibility or resistance to
parasite infections either by the effects of (1) specific MHC alleles which are selected frequency-dependent in space and time or (2) a heterozygote or divergent allele advantage. So far, most studies have focused only on structural variance in co-evolutionary processes although this might not be the only trait subject to natural selection. In the present study, we analysed structural variance stretching from exon1 through exon3 of MHC class II
DRB genes as well as genotypic expression variance in relation to the gastrointestinal helminth prevalence and
infection intensity in wild yellow-necked mice (Apodemus flavicollis). We found support for the functional importance of specific alleles both on the sequence and expression level. By resampling a previously investigated study population we identified specific MHC alleles affected by temporal shifts in parasite pressure and recorded associated changes in allele frequencies. The allele Apfl-
DRB*23 was associated with resistance to
infections by the oxyurid nematode Syphacia stroma and at the same time with susceptibility to
cestode infection intensity. In line with our expectation, MHC
mRNA transcript levels tended to be higher in cestode-infected animals carrying the allele Apfl-
DRB*23. However, no support for a heterozygote or divergent allele advantage on the sequence or expression level was detected. The individual
amino acid distance of genotypes did not explain individual differences in parasite loads and the genetic distance had no effect on MHC genotype expression. For ongoing studies on the functional importance of expression variance in parasite resistance, allele-specific expression data would be preferable.