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.
Highlights
The basis of adaptive rather than neutral genetic variation has become increasingly accessible in recent years as loci under selection are identified and characterised
N is sample size and N alleles is mean number of alleles with standard deviations given for microsatellite loci, NP is the number of population- specific alleles occurring only in one population, R is the allelic richness adjusted to the minimum sample size of nine individuals
As FST may be affected by highly variable markers such as microsatellites we calculated DEST [78] in GenAlEx 6.5b3 [79], [80]
Summary
The basis of adaptive rather than neutral genetic variation has become increasingly accessible in recent years as loci under selection are identified and characterised. Some of the most popular genes used in this context are those belonging to the major histocompatibility complex (MHC). These genes play an important role in the adaptive immune response of vertebrates. Exon 2 encodes the b - 1 domain which includes most of the antigen binding sites (ABS) of the beta domain and is the most polymorphic region of the gene [2], [3]. Due to high selective pressure on MHC genes, variation tends to be high, at the ABS [4]. These sites encode amino acid residues involved in the recognition and binding of foreign peptides [5]
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