Somatic cell variation during uninterrupted growth of Neurospora crassa in continuous growth tubes.

Continuous growth of Neurospora crassa was studied to determine whether the organism was capable of uninterrupted hyphal elongation, whether there was any evidence of homeostatic mechanisms controlling growth over extended periods and, particularly, whether spontaneous mutants with suppressive phenotypes were capable of being expressed. Parallel cultures, A and B, having a common origin, were incapable of uninterrupted growth but permanent cessations of growth were not encountered. The A-culture showed frequent stops of long duration whereas B had infrequent stops of short duration. This stop-start growth behaviour in the Aand Bcultures was determined by two different cytoplasmic factors. Both extranuclear mutants show decreased cytochrome-c oxidase activity (Bertrand & Pittenger, unpublished). These stopper phenotypes may be similar to ‘vegetative death’ in Aspergillus and ‘senescence’ in Podospora. Both Aand B-cultures showed several major increases and decreases in rate, in addition to more frequent minor fluctuations. The general constancy of growth, however, and the routine restoration of growth after cessations, as well as following either transient or long-term growth rate decreases, indicated that homeostatic mechanisms were capable of buffering the organisms against most deleterious intracellular effects. Two spontaneous nuclear mutants with decreased growth rates as homokaryons also accumulated during growth. These altered nuclear types, whose growth rates were clearly non-adaptive, nevertheless had selective values. Their proportions increased significantly during growth to the extent of either completely displacing the original type or of reaching a high enough proportion so that growth rates were decreased and the morphology of the cultures altered. These nuclear and extranuclear mutants appeared to have a selective advantage because of suppressive characteristics.