Common chimpanzees have greater diversity than humans at two of the three highly polymorphic MHC class I genes

MHC class I polymorphism improves the defense of vertebrate species against viruses and other intracellular pathogens. To see how polymorphism at the same class I genes can evolve in different species we compared the MHC-A, M HC-B, and MHC-C loci of common chimpanzees and humans. Diversity in 23 Patr -A. 32 Patr-B, and 18 Patr-C alleles obtained from study of 48 chimpanzees was compared to diversity in 66 HLA-A, 149 HLA-B, and 41 HLA-C alleles obta ined from a study of over 1 million humans. At each locus. alleles group hi erarchically into families and then lineages. No alleles or families are sh ared by the two species, commonality being seen only at the lineage level. The overall nucleotide sequence diversity of MHC class I is estimated to be greater for modern chimpanzees than humans, Considering the numbers of lin eages, families, and alleles, Patr-B and Patr-C have greater diversity than the HLA-B and HLA-C, respectively. In contrast, Patr-A has less polymorphi sm than HLA-A, due to the absence of A2 lineage alleles, The results are co nsistent: with ancestral humans having passed through a narrower population bottleneck than chimpanzees, and with pathogen-mediated selection having f avored either preservation of A2 lineage alleles on the human line and/or t heir extinction on the chimpanzee line.