EVOLUTION OF FITNESS IN EXPERIMENTAL POPULATIONS OF VESICULAR STOMATITIS-VIRUS

The evolution of fitness in experimental clonal populations of vesicul ar stomatitis virus (VSV) has been compared under different genetic (f itness of initial clone) and demographic (population dynamics) regimes . In spite of the high genetic heterogeneity among replicates within e xperiments, there is a clear effect of population dynamics on the evol ution of fitness. Those populations that went through strong periodic bottlenecks showed a decreased fitness in competition experiments with wild type. Conversely, mutant populations that were transferred under the dynamics of continuous population expansions increased their fitn ess when compared with the same wild type. The magnitude of the observ ed effect depended on the fitness of the original viral clone. Thus, h igh fitness clones showed a larger reduction in fitness than low fitne ss clones under dynamics with included periodic bottleneck. In contras t, the gain in fitness was larger the lower the initial fitness of the viral clone. The quantitative genetic analysis of the trait ''fitness '' in the resulting populations shows that genetic variation for the t rait is positively correlated with the magnitude of the change in the same trait. The results are interpreted in terms of the operation of M ULLER's ratchet and genetic drift as opposed to the appearance of bene ficial mutations.