Inducible Mutability and Components of Adaptation Figure 4. Relative fitness of ‘round’ (R) isolates from UVR lineages 25-32. Relative fitness was measured by direct competition with the ancestor under (A) non-UVR and (B) UVR conditions. Single isolates of P. cichorii 302959 exhibiting the ‘round’ colony morphology were taken after 250 (open bars) and 500 (shaded bars) generations of experimental evolution. Fitness values are means and error bars represent standard error of the mean. Values are significant by two-tailed independent t-test (d.f. = 5, a = 0.05) where indicated (* P,0.05, ** P,0.01, *** P,0.001). P-paired values correspond to two-tailed paired t-tests (d.f. = 5, a = 0.05) between relative fitness values for isolates from generation 250 and 500. doi:10.1371/journal.pone.0015975.g004 at generation 250 and 21% at generation 500 (Figure 8A and B). Lag times of population samples were further reduced under UVR conditions by an average of 32% at generation 250 and 34% at generation 500 (Figure 8A and B). The lag times of isolates from UVR lineages again exhibited similar patterns of improvement significantly with greater average reductions under UVR condi- tions at both generation 250 and 500 (Figure 8A and B). Conversely, isolates from non-UVR lineages exhibited comparable reductions in lag times under non-UVR and UVR conditions at both generation 250 and 500 (Figure 8A and B). A complete list of doubling and lag times for each population sample and single- colony isolate is available in Tables S1 and S2. UVR Tolerance Changes in UVR tolerance could lead to changes in relative fitness under UVR conditions. Therefore, the UVR tolerance of population samples and isolates from each UVR and non-UVR lineages taken at generations 250 and 500 was determined following a single dose of UVC. An elevated dose of ,140 J m22 s21 was used to resolve subtle differences in UVR tolerance that could not be distinguished at the lower dosage used during lineage propagation. The ancestral P. cichorii 302959 strain exhibited a 0.7% survival rate at the elevated dose. Population samples from UVR lineages displayed the most consistent improvements in UVR tolerance with average survival rates of 4.3% and 4.0% at generations 250 and 500, respectively. R and F morphotype isolates exhibited a mixture of UVR tolerance phenotypes that were higher, lower, or comparable to that of the ancestor. A complete list of UVR tolerance measures for each population sample and isolate is available in Table S3. Relative fitness trajectories To track changes in relative fitness during lineage growth over the course of 500 generations, preserved population samples from four randomly selected lineages were obtained at different intervals and competed against the P. cichorii 302959 ancestor under both non-UVR and UVR conditions. The resulting fitness trajectories are plotted in Figure 8. UVR lineages 26 and 30 first exhibited fitness gains under their native, UVR conditions before later gaining fitness under the alternate, non-UVR conditions (Figure 9A and B). The relative fitness of UVR lineage 26 remained comparable under both non-UVR and UVR conditions PLoS ONE | www.plosone.org 6 January 2011 | Volume 6 | Issue 1 | e15975