An analysis of spontaneous reversion in conidial spores and ascospores of Neurospora crassa

Cultures of a serine auxotroph of Neurospora crassa, ser (JBM 4-13), demonstrate a high frequency of spontaneous reversion to prototrophy. This study examines two questions: First, whether the phenomenon of spontaneous reversion occurs in all vegetative cultures and, second, whether reversion occurs in stored ascospores. Eight vegetative cultures, which had not shown reversion from ser (JBM 4-13) to wild-type in a previous study, were repeatedly tested for the presence of revertant nuclei. Each of the eight stocks ultimately displayed a high frequency of revertant nuclei during a study of serial subcultures of the mutants. Reversion of ascospores stored in water at different temperatures was examined in the second part of this study. Results obtained over a twenty-week period indicated that reversion of ser (JBM 4-13) does not occur at any temperature in stored ascospores. During the course of this study freeze-drying was to be used to exclude the possibility of DNA synthesis during storage. This treatment was found to result in total inviability of ascospores. Separate factors involved in the freeze-drying technique were examined to determine the cause of lethality. Results reported in this thesis indicate that reversion of ser (JBM 4-13) is a property of all vegetative cultures of the mutant and does not occur in stored ascospores.