Reverse-Mutation and Adaptation in Leucineless Neurospora

Abstract

Mutations affecting specific steps in biochemical syntheses have been produced in Neurospora crassa by x-rays and ultra-violet radiation.1In crosses with wild-type molds, these mutations segregate in a Mendelian fashion and hence involve the alteration of single determinant factors.Since many of the mutants seem to completely lack a specific synthetic ability it was not certain whether they involved small chromosomal aber- rations such as deficiencies, or the inactivation of genes which still self- duplicate.Up to the present time no demonstration of back-mutation, which would afford indirect e'vidence on this question, has been made for any of the biochemical mutants of Neurospora.On the basis of the evi- dence presented below we conclude that reverse-mutation to the wild-type allele does occur.Strain 33757, which originated from a culture treated with ultra-violet light, is unable to synthesize the amino acid, leucine, an inability which has been shown by Regnery2 to be caused by a difference from wild type in a single factor.In the course of the development of an assay method for leucine by the use of strain 33757 it was noted that the weights of some cul- tures were unusually high and did not bear the typical relationship to leucine in the medium which characterizes the growth of this mutant strain.The phenomenon was called adaptation.3 Genetic and physiological- studies have been made on three independently adapted strains of 33757.4Adapted strains a and ,B were derived as follows from a leucineless albino- marked stock of mating type A (33757-4637-A).Conidia were inoculated into flasks containing 0.25 mg.I (+) leucine per 50-ml.medium and were incubated at 30°C. for 8'/2 days.Samples were taken from two cultures which subsequently proved to have unusually high weights, and inoculated into minimal medium.From this time, adapted cultures a and ,B were PROC.N.