Gene duplication, allelic diversity, selection processes and adaptive value of MHC class II DRB genes of the bank vole, Clethrionomys glareolus

Diverse MHC IIB allele repertoire increases parasite resistance and body condition in the Long-tailed giant rat (Leopoldamys sabanus)

BACKGROUND

Genes of the major histocompatibility complex (MHC) code for key functions in the adaptive immune response of vertebrates and most of them show exceptionally high polymorphism. This polymorphism has been associated with the selection by diverse and changing parasite communities. We analysed MHC class IIB diversity, gastrointestinal parasite load and body condition in the wild ranging tropical rat Leopoldamys sabanus (Thomas, 1887) under natural selection conditions in a highly variable rainforest environment in Borneo to explore the mechanisms that maintain these high levels of genetic polymorphism.

RESULTS

Allelic diversity was determined via SSCP and sequencing, and parasite screening was done through non-invasive faecal egg count. The detected alleles showed expected high levels of polymorphism and balancing selection. Besides a clear advantage for more diverse MHC genotypes in terms of number of alleles, reflected in better body condition and resistance against helminth infection, our data also suggested a positive effect of MHC allele divergence within an individual on these parameters.

CONCLUSION

In accordance with the heterozygote advantage hypothesis, this study provides evidence for an advantage of more diverse MHC genotypes. More specifically, the potential negative relation between individual allele divergence and number of parasite species is in line with the 'divergent allele advantage' hypothesis.

Opposites attract: MHC-associated mate choice in a polygynous primate

We investigated reproduction in a semi-free-ranging population of a polygynous primate, the mandrill, in relation to genetic relatedness and male genetic characteristics, using neutral microsatellite and major histocompatibility complex (MHC) genotyping. We compared genetic dissimilarity to the mother and genetic characteristics of the sire with all other potential sires present at the conception of each offspring (193 offspring for microsatellite genetics, 180 for MHC). The probability that a given male sired increased as pedigree relatedness with the mother decreased, and overall genetic dissimilarity and MHC dissimilarity with the mother increased. Reproductive success also increased with male microsatellite heterozygosity and MHC diversity. These effects were apparent despite the strong influence of dominance rank on male reproductive success. The closed nature of our study population is comparable to human populations for which MHC-associated mate choice has been reported, suggesting that such mate choice may be especially important in relatively isolated populations with little migration to introduce genetic variation.

Correlated evolution of nucleotide substitution rates and allelic variation in Mhc-DRB lineages of primates

BACKGROUND

The major histocompatibility complex (MHC) is a key model of genetic polymorphism. Selection pressure by pathogens or other microevolutionary forces may result in a high rate of non-synonymous substitutions at the codons specifying the contact residues of the antigen binding sites (ABS), and the maintenance of extreme MHC allelic variation at the population/species level. Therefore, selection forces favouring MHC variability for any reason should cause a correlated evolution between substitution rates and allelic polymorphism. To investigate this prediction, we characterised nucleotide substitution rates and allelic polymorphism (i.e. the number of alleles detected in relation to the number of animals screened) of several Mhc class II DRB lineages in 46 primate species, and tested for a correlation between them.

RESULTS

First, we demonstrate that species-specific and lineage-specific evolutionary constraints favour species- and lineage-dependent substitution rate at the codons specifying the ABS contact residues (i.e. certain species and lineages can be characterised by high substitution rate, while others have low rate). Second, we show that although the degree of the non-synonymous substitution rate at the ABS contact residues was systematically higher than the degree of the synonymous substitution rate, these estimates were strongly correlated when we controlled for species-specific and lineage-specific effects, and also for the fact that different studies relied on different sample size. Such relationships between substitution rates of different types could even be extended to the non-contact residues of the molecule. Third, we provide statistical evidence that increased substitution rate along a MHC gene may lead to allelic variation, as a high substitution rate can be observed in those lineages in which many alleles are maintained. Fourth, we show that the detected patterns were independent of phylogenetic constraints. When we used phylogenetic models that control for similarity between species, due to common descent, and focused on variations within a single lineage (DRB1*03), the positive relationship between different substitution rates and allelic polymorphisms was still robust. Finally, we found the same effects to emerge in the analyses that eliminated within-species variation in MHC traits by using strictly single population-level studies. However, in a set of contrasting analyses, in which we focused on the non-functional DRB6 locus, the correlation between substitution rates and allelic variation was not prevalent.

CONCLUSION

Our results indicate that positive selection for the generation of allelic polymorphism acting on the functional part of the protein has consequences for the nucleotide substitution rate along the whole exon 2 sequence of the Mhc-DRB gene. Additionally, we proved that an increased substitution rate can promote allelic variation within lineages. Consequently, the evolution of different characteristics of genetic polymorphism is not independent.

MHC haplotype involvement in avian resistance to an ectoparasite

Research on immune function in evolutionary ecology has frequently focused on avian ectoparasites (e.g., mites and lice). However, host immunogenetics involved with bird resistance to ectoparasites has not been determined. The critical role of the major histocompatibility complex (MHC) in adaptive immunity and high genetic variation found within the MHC make this gene complex useful for exploring the immunogenetic basis for bird resistance to ectoparasites. The objective of this study was to determine if the avian MHC influenced resistance to a blood-feeding ectoparasite. Four congenic lines of chickens, differing only at the MHC, were comparatively infested with a cosmopolitan ectoparasite of birds-northern fowl mite (NFM)-which is also a serious pest species of poultry. Mite infestations were monitored over time and mite densities (weekly and maximum) were compared among lines. Chickens with the MHC haplotype B21 were relatively resistant to NFM, compared with birds in the B15 congenic line (P < 0.02). To test for similar effects in an outbred genetic background, a separate experiment was performed with 107 commercial chickens (white leghorn, W-36 strain) infested with NFM. Hens were genotyped using a MHC microsatellite marker (LEI0258) and associations between MHC haplotype and NFM density were tested. The highest peak NFM populations occurred more often on hens with the B15 haplotype versus the B21 haplotype (P = 0.012), which supported the results of the congenic study. These data indicate the avian MHC influences ectoparasite resistance, which is relevant to disease ecology and avian-ectoparasite interaction.

Association between the DQA MHC class II gene and Puumala virus infection in Myodes glareolus, the bank vole

Puumala virus (PUUV) is a hantavirus specifically harboured by the bank vole, Myodes (earlier Clethrionomys) glareolus. It causes a mild form of hemorrhagic fever with renal syndrome (HFRS) in humans, called Nephropathia epidemica (NE). The clinical severity of NE is variable among patients and depends on their major histocompatibility complex (MHC) genetic background. In this study we investigated the potential role of class II MHC gene polymorphism in the susceptibility/resistance to PUUV in the wild reservoir M. glareolus. We performed an association study between the exon 2 of the DQA gene and PUUV antibodies considering a natural population of bank voles. Because immune gene polymorphism is likely to be driven by multiple parasites in the wild, we also screened bank voles for other potential viral and parasitic infections. We used multivariate analyses to explore DQA polymorphism/PUUV associations while considering the potential antagonist and/or synergistic effects of the whole parasite community. Our study suggests links between class II MHC characteristics and viral infections including PUUV and Cowpox virus. Several alleles are likely to be involved in the susceptibility or in the resistance of bank voles to these infections. Alternatively, heterozygosity does not seem to be associated with PUUV or any other parasite infections. This result thus provides no evidence in favour of the hypothesis of selection through overdominance. Finally this multivariate approach reveals a strong antagonism between ectoparasitic mites and PUUV, suggesting direct or indirect immunogenetic links between infections by these parasites. Other datasets are now required to confirm these results and to test whether the associations vary in space and/or time.