Evolution of Fusarium graminearum A3/5 grown in a glucose-limited chemostat culture at a slow dilution rate

The evolution of Fusarium graminearum A3/5 grown in a glucose-limited chemostat at a dilution rate of 0.05 h-1 (doubling time of 13.9 h) was followed for 957 h or 69 generations. Periodic selection of advantageous mutants was monitored in the culture by determining increases and decreases in the concentration of cycloheximide-resistant macroconidia in the population. Six peaks in the concentration of cycloheximide-resistant macroconidia were observed representing five adaptive changes in the population; on average, an adaptive change occurred once every 148 ± 22 h (mean ± SE). The selection coefficient of strains present at the start of each increase in the concentration of cycloheximide-resistant macroconidia (i.e. after the establishment of a new advantageous strain) was determined relative to A3/5 and was found to increase progressively with time. When grown at a dilution rate of 0.05 h-1, the strain (A28-S) isolated from the last adaptive peak had a selection coefficient of 0.023 h-1 relative to A3/5, but A28-S lost its selective advantage when grown at a dilution rate of about 0.11 h-1 and was at a selective disadvantage when grown at a dilution rate higher than 0.11 h-1. The K(m) value (12 ± 5 μM) for uptake of glucose by A28-S was significantly lower than that for A3/5. The spontaneous mutation rate from cycloheximide sensitivity to cycloheximide resistance was estimated to be 1.8 (± 0.2) x 10-6 h-1 or 2.5 x 10-5 generation-1. The culture initially contained about 1 x 106 macroconidia ml-1 but this decreased with time until, at about 800 h, the culture contained only about 1 x 104 macroconidia ml-1. No highly branched (colonial) mutants were observed in glucose-limited cultures at dilution rates of 0.05 h-1, even though the evolution of the population was followed for a further 1345 h in a second chemostat, making a total evolutionary period of 2207 h or 159 generations.