EVOLUTION OF FUSARIUM-GRAMINEARUM A3 5 GROWN IN A SERIES OF GLUCOSE-LIMITED CHEMOSTAT CULTURES AT A HIGH DILUTION RATE/

The evolution of Fusarium graminearum A3/5 was studied in a series of glucose-limited chemostat cultures grown at a dilution rate of 0.18 h( -1): the second chemostat was inoculated with macroconidia harvested f rom a sub-culture derived from the first chemostat, and the third chem ostat was inoculated with macroconidia harvested from a sub-culture de rived from the second chemostat. Using this method it was possible to follow the evolution of F. graminearum for ca 3 months (531 generation s). Periodic selection of advantageous mutants was detected in the pop ulation by monitoring increases and decreases in the proportion of cyc loheximide-resistant macroconidia; 15 peaks of cycloheximide resistanc e were observed in the three cultures representing 14 adaptive changes with an adaptive change occurring in the population on average once e very 135 +/- 10 h (mean +/- SE.). After 648 h of cultivation, a highly branched (colonial) mutant was detected in the population and this mu tant increased in concentration (with a selection coefficient of 0.016 h(-1) compared with the parental strain) and eventually formed 82-92% of the population but never completely supplanted the sparsely-branch ed population. A second mutant (A21-XS), more sparsely branched than A 3/5, was isolated at 1277 h.