Exponential fitness gains of RNA virus populations are limited by bottleneck effects

Fitness is a parameter that quantitatively measures adaptation of a virus t o a given environment. We have previously reported exponential fitness gain s of large populations of vesicular stomatitis virus replicating in a const ant environment (I. S. Novella et al., Proc. Natl. Acad. Sci. USA 92:5841-5 844, 1995). In this paper, we report that during long-term passage of such large viral populations, fitness values reached a high-fitness plateau duri ng which stochastic fitness variations were observed. This effect appears l ikely to be due to bottleneck effects on very high fitness populations.