MHC class I genes evolve by recombination, largely within loci, and selection (presumably pathogen-driven) maintains these new alleles in the population. Although chimpanzees and humans share an A locus allelic lineage, the B locus molecules of the chimpanzee were less similar to human B locus molecules. The A and B locus molecules in rhesus and cynomolgus monkeys were even less similar to their human counterparts, with little conservation of allelic lineages between macaques and humans. In contrast to the instability of MHC class I alleles and allelic lineages, the MHC class I loci themselves are well conserved during the evolution of Great Ape and Old World primates. Homologues of HLA-A, -B, -E, and -F have been found in macaques. The C locus, however, has only been found in gorillas and chimpanzees, whereas in orangutans and rhesus monkeys it is possible that the A and B loci have been duplicated. Classical New World monkey MHC class I genes are all more similar to the nonclassical HLA-G gene and a nonclassical F-like gene is present in the cotton-top tamarin. Duplication and either subsequent deletion or expansion of MHC class I loci, therefore, appear to be the modus operandi of the evolution of these genes in primates.
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