Genetic diversity and differentiation at MHC genes in island populations of tuatara (Sphenodon spp.)

Neutral genetic markers are commonly used to understand the effects of fragmentation and population bottlenecks on genetic variation in threatened species. Although neutral markers are useful for inferring population history, the analysis of functional genes is required to determine the significance of any observed geographical differences in variation. The genes of the major histocompatibility complex (MHC) are well-known examples of genes of adaptive significance and are particularly relevant to conservation because of their role in pathogen resistance. In this study, we survey diversity at MHC class I loci across a range of tuatara populations. We compare the levels of MHC variation with that observed at neutral microsatellite markers to determine the relative roles of balancing selection, diversifying selection and genetic drift in shaping patterns of MHC variation in isolated populations. In general, levels of MHC variation within tuatara populations are concordant with microsatellite variation. Tuatara populations are highly differentiated at MHC genes, particularly between the northern and Cook Strait regions, and a trend towards diversifying selection across populations was observed. However, overall our results indicate that population bottlenecks and isolation have a larger influence on patterns of MHC variation in tuatara populations than selection.