SIZE OF GENETIC BOTTLENECKS LEADING TO VIRUS FITNESS LOSS DETERMINED BY MEAN INITIAL POPULATION FITNESS

Genetic bottlenecks are important events in the genetic diversificatio n of organisms and colonization of new ecological niches. Repeated bot tlenecking of RNA viruses often leads to fitness losses due to the ope ration of Muller's ratchet. Herein we use vesicular stomatitis virus t o determine the transmission population size which leads to fitness de creases of virus populations. Remarkably, the effective size of a gene tic bottleneck associated with fitness loss is greater when the fitnes s of the parental population increases. For example, for starting viru s populations with low fitness, population transfers of five-clone-to- five-clone passages resulted in a fitness increase. However, when a pa rental population with high fitness,vas transferred, 30-clone-to-30-cl one passages were required simply to maintain fitness values.