MHC variability, endoparasites and fitness in brown hares

The major histocompatibility complex (MHC) is a genetic region composed of a set of genes with immunological and non-immunological functions. MHC class II molecules are expressed on anti-gen presenting cells, such as macrophages, B cells and dendritic cells, bind foreign molecules and present them to CD4+ helper T cells. In natural animal populations, MHC class II genes frequently show pronounced polymorphism particularly in those parts that code for the peptide-binding region. This is hypothesised to result from selection for variation. However, apart from some few studies on the relationship between MHC alleles or genotypes and infectious or parasitic diseases, it is not well understood how the many MHC genes interact, and what meaning they have for the individual fitness under natural conditions in wild living populations, under multiple challenge from infectious and parasitic agents. In this project we use brown hares (Lepus europaeus) as a mammalian model to assess and analyse the consequences of sequence variation of two MHC class II genes (DRB, DQB) on parasite burden and other fitness parameters in natural populations. The young of this species live above ground from birth without any special protection by the mother. Their sudden exposure to above ground environmental challenges, including harsh weather conditions and various pathogens, immediately after the intra-uterine life, together with their rapid ontogenetic development, might suggest a very efficient immune system to guarantee sufficient energy allocation to growth. To understand the meaning of allelic MHC variability we investigate relationships between sequence variation at the studied loci and individual endopara-site load, various fitness parameters such as body condition and female reproductive success, as well as CD4+ T-cell counts. To account for possible environmental (e.g., population-specific parasite communities) or population genetic and phylogeographic influences on such relation- ships, we study two populations in Austria and the UK. The direct envolvement of MHC genes in the immune response is examined for the first time ever in a wildlife species by measuring levels of gene expression and associated CD4+ T cell reaction after experimentally infecting leverets with coccidia (Eimeria sp.). Under the heterozygosity advantage hypothesis we should expect a negative relationship between MHC heterozygosity and parasite burden and a positive relationship for all other fitness parameters. We expect increased gene expression follow-ing experimental infections, with higher expression levels in outbred hares relative to full-sibs.

The link between adaptive genetic variation, individual fitness, and wildlife population dynamics is fundamental to the study of ecology and evolutionary biology. In this context molecular markers associated with host-pathogen interactions are of great interest to understand the interplay between population dynamics and natural selection. We have studied the effects of allelic variation of three MHC (major histocompatibility complex) class II genes (DQA, DQB, DRB) on individual fitness components of brown hares, such as gastro-intestinal parasite load, body conditon, and female reproductive performance. MHC-genes are important in the initiation of the immune response to viral or bacterial infections and to fight parasitic diseases. Usually, MHC-gene diversity is high in natural populations of vertebrates, but both the reasons and consequences for individual fitness are still under debate. By studying hares from eastern Austria and northern Belgium we accounted for possible spatial/ecological variation of MHC effects on fitness, as eastern Austria has subcontinental and northern Belgium oceanic/atlantic climate. We found no effect of MHC-genes on Trichostrongylus retortaeformis, a parasitic nematode that showed a wide range of intensity of infestation of digestive tracts of the hares. For coccidia (protozoan gut parasites) we found a significant negative effect of MHC heterozyogisity on intensity of infection; particularly so for the DQA and the DQB loci. Interestingly, the DQA-1 allele, which varied also strongly among populations and across the two regions, showed a significant additive effect on coccidia loads, with homozygotes for DQA-1 having the highest coccidia loads, hares possessing one DQA-1 allele having medium levels, and hares without DQA-1 having the lowest average count. These effects were independent of other variables such as sex, age, body weight, body size, and overall individual heterozygosity as estimated by 13 microsatellite markers. Female reproduction was significantly influenced by the DQA locus, but not by the DRB locus. Females with homozygous genotypes at the DQA locus had a significantly higher rate not to reproduce throughout one entire reproductive period than females with a heterozygous DQA-genotype. This effect was, however, only observed in the Belgium hares. In addition, there was a tendency for females from both Austria and Belgium to have higher numbers of offspring per reproduction period when having a heterozygous DQA-genotype. The DQA gene had a positive effect on body condition of the hares, which we measured by weighing the retroperitoneal fat deposits: hares possessing two DQA- 1 alleles had the highest body conditon levels, those possessing one (in heterozygous genotypes) had medium body condition, and those with no DQA-1 allele had lowest body condition. Overall, we could demonstrate both allele and genotype effects for two out of three MHC loci, partly acting not in parallel and only regionally on diverse fitness paramters of brown hares. These findings aid in understanding and interpreting variation at MHC loci and its consequences for wild living vertebrates. It underlines the meaning of conserving genetic diversity in wild living populations, particularly for genes under strong selection.