Faculty Opinions recommendation of Quasispecies diversity determines pathogenesis through cooperative interactions in a viral population.

An RNA virus population does not consist of a single genotype; rather, it is an ensemble of related sequences, named quasispecies 1-4.Quasispecies arise from rapid genomic evolution powered by the high mutation rate of RNA viral replication 5-8.While a high mutation rate is dangerous for a virus as it results in nonviable individuals, it has been hypothesized that high mutation rates create a "cloud" of potentially beneficial mutations at the population level, which afford the viral quasispecies a greater probability to evolve and adapt to new environments and challenges during infection 4,9-11.Importantly, mathematical modelling predicts that the viral quasispecies is not simply a collection of diverse mutants but a group of interactive variants, which together contribute to the characteristics of the population 4,12.In this view, viral populations, rather than individuals, are the target of evolutionary selection 4,12.To test this hypothesis we examined whether limiting genomic diversity affects viral pathogenesis.We find that poliovirus carrying a high fidelity polymerase replicates at wildtype levels but generates less genomic diversity and is unable to adapt to adverse growth conditions.In infected animals, the reduced viral diversity led to loss of neurotropism and an attenuated pathogenic phenotype.Strikingly, expanding quasispecies diversity of the high fidelity virus by chemical mutagenesis prior to infection restored neurotropism and pathogenesis.Analysis of viruses isolated from brain provides direct evidence for complementation between members within the quasispecies, indicating that selection indeed occurred at the population level rather than on individual mutants.Our study provides direct evidence for a fundamental prediction of the quasispecies theory and establishes a compelling link between mutation rate, population dynamics and pathogenesis.To examine the biological role of viral quasispecies we searched for viruses carrying a polymerase with enhanced fidelity, which should decrease genomic diversity and restrict quasispecies complexity.To this end, we isolated poliovirus resistant to ribavirin (Supplementary Fig. S1), a mutagen that increases mutation frequency of poliovirus replication above the tolerable error threshold and drives the virus into viral extinction 13,14.The ribavirinresistant mutant replicated efficiently in the presence of ribavirin, producing over 300-fold more virus than wildtype (Supplementary Fig. S2a).The ribavirin-resistant phenotype is determined by a single point mutation, Gly-64 to Ser (G64S), within the finger domain of the viral polymerase 15.Interestingly, the same mutation was independently isolated in another screen for ribavirin-resistant polioviruses 16, suggesting that there are limited mechanistic avenues to overcome the mutagenic effects of ribavirin.