Radiation-induced genetic mosaicism and lethal sectoring in yeast

Abstract

The induction of masaics for recessive lethal mutations by X-rays was studied in a diploid strain of Saccharomyces cerevisiae. Mosaicism was detected by genetic analysis (frequencies of live and dead spores within asci) of individual third generation vegetative lines of descent from irradiated cells. Only about 30% of radiation-induced mutants were mosaic, and most if not all of this mosaicism was exposed at the first post-irradiation cell division. It was concluded that the genetic effects of X-rays ussually appear as lesions in both original strands of DNA as well as in both newly replicated strands. Lethal sectoring, the presence of sub-clones of dead cells, was common in the early generations of macro-colonies developing from X-irradiated cells. No evidence could be found that this mosaicism for dominant lethality was genetic in origin. However, the frequency of recessive lethals was higher among sectored colonies. It was demonstrated that lethal sectoring grossly distorts the estimate of genetic masiacism as inferred from the relative frequencies of mosaic and wholly colonies.