A UNIQUELY HIGH-LEVEL OF RECOMBINATION AT THE HLA-B LOCUS

Major histocompatibility complex (MHC) loci are some of the most polym orphic genes in the animal kingdom. Recently, it has been suggested th at although most of the human MHC loci are relatively stable, the HLA- B locus can undergo rapid changes, especially in isolated populations. To investigate the mechanisms of HLA-B evolution we have compared the sequences of 19 HLA-B homologues from chimpanzees and bonobos to 65 H LA-B sequences. Analysis of the chimpanzee and bonobo HLA-B homologues revealed that despite obvious similarities between chimpanzee and hum an alleles in exon 2, there was little conservation of exon 3 between humans and the two chimpanzee species. This finding suggests that, unl ike all other HLA loci, recombination has characterized the HLA-B locu s and its homologues for over 5 million years.