EVOLUTIONARY DYNAMICS OF FITNESS RECOVERY FROM THE DEBILITATING EFFECTS OF MULLER'S RATCHET

The great adaptability shown by RNA viruses is a consequence of their high mutation rates. The evolution of fitness in a severely debilitated, clonal population of the nonsegmented ribovirus vesicular stomatitis virus (VSV) has been compared under five different demographic regimes, ranging from severe serial bottleneck passages (one virion) to large population passages (10⁵ virions or more) under similar environmental conditions (cell culture type and temperature). No matter how small the bottleneck, the fitness of the evolved populations was always higher than the fitness of the starting population; this result is clearly different from that previously reported for viruses with higher fitness. The reattainment of fitness under a regime of serial population passages showed two main characteristics: (1) the rate of adaptation was higher during early passages; and (2) a maximum fitness value was reached after a large number of passages. The maximum fitness reached by this initially debilitated clone was similar to the fitness of wild-type virus. The practical implications of these findings in the design of vaccines using attenuated viruses are also discussed.