Population genetic and functional analysisof the B4galnt2 gene in the genus Mus(Rodentia; Muridae)
B4galnt2 is a blood group-related glycosyltransferase expressed in the gastrointestinal (GI) tract of most mammals. However, cis-regulatory variation at B4galnt2 in house mice leads to different tissue-specific expression patterns affecting intestinal epithelium and vascular endothelium according to the alleles present in the direct upstream region (”C57BL6/J”’ or ”‘RIIIS/J” allele, respectively). Blood vessel expression of B4galnt2 leads to a phenotype in mice very similar to a common human bleeding disorder, von Willebrand disease, but the consequences of altered expression in the intestine are unknown. This dissertation combines poulation genetics with metagenomics with respect to the B4galnt2 gene in natural populations of mice and enlightens the role of biogeography in shaping intestinal micriobial communities in general. The population genetic study of the B4galnt2 gene in all three subspecies of M. musculus (i.e. M. m. domesticus, M. m. musculus and M. m. castaneus) and the sister species M. spretus revealed the long-term maintenance of different allele classes present in the direct upstream region of B4galnt2. Varying expression patterns could be identified to be present for > 2.8 MY, since the divergence of M. famulus. The finding, that gut expression was conserved in all mice exhibiting the C57BL6/J allele lead to the suggestion the the gut is a likely target of selection. It is known, that glycosylation profiles in the GI tract can influence both symbiotic/commensal and pathogenic bacteria. Together with the long-term maintenance of alleles conferring differences in B4galnt2 expression suggests that host-pathogen interactions in the gut are likely involved. As not much was known about the biogeographic influences on the intestinal microbiota we described and thus shed light on the effect of different factors (i.e. geography, host population structure, maternal transmission) on them. We performed a survey of eight house mouse populations throughout western Europe, by applying high throughput pyrosequencing of the bacterial 16S rRNA gene, we obtained the microbiota composition of those mice, microsatellites were used for estimating hosts population structure and sequencing of the mitochondrial D-loop region for the inference of maternal inheritance. Geography was found to be the most dominant factor shaping the bacterial composition, followed by host population structure and the interaction of both. Additionally we could identify several bacterial ”species” which showed significant correlation to the underlying population structure of their host, maternal lineages and geography. We also performed a survey of B4galnt2 allele frequencies of the eight sampled populations in France and Germany. We detected a clear pattern of allele frequency distribution with a clear decline of the RIIIS/J allele frequency in central France, where we would thus locate the selective pressure(s). To shed light on the population dynamics concerning B4galnt2 allele frequencies we analysed the frequency distribution pattern according to population substructure (as estimated by microsatellites and mtDNA). To follow up on the question of possible phenotypic consequences of B4galnt2 expression, likely on the intestinal microbiota, we analysed the gut microbiota composition according to B4galnt2 genotype by keeping in mind the newly gained insights of Chapter 2. This survey allowed us to infer that local adaptation is the most likely explanation for these dramatic differences in allele frequencies, rather than population substructure (e.g. due to different colonization waves). We could also confirm the influence of B4galnt2 expression on the microbiota composition in wild-caught mice as already described for lab mice. A direct pathogen-genotype association could not be detected, which might be due to our sampling size. By identifying the long-term maintenance of different B4galnt2 expression patterns and elucidating the role of biogeography on the intestinal microbiota we were able to further characterize the evolutionary forces acting at B4galnt2. We shed light on the complex dynamics acting at the B4galnt2 gene, but, until the beneficial consequence(s) of exhibiting the RIIIS/J allele is known, the story of B4galnt2 will remain a mystery and has to be further explored.
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