DEVELOPMENT OF MORPHOLOGICAL HETEROGENEITY IN GLUCOSE-LIMITED CHEMOSTAT CULTURES OF ASPERGILLUS-ORYZAE

Aspergillus oryzae was grown for 39 d at 34-degrees-C in a glucose-lim ited chemostat culture at a dilution rate of ca 0.18 h-1 (doubling tim e of ca 3.85 h). During this time the parental strain was displaced an d the culture became morphologically heterogeneous. Four morphological mutants were recognized: a non-sporing flat mutant, a densely sporing semi-colonial mutant, a colonial mutant and a fluffy mutant. On agar medium the flat mutant (hyphal growth unit length, G = 273 mum) was mo re sparsely branched than the parental strain (G = 192 mum), and the d ensely sporing semi-colonial mutant (G = 62 mum) was more highly branc hed than the parental strain. The parental strain was initially displa ced from the culture by the flat mutant, then by the densely sporing s emi-colonial mutant, and at the end of the experiment the flat and den sely sporing semi-colonial mutants made up ca 65% and ca 35% of the po pulation respectively. Some of the morphological heterogeneity observe d may have been derived from biomass which periodically accumulated on the internal surfaces of the vessel, but under the prevailing environ mental conditions the densely sporing semi-colonial mutant had a selec tive advantage (selection coefficient = 0.04 h-1) over the parental st rain. A second chemostat inoculated with mycelium derived from uv-irra diated conidia initially showed greater morphological heterogeneity th an the one inoculated with non-irradiated conidia, but eventually this chemostat also was dominated by a non-sporing flat mutant which forme d 75-80% of the population.