Drift Rather than Selection Dominates MHC Class II Allelic Diversity Patterns at the Biogeographical Range Scale in Natterjack Toads Bufo calamita

Abstract

Study of major histocompatibility complex (MHC) loci has gained great popularity in recent years, partly due to their function in protecting vertebrates from infections. This is of particular interest in amphibians on account of major threats many species face from emergent diseases such as chytridiomycosis. In this study we compare levels of diversity in an expressed MHC class II locus with neutral genetic diversity at microsatellite loci in natterjack toad (Bufo (Epidalea) calamita) populations across the whole of the species’ biogeographical range. Variation at both classes of loci was high in the glacial refugium areas (REF) and much lower in postglacial expansion areas (PGE), especially in range edge populations. Although there was clear evidence that the MHC locus was influenced by positive selection in the past, congruence with the neutral markers suggested that historical demographic events were the main force shaping MHC variation in the PGE area. Both neutral and adaptive genetic variation declined with distance from glacial refugia. Nevertheless, there were also some indications from differential isolation by distance and allele abundance patterns that weak effects of selection have been superimposed on the main drift effect in the PGE zone.

Selection outweighs drift at a fine scale: Lack of MHC differentiation within a family living lizard across geographically close but disconnected rocky outcrops

“…Genetic differentiation of E. stokesii at microsatellites but not the MHC is consistent with patterns found for wolverines (Rico et al., ), house sparrows (Bichet et al., ), black grouse (Strand et al., ) and a molly (Tobler et al., ). On the other hand, our results contrast with cases where MHC differentiation has been stronger compared to microsatellites (Ekblom et al., ; Kyle et al., ; Loiseau et al., ; Miller, Kaukinen, Beacham, & Withler, ) and cases where both the MHC and microsatellites exhibited similar patterns of differentiation (Boyce et al., ; Zeisset & Beebee, ). However, all the aforementioned comparative studies of MHC and microsatellite differentiation sampled multiple populations across broad spatial scales.…”

Effects of parasite and historic driven selection on the diversity and structure of a MHC-II gene in a small mammal species (Peromyscus leucopus) undergoing range expansion

“…In eastern-North America, the species is known to expand its range at a rate of 10-15 km per year (Roy-Dufresne et al 2013;Myers et al 2009). Our data confirms that this expansion is associated with a loss of neutral as well as selected genetic diversity and may be explained by neutral processes such as genetic drift as observed in (Zeisset and Beebee 2014). The detected loss of DRB genetic diversity may furthermore be explained by an additional process, the positive selection that we observed on the margin of the species distribution.…”

Spatial pattern of genetic diversity and selection in the MHC class II DRB of three Neotropical bat species

“…Although pathogen-driven selection is known to be crucial in MHC diversity [98], demographic processes such as genetic drift, bottlenecks, expansions, fragmentation and geographic isolation have been demonstrated to also participate in the shaping of this extensive polymorphism [15, 17, 99, 100]. Evolutionary analyses of MHC DRB alleles for the three species did not reveal any area-dependent clustering effect.…”