SUBCLONAL COMPONENTS OF CONSENSUS FITNESS IN AM RNA VIRUS CLONE

Most RNA virus populations exhibit extremely high mutation frequencies which generate complex, genetically heterogeneous populations referre d to as quasispecies. Previous work has shown that when a large spectr um of the quasispecies is transferred, natural selection operates, lea ding to elimination of noncompetitive (inferior) genomes and rapid gai ns in fitness. However, whenever the population is repeatedly reduced to a single virion, variable declines in fitness occur as predicted by the Muller's ratchet hypothesis. Here, we quantitated the fitness of 98 subclones isolated from an RNA virus clonal population. We found a normal distribution around a lower fitness, with the average subclone being less fit than the parental clonal population. This finding demon strates the phenotypic diversity in RNA virus populations and shows th at, as expected, a large fraction of mutations generated during virus replication is deleterious, This clarifies the operation of Muller's r atchet and illustrates why a large number of virions must be transferr ed for rapid fitness gains to occur. We also found that repeated genet ic bottleneck passages can cause irregular stochastic declines in fitn ess, emphasizing again the phenotypic heterogeneity present in RNA vir us populations. Finally, we found that following only 60 h of selectio n (15 passages in which virus yields were harvested after 4 h), RNA vi rus populations can undergo a 250% average increase in fitness, even o n a host cell type to which they were already well adapted. This is a remarkable ability; in population biology, even a much lower fitness g ain (e.g., 1 to 2%) can represent a highly significant reproductive ad vantage. We discuss the biological implications of these findings for the natural transmission and pathogenesis of RNA viruses.