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) w as followed for 957 h or 69 generations. Periodic selection of advanta geous mutants was monitored in the culture by determining increases an d decreases in the concentration of cycloheximide-resistant macroconid ia in the population. Six peaks in the concentration of cycloheximide- resistant macroconidia were observed representing five adaptive change s in the population; on average, an adaptive change occurred once ever y 148 +/- 22 h (mean +/- se). The selection coefficient of strains pre sent at the start of each increase in the concentration of cycloheximi de-resistant macroconidia (i.e. after the establishment of a new advan tageous strain) was determined relative to A3/5 and was found to incre ase 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 sele ction 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 high er than 0.11 h(-1), The K-m value (12 +/- 5 mu M) for uptake of glucos e by A28-S was significantly lower than that for A3/5. The spontaneous mutation rate from cycloheximide sensitivity to cycloheximide resista nce was estimated to be 1.8 (+/- 0.2) x 10(-6) h(-1) or 2.5 x 10(-5) g eneration(-1). The culture initially contained about 1 x 10(6) macroco nidia ml(-1) but this decreased with time until, at about 800 h, the c ulture contained only about 1 x 10(4) macroconidia ml(-1). No highly b ranched (colonial) mutants were observed in glucose-limited cultures a t dilution rates of 0.05 h(-1), even though the evolution of the popul ation was followed for a further 1345 h in a second chemostat, making a total evolutionary period of 2207 h or 159 generations.