BACKGROUND: The major histocompatibility complex (MHC) is an important component of the vertebrate immune
system and is frequently used to characterise adaptive variation in wild populations due to its co-evolution with
pathogens. Passerine birds have an exceptionally diverse MHC with multiple gene copies and large numbers of
alleles compared to other avian taxa. The Nesospiza bunting species complex (two species on Nightingale Island;
one species with three sub-species on Inaccessible Island) represents a rapid adaptive radiation at a small, isolated
archipelago, and is thus an excellent model for the study of adaptation and speciation. In this first study of MHC in
Nesospiza buntings, we aim to characterize MHCIIß variation, determine the strength of selection acting at this gene
region and assess the level of shared polymorphism between the Nesospiza species complex and its putative sister
taxon, Rowettia goughensis, from Gough Island.
RESULTS: In total, 23 unique alleles were found in 14 Nesospiza and 2 R. goughensis individuals encoding at least
four presumably functional loci and two pseudogenes. There was no evidence of ongoing selection on the peptide
binding region (PBR). Of the 23 alleles, 15 were found on both the islands inhabited by Nesospiza species, and
seven in both Nesospiza and Rowettia; indications of shared, ancestral polymorphism. A gene tree of Nesospiza
MHCIIß alleles with several other passerine birds shows three highly supported Nesospiza-specific groups. All R.
goughensis alleles were shared with Nesospiza, and these alleles were found in all three Nesospiza sequence groups
in the gene tree, suggesting that most of the observed variation predates their phylogenetic split.
CONCLUSIONS: Lack of evidence of selection on the PBR, together with shared polymorphism across the gene tree,
suggests that population variation of MHCIIß among Nesospiza and Rowettia is due to ancestral polymorphism rather
than local selective forces. Weak or no selection pressure could be attributed to low parasite load at these isolated
Atlantic islands. The deep divergence between the highly supported Nesospiza-specific sequence Groups
2 and 3, and the clustering of Group 3 close to the distantly related passerines, provide strong support for preserved
ancestral polymorphism, and present evidence of one of the rare cases of extensive ancestral polymorphism in birds.