Complex events in the evolution of the haptoglobin gene cluster in primates

Southern blot analyses of genomic DNA show that new world monkeys have only one haptoglobin gene but that chimpanzees, gorillas, orangutans, and old world monkeys have three. Humans have two: haptoglobin (Hp) and haptoglobin-related (Hpr). These observations suggest that a triplication of the haptoglobin locus occurred after the divergence of the new world monkeys, followed by a deletion of one locus in humans. To investigate these events, we have cloned the haptoglobin gene cluster in chimpanzee. The organization of the Hp and Hpr genes in chimpanzees is the same as in humans, including a retrovirus-like sequence in the first intron of Hpr. The third gene, which we name Hpp for haptoglobin primate, is 16 kilobases downstream of Hpr. A second copy of the retrovirus-like sequence occurs between Hpr and Hpp. The nucleotide sequence of the chimpanzee Hpp gene suggests that it may code for a functional protein, but the chimpanzee Hpr gene has a single base deletion in exon 5 that causes a frameshift. Comparison of the human and chimpanzee sequences suggests that the human Hpr gene was generated by a homologous unequal crossover between ancestral Hpr and Hpp genes. The crossover point lies within a 1.3-kilobase region containing exon 5 and 500 nucleotides 3' to the genes, but the exact point is obscured by a subsequent gene conversion event.