Hybrid Origin of SIV in Chimpanzees.

The ancestry of human immunodeficiency virus-1 (HIV-1) has been traced to simian immunodeficiency virus (SIVcpz) infecting chimpanzees (Pan troglodytes) in west central Africa, but the origin of SIVcpz itself remains unknown. Species- specific strains of SIV have been identified in more than 20 species of African primates. The known strains of SIV cluster into six distinct major lineages, whose order of divergence has been difficult to disentangle. SIV phylogeny is complicated by a number of viruses that occupy different positions in the evolutionary tree dependent on the region of genome analyzed, indicative of recombination events among viruses from different major lineages. Two recently characterized viruses, SIVrcm, from red-capped mangabeys (Cercocebus torquatus), and SIVgsn, from greater spot-nosed monkeys (Cercopithecus nictitans), are most closely related to SIVcpz, but only in certain genomic regions, and so both have been interpreted as recombinant viruses. The finding of a hybrid origin of SIV in chimpanzees has important implications.