Mhc class II genes of Darwin’s Finches: divergence by point mutations and reciprocal recombination

Abstract

The 15 species of Darwin’s Finches are one of the classical examples of adaptive radiation — the divergence of a single ancestral species into multiple forms adapted to various ecological niches. As such they provide an opportunity to study the evolution of the major histocompatibility complex (Mhc) genes in the period of adaptive radiation. In the present study two families of the Ground Finch Geospiza scandens from the Daphne Major Island in the Galapagos Archipelago were sampled and exon 2 of their Mhc class II B genes was sequenced. The sequences indicate that unlike the domestic fowl with its compact, gene-poor Mhc, the Mhc of the Darwin’s Finches contains more than half a dozen of class II B loci alone. Roth the exon 2 and intron 2 sequences suggest that the genes at the different loci are closely related and that some of them may have diverged from one another during the adaptive radiation of the Darwin’s Finches on the Galapagos Archipelago. The intron 2 sequences seem to have arisen by multiplication of a hexamer TCCCAG, ultimately reaching the length of ~ 2 kb. The repeats are still clearly recognizable in the introns of the genes at different loci. Sequence motifs in exon 2 of the class II B genes specify the polymorphic residues in the putative peptide-binding region (PBR) of the β2 domain. The evolution of the motifs can be explained by the sequential accumulation of point mutations and their incorporation (or fixation) by balancing selection. The divergence of the individual class II B genes is best explained by this process combined with occasional reciprocal recombination that shuffled the motifs.