Fast-evolving MHC class I polymorphism serves to diversify NK cell and CD8 T cell
responses in individuals, families, and populations. As only chimpanzee and bonobo have
strict orthologs of all HLA class I, their study gives unique perspective on the human
condition. We defined polymorphism of Papa-B, the bonobo ortholog of
HLA-B, for six wild bonobo populations. Sequences for
Papa-B exon 2 and 3 were determined from the genomic DNA in 255 fecal
samples, minimally representing 110 individuals. Twenty-two Papa-B
alleles were defined, each encoding a different Papa-B protein. No Papa-B is identical to
any chimpanzee Patr-B, human HLA-B, or gorilla Gogo-B. Phylogenetic analysis identified a
clade of MHC-B, defined by residues 45–74 of the α1 domain, which is
broadly conserved among bonobo, chimpanzee, and gorilla. Bonobo populations have
3–14 Papa-B allotypes. Three Papa-B are in all populations, and they are each of a
different functional type: allotypes having the Bw4 epitope recognized by killer cell
immunoglobulin-like receptors (KIR) of NK cells, allotypes having the C1 epitope also
recognized by KIR, and allotypes having neither epitope. For population ML these three
Papa-B are the only Papa-B allotypes. Although small in number, their sequence divergence
is such that the nucleotide diversity (mean p-distance) of Papa-B in ML
is greater than in the other populations, and also greater than expected for random
combinations of three Papa-B. Overall, Papa-B has
substantially less diversity than Patr-B in chimpanzee subspecies and
HLA-B in indigenous human populations, consistent with bonobo having
experienced narrower population bottlenecks.