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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-, Mg(2+)-, ammonium-, and sulphate-limited cultures), suggesting that it has a higher maximum specific growh rate than A3/5. Compared with A3/5, C106 was positively selected for in Mg(2+)-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 Mg(2+). However, in batch culture, C106 and A3/5 had the same (15 microM) appaent K(s) value for Mg(2+). C106 was replaced (negative selection coefficient) by A3/5 in gluose-, ammonium-, and phsophate-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 phsophate-limited cultures. Because A3/5 and CC1-1 had similar K(m) values (30 microM) 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.

By monitoring increases and decreases in the proportion of cycloheximide-resistant macroconidia, periodic selection was observed in populations of the filamentous fungus Fusarium graminearurn, grown in glucose-limited chemostat cultures. The results indicated that periodic selection of advantageous mutants of F. graminearurn occurred at intervals of about 124 h at both high (D = 0.19 h-', approximately 34 generations) and low (D = 0-06 h-', approximately 11 generations) dilution rates. Several 'adaptive' peaks (each indicating the appearance of an advantageous mutation) were observed before morphological (highly branched) mutants appeared in the populations; these mutants have previously been observed to have a selective advantage over the parental strain. At intervals, macroconidia harvested from the chemostat were used to inoculate plates of non-antibioticcontaining agar medium, and it was possible to monitor periodic selection in the original chemostat culture using second generation macroconidia harvested from these cultures. The proportion of cycloheximide-, potassium chlorate-, and p-fluoro-DL-phenylalanine-resistant macroconidia in these second generation macroconidia changed in a pattern similar to that observed when monitoring the proportion of cycloheximide-resistant macroconidia in the first generation population harvested directly from the chemostat. The experiments demonstrated that populations of filamentous fungi are heterogeneous and that much of this heterogeneity may already be present at the end of batch growth, i.e. before the onset of continuous cultivation.

The response of iusarium gramineanrm to choline, acetylcholine and a number of related analogues was investigated and their ability to induce a morphological response quantified. A number of mutants resistant to the alkylating agent nitrogen mustard (nim strains) were generated and found to have lost the ability to transport choline. These mutants were found to be insensitive to choline and acetylcholine but not to betaine, ethanolamine and other analogues. In addition, the nonlcompetitive inhibitor hemicholinium-3 was also found to reduce the morphological effect of choline, proving that transport of choline into the hypha is essential for the morphological response. Acetylcholinesterase inhibitors blocked the morphological response to acetylcholine but had no effect on the response to choline, suggesting the presence of a membrane-or wall-bound acetylcholinesterase that hydrolyses acetylcholine to choline which subsequently induces the morphological response. Studies on the in who chitin synthase activity revealed that addition of choline caused a transient 75% increase in chitin synthase activity within 30 s, the rate rapidly returning to that observed before the addition of choline.No such effect was observed with the nim mutants.

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