A genetic study of revertants of an unstable serine-requiring mutant of Neurospora crassa

A serine auxotroph of Neurospora crassa, ser (JBM 4-13), exhibits genetic instability, reverting to prototrophy at high frequency. This study examines reversion in a mutant stock and the genetic behavior of serine-independent cultures derived from that reversion. Individually isolated ascospores of the mutant, as well as subcultures of individual progeny, exhibited spontaneous reversion frequencies ranging from 8 X 10-7 to 3.7 x 10- 2 . The great variability in reversion frequency and variability in both mutant and revertant colony size were notable properties. All revertants tested demonstrated qualitatively similar growth characteristics and identical genetic behavior. Three revertants were studied genetically. Revertant #9 had behaved in earlier crosses as if it were a heterocaryon. In this study, heterocaryosis was verified and the revertant was reisolated and crossed to wild-type. Newly isolated revertants 116Rl and 123R6 were made homocaryotic by backcrossing to a mutant stock; they were then outcrossed to wild-type. No serine-requiring progeny were isolated from all spores examined for the crosses of the revertants to wild-type. These results indicated that the reversion of ser (JBM 4-13) involves either back mutation or a closely linked suppressor. The behavior of ser (JBM 4-13) is examined in the light of several models proposed to account for genetic instability in Neurospora and other organisms.