Genetic variability

HIV differs from many viruses in that it has very high genetic variability. This diversity is a result of its fast replication cycle, with the generation of 10⁹ to 10¹⁰ virions every day, coupled with a high mutation rate of approximately 3 x 10^-5 per nucleotide base per cycle of replication and recombinogenic properties of reverse transcriptase.This complex scenario leads to the generation of many variants of HIV in a single infected patient in the course of one day.This variability is compounded when a single cell is simultaneously infected by two or more different strains of HIV. When simultaneous infection occurs, the genome of progeny virions may be composed of RNA strands from two different strains. This hybrid virion then infects a new cell where it undergoes replication. As this happens, the reverse transcriptase, by jumping back and forth between the two different RNA templates, will generate a newly synthesized retroviral DNA sequence that is a recombinant between the two parental genomes.This recombination is most obvious when it occurs between subtypes.

The closely related simian immunodeficiency virus (SIV) exhibits a somewhat different behavior: in its natural hosts, African green monkeys and sooty mangabeys, the retrovirus is present in high levels in the blood, but evokes only a mild immune response,does not cause the development of simian AIDS,and does not undergo the extensive mutation and recombination typical of HIV.By contrast, infection of heterologous hosts (rhesus or cynomologus macaques) with SIV results in the generation of genetic diversity that is on the same order as HIV in infected humans; these heterologous hosts also develop simian AIDS.The relationship, if any, between genetic diversification, immune response, and disease progression is unknown.

Three groups of HIV-1 have been identified on the basis of differences in env: M, N, and O.Group M is the most prevalent and is subdivided into eight subtypes (or clades), based on the whole genome, which are geographically distinct.The most prevalent are subtypes B (found mainly in North America and Europe), A and D (found mainly in Africa), and C (found mainly in Africa and Asia); these subtypes form branches in the phylogenetic tree representing the lineage of the M group of HIV-1. Coinfection with distinct subtypes gives rise to circulating recombinant forms (CRFs). In 2000, the last year in which an analysis of global subtype prevalence was made, 47.2 percent of infections worldwide were of subtype C, 26.7 percent were of subtype A/CRF02_AG, 12.3 percent were of subtype B, 5.3 percent were of subtype D, 3.2 percent were of CRF_AE, and the remaining 5.3 percent were composed of other subtypes and CRFs.Most HIV-1 research is focused on subtype B; few laboratories focus on the other subtypes.The genetic sequence of HIV-2 is only partially homologous to HIV-1 and more closely resembles that of SIV than HIV-1.