FIG. 2.
Initial fitness values determine the effective size of a bottleneck regarding the maintenance of fitness values. We used published data from the work of Novella et al. (25) to obtain the plot shown. To fit our data, the best regression value (R = 0.981) was obtained by using an exponential fit E = 1.675 × 100.43W, where E is the effective size of a bottleneck and W is the initial fitness of the population. With a population size of 2 × 105 infectious units, bottleneck effects are expected when fitness reaches values of about 12 (broken lines), and our present data fit this model. Bottleneck sizes were calculated by averaging the results from several replicas. For instance, after 20 consecutive 5-plaque-to-5-plaque passages of MARM U (W = 1.0), the resulting fitness values of the six replicas ranged from 0.81 to 2.03. Overall, there were no significant fitness changes, but particular populations may show an increase, a decrease, or no change in fitness. This means that in a particular large population, bottleneck effects might be observed at fitness values lower or higher than 12.