Evolution of the mammalian MHC: natural selection, recombination, and convergent evolution

The genes that encode molecules involved in antigen presentation within the class I and class II regions of the mammalian major histocompatibility com plex (MHC) include several that are highly polymorphic. There is evidence t hat this polymorphism is maintained by positive selection, most likely over dominant selection, relating to their role in presenting foreign peptides t o T cells. This selection can maintain allelic lineages for much longer per iods of time than neutral polymorphisms are expected to last, but sharing o f polymorphic amino acid motifs among species of different mammalian orders is due to independent (or convergent) evolution rather than common ancestr y. It has been suggested that interallelic recombination (gene conversion) plays a role in enhancing polymorphism, but there is evidence of striking d ifferences among loci with respect to the rare at which such recombination has contributed to current polymorphism. Recent attempts to interpret linka ge relationships in the MHC region as evidence of ancient genomic duplicati ons are not supported by phylogenetic analysis. Rather, natural selection m ay have played a role in the linkage of other genes to those of the MHC.