An inquiry into high revertant frequency in Neurospora crassa

The serine auxotroph, ser (JBM 4-13), appeared to be highly unstable and displayed a highly variable revertant frequency of 1 x 10-6 to 2.8 x 10-1. The mutant's characteristics were examined with respect to the properties of unstable genes predicted by current models explaining genetic instability. After a sustained effort, no auxotrophic strain could be isolated with a non-variable revertant frequency. Time and temperature were tested for their ability to alter revertant frequency. A revertant was isolated for comparison with the auxotroph and with a nutritionally wild-type strain for the effects of time and serine concentration on growth. Both the auxotroph and the wild-type strain were exposed to UV irradiation to determine dose response curves as well as the effect of UV irradiation on the revertant frequency in the auxotroph. Mixed cultures of known ratios of revertants to auxotrophs were grown at various serine concentrations to determine the effect on the revertrant frequency. The results of these studies indicate that it is unlikely that the high revertant frequency is due to extrinsic mechanisms such as controlling elements, episome-like factors, mutator genes, virus particles or defective repair mechanisms. Genetic instability due to an intrinsic factor has not been excluded by the present study; however, considerable evidence was obtained indicating that the high revertant frequency is due to a selective advantage of the revertant over the auxotroph in mixed cultures grown on low concentrations of serine supplement. The selective advantage of the revertant over the auxotroph was not apparent at higher concentrations of serine. The variability in the revertant frequency between cultures could be a result of differences in the time at which the reversion event occurred in the individual cultures.