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

¹School of Biological Sciences, 1.800 Stopford Building, University of Manchester, Manchester M13 9PT, UK
²Manchester Biotechnology Centre, University of Manchester Institute of Science and Technology, PO Box 88, Manchester M60 1QD, UK

Author for correspondence: Marilyn G. Wiebe. Tel: +44 61 275 3918. Fax: +44 61 275 5656.

ABSTRACT

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^-1relative to A3/5, but A28-S lost its selective advantage when grown at a dilution rate of about 0.11 h^-1and was at a selective disadvantage when grown at a dilution rate higher than 0.11 h^-1. The KM 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^-6h^-1or 2.5 x 10^-5generation^-1. The culture initially contained about 1 x 10⁶macroconidia ml^-1 but this decreased with time until, at about 800 h, the culture contained only about 1 x 10⁴macroconidia ml^-1. No highly branched (colonial) mutants were observed in glucose-limited cultures at dilution rates of 0.05 h^-1even 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.