Figure 1

Effects of applying mutation filter to number of substitutions observed. To exclude nucleotide variations arising from sequencing error or other artifacts, we include only variations that are present in at least a number of different isolates. The graph shows an exponential decrease in the number of nucleotide variations detected with increasing filter stringency. The curve essentially flattens around 3, which corresponds to the filtering scheme that considers a sequence variation as a real mutation if it is shared by more than two isolates.