Nutrient-dependent selection of morphological mutants of Fusarium graminearum A3/5 isolated from long-term continuous flow cultures

Highly branched (colonial)mutants (MC1-1-, CC1-1, and C106) of Fusarium graminearum A3/5 were each grown with the parental strain (A3/5) in continuous flow cultures at high and low dilution rates using a variety of nutrient limitations. MC1-1 replaced A3/5 in all nutrient-limited cultures tested (glucose-, Mg2+-, ammonium-, and sulphate-limited cultures), suggesting that it has a higher maximum specific growth rate than A3/5. Compared with A3/5, C106 was positively selected for in Mg2+-limited cultures and its selection coefficient was higher at low than at high dilution rates, suggesting that, compared with A3/5, it has a reduced saturation constant (K(s)) for Mg2+. However, in batch culture, C106 and A3/5 had the same (15 μM) apparent K(s) value for Mg2+. C106 was replaced (negative selection coefficient) by A3/5 in glucose-, ammonium-, and phosphate-limited continuous flow cultures, but was neither at an advantage nor a disadvantage (i.e., it behaved as a neutral mutation) in sulphate-limited cultures. CC1-1 replaced A3/5 when they were grown together in glucose-, maltose-, or ribose-limited continuous flow cultures, but not in fructose-, xylose-, ammonium-, or phosphate-limited cultures. Because A3/5 and CC1-1 had similar K(m) values (30 μM) for glucose, and because the selective advantage of CC1-1 was maintained in maltose-limited cultures (maltose was not hydrolyzed extracellularly), it was concluded that the selective advantage of CC1-1 did not result from it having a lower K(s) for glucose than the parental strain. Rather, the data suggested that the activity of phosphoketopentoepimerase may be altered by the CC1-1 mutation.