PH OSCILLATIONS AND CONSTANT LOW PH DELAY THE APPEARANCE OF HIGHLY BRANCHED (COLONIAL) MUTANTS IN CHEMOSTAT CULTURES OF THE QUORN(R) MYCO-PROTEIN FUNGUS, FUSARIUM-GRAMINEARUM A3 5/

At pH 5.8, highly branched (colonial) mutants appear in glucose-limite d chemostat cultures of Fusarium graminearum A3/5 after ca. 400 h (ca. 107 generations) of growth. The appearance of these mutants was delay ed by up to 144 h (45 generations) when the culture was switched at in tervals of 120 h between pH 4.8 and 6.6. The concentration of cyclohex imide-resistant macroconidia in the culture was used as an indicator o f the period ic selection of advantageous mutants and it was found tha t, in chemostat populations subjected to pH oscillations, the interval (210 +/- 20 h) between peaks was nearly double that observed in chemo stat populations cultured at constant pH (124 +/- 12 h at constant pH 5.8 and 120 h +/- 17 h at constant pH 4.5), indicating that the popula tion evolved more slowly under oscillating pH than under constant pH. When grown in mixed culture with the parental strain (A3/5), the selec tive advantage of two colonial mutants isolated from chemostat culture s grown under conditions of oscillating pH was found to be pH dependen t. Compared to cultures grown at constant pH 5.8, a delay of ca. 312 h (87 generations) in the appearance of colonial mutants was observed w hen F. graminearum A3/5 was grown in glucose-limited chemostat culture at constant pH 4.5. (C) 1996 John Wiley & Sons, Inc.