Figure 4.

A schematic evolutionary model comparing the EIAV virulent strain to the vaccine strain during long-term passage in vitro. The virulent virus strains are presented with black circles, and the avirulent virus strains are presented with red circles. In this model, the env-Δ236D-phenotype sequences preexisted in EIAV_LN40 as a minority population. With serial passages in vitro, sequences containing the env-Δ236D-phenotype gradually became the predominant EIAV quasispecies and generated high replicative fidelity under the new environmental selection conditions. Virulence was simultaneously attenuated as a result, as discussed in the text.