Different evolution of simian immunodeficiency virus in a natural host and a new host

To address the mechanisms of host-virus adaptation and pathogenesis of lentiviral infections, we compared the evolution of the same isolate of simian immunodeficiency virus (SIVsmm9) in two different situations: nonpathogenic infection of its natural host, the sooty mangabey, and AIDS-inducing infection of a new host, the rhesus macaque. Samples were obtained at 6, 12, and 23 or 30 months postinfection from three animals of each species. Sequences were derived from the V1 and V2 domains of the surface glycoprotein. In the macaques, we observed specific variations absent from all mangabey samples, indicating that different host species select different virus variants. In the macaques, we also observed a different shape in the phylogenetic tree, a lower divergence of sibling sequences, and a lower synonymous/nonsynonymous change ratio than in the mangabeys. This suggests that the viral population is larger and submitted to weaker selection pressures when host-virus adaptation is achieved, such as in the mangabey.