CROSS-SPECIES TRANSMISSION AND RECOMBINATION OF AIDS VIRUSES

Acquired Immune Deficiency Syndrome (AIDS) is caused by two different Human Immunodeficiency Viruses, HIV-1 and HIV-2. Closely related virus es (SIVs) are found in many species of non-human primates. Phylogeneti c analyses indicate that cross-species transmission events have been q uite frequent. Both HIV-1 and HIV-2 appear to have resulted from multi ple transfers of lentiviruses naturally infecting other primates; the source of HIV-2 appears to have been sooty mangabeys, whereas for HIV- 1 the source may have been chimpanzees. Phylogenetic analyses also pro vide evidence that recombination has occurred between divergent viruse s in vivo. Evolutionary trees based on various regions of the viral ge nome generally have consistent branching orders. However, some isolate s fall into significantly different phylogenetic positions, indicating that their genomes are mosaics of sequences with different evolutiona ry histories. This implies that co-infection with highly divergent vir al strains can occur in HIV-infected humans and SIV-infected primates; this could lead to the generation of hybrid genomes with significantl y altered biological properties, and also has important implications f or HIV vaccine development programmes.