Simultaneous detection of changes in chromosome number, gene conversion and intergenic recombination during mitosis of Saccharomyces cerevisiae: spontaneous and ultraviolet light induced events.

We have employed a hyperhaploid strain of Saccharomyces cerevisiae disomic for chromosome VII to monitor spontaneous and ultraviolet light induced restitution of haploidy (chromosomal loss and/or nondisjunction), mitotic gene conversion and mitotic intergenic recombination. The disomic chromosomal pair incorporates six heterozygous markers, including cyh2 (r), distributed on both sides of the centromere. Cycloheximide resistant segregants of spontaneous origin were analyzed to calculate the spontaneous mitotic rates of restitution of haploidy, intergenic recombination and gene conversion that result in expression of the cyh2 (r) mutation. Restitution of haploidy was found to be the most common source of spontaneously arising cycloheximide resistant segregants. In contrast, those induced by ultraviolet light resulted most frequently from gene conversion of CYH2 (s) to cyh2 (r). The chromosome VII hyperhaploid system provides a sensitive method to detect the aneugenic and recombinagenic effects of suspect chemical and physical agents.