Analysis of HIV-1 env gene sequences reveals evidence for a low effective number in the viral population.

Selection is usually considered to be the dominant force controlling viral variation; the large population sizes suggest that deterministic population genetic models are appropriate. To investigate their validity for HIV, samples of env gene sequences were tested for departure from neutrality because of mutation-selection balance. None of the samples departed significantly when tested as nucleotide sequences. At the amino acid level, significantly elevated diversity was detected in two samples within, but not outside, the V3 loop. The effective population number has been estimated (using a phylogenetic method) to be close to 10(3). Estimates from nucleotide diversity are about 2-fold lower. The low value of the effective population number might arise from high variability in progeny number between infected cells, from the expansion in population number from a small inoculum as the virus is transmitted between hosts, or from variable selection at linked sites. These results suggest that the population genetics of HIV are best described by stochastic models.