Determination of Radiation-induced Mutation Rates of Recessive Lethal Alleles in Saccharomyces

Abstract

COMPARING X-ray inactivation curves of haploid and diploid strains of Saccharomyces, Latarjet and Ephrussi¹ were the first to suggest that radiation inactivation of yeast cells may be caused by an induction of recessive and dominant lethal mutations. The first evidence of X-ray-induced recessive and dominant lethal mutations was obtained by Mortimer² by means of tetrad analysis and specific mating experiments respectively. Recently the number of loci which may be induced by ultra-violet or X-rays to mutate to corresponding recessive lethal alleles as well as the average mutation rate of these loci has repeatedly been estimated after taking into consideration inactivation by dominant lethal mutations and other lethal effects^3,4. Although the mean mutation rates determined in different laboratories do show a good correspondence, they are much higher than ultra-violet or X-ray-induced mutation rates from auxotrophic to prototrophic conditions or vice versa^4,5. This striking difference in mutation rates demands a new independent investigation of the frequencies of radiation-induced recessive lethal mutations. If the estimated high mean mutation rates of recessive lethal mutations do correspond with reality, an investigation of specific mutation rates of a number of individual loci should reveal certain loci exhibiting high mutation rates.