Chimpanzee MHC class I A locus alleles are related to only one of the six families of human A locus alleles.

There are nearly 50 alleles at the highly polymorphic HLA-A class I locus that fall into six distinct families. To determine the allelic repertoire and the mechanism of generation of diversity of the A locus in primates we have analyzed A locus alleles from 28 apparently unrelated chimpanzees (Pan troglodytes) and bonobos (Pan paniscus). We have, therefore, compared the sequences of 19 HLA-A homologues from chimpanzees and bonobos to 42 HLA-A sequences. HLA-A homologues were well preserved in chimpanzees and bonobos with very few new substitutions present in the A locus alleles of both species of chimpanzee. Surprisingly, all chimpanzees and bonobos expressed A locus alleles related to only one of the six families of human HLA-A alleles. This suggests that the common ancestor of these two species either passed through a genetic bottleneck or that selection has favored the maintenance of the HLA-A1, -A3, -A11 family in chimpanzees.