Evidence for negative selective pressure in HIV-2 evolution in vivo

In three patients, the average frequency of synonymous substitutions (dS) was significantly higher than the nonsynonymous substitutions (dN) whereas in the fourth patient the dN/dS ratio approached the unity. These data demonstrate that negative selective pressure determines the evolution of the HIV-2 env C2-C3 region in vivo. Our results suggest that throughout HIV-2 infection low virus adaptation to strong selective pressures (e.g. immune pressure) promotes the predominance of a few optimally adapted forms.