REPEATED TRANSFER OF SMALL RNA VIRUS POPULATIONS LEADING TO BALANCED FITNESS WITH INFREQUENT STOCHASTIC DRIFT

The population dynamics of RNA viruses have an important influence on fitness variation and, in consequence, on the adaptative potential and virulence of this ubiquitous group of pathogens. Earlier work with ve sicular stomatitis virus showed that large population transfers were r eproducibly associated with fitness increases, whereas repeated transf ers from plaque to plaque (genetic bottlenecks) lead to losses in fitn ess. We demonstrate here that repeated five-plaque to five-plaque pass age series yield long-term fitness stability, except for occasional st ochastic fitness jumps. Repeated five-plaque passages regularly altern ating with two consecutive large population transmissions did not caus e fitness losses, but did limit the size of fitness gains that would o therwise have occurred. These results underscore the profound effects of bottleneck transmissions in virus evolution.