Figure 1. Phylogenetic tree of human and simian lentiviruses.

The genetic similarity between different HIV and simian immunodeficiency virus (SIV) strains was compared by aligning the full genome sequences of 87 human and simian lentiviruses using ClustalX v.1.83 (the accession numbers are available on request). Phylogenetic trees based on nucleotide distance were constructed by neighbour-joining methods as implemented in ClustalX with 1,000 bootstrap resamplings (not presented) and schematically represented with the TreeView program. HIV-2 and HIV-1 share only 50–60% sequence identity and cluster at distinct locations on the phylogenetic tree whereas SIV_cpz branches out from the root of the HIV-1 groups. The origins of these HIV-1 groups in southern Cameroon have recently been described and indicate two probable jumps from chimpanzee (groups M and N) and gorilla (group O) species. HIV-1 M subtypes probably evolved from a discrete introduction into the human population and then diverged into different subtypes. The subtypes defined as 'A-like' describe HIV-1 isolates with sequences that map phylogenetically more to subtype A than to any other subtype. For example, the recombinant form CRF02_AG (such as 02 AG.NG.IBNG in the HIV-1 group M A-like cluster) has longer genomic segments that are more related to subtype A than to subtype G. M, main; N, new.