Species-specific evolution of MHC class I genes in the higher primates.

Humans express three highly polymorphic 'classical' (HLA-A,B and C) and three conserved 'non-classical' (HLA-E, F and G) MHC class I genes. Their comparison with the MHC class I genes of apes and monkeys reveals the differential extent to which MHC class I genes have been preserved during primate evolution. African apes have orthologues of all six human genes, and although allelic lineages of the A and C loci are shared, these species share none of the human alleles. In Asian apes, several MHC class I genes show significant differences from the human genes, a trend which continues with the Old World monkeys, and even more so in the New World monkeys, where E and F are the only human gene orthologues. The C locus is confined to humans and apes. Multiple A-related and B-related loci have been identified in apes and Old World monkeys showing that duplication of these loci has been a common event during primate evolution. Certain of the daughter loci exhibit low polymorphism, suggesting they have adopted a non-classical function. The differing rates at which MHC class I genes have evolved during primate evolution likely reflects their differing functions in the immune response.