Nonrandomly-associated forward mutation and mitotic recombination yield yeast diploids homozygous for recessive mutations

Abstract

We have employed the analysis of spontaneous forward mutations that confer the ability to utilize L-α-aminoadipate as a nitrogen source (α-Aa⁺) to discern the events that contribute to mitotic segregation of spontaneous recessive mutations by diploid cells. α-Aa^- diploid cells yield α-Aa⁺ mutants at a rate of 7.8±3.6×10^-9. As in haploid strains, approximately 97% (30/31) of α-Aa⁺ mutants are spontaneous lys2-x recessive mutations. α-Aa⁺ mutants of diploid cells reflect mostly the fate of LYS2/lys2-x heterozygotes that arise by mutation within LYS2/LYS2 populations at a rate of 1.2±0.4×10^-6. Mitotic recombination occurs in nonrandom association with forward mutation of LYS2 at a rate of 1.3±0.6×10^-3. This mitotic recombination rate is tenfold higher than that of a control LYS2/lys2-1 diploid. Mitotic segregation within LYS2/lys2-x subpopulations yields primarily lys2-x/lys2-x diploids and a minority of lys2-x aneuploids. Fifteen percent of lys2-x/lys2-x diploids appear to have arisen by gene conversion of LYS2 to lys2-x; 85% of lys2-x/lys2-x diploids appear to have arisen by mitotic recombination in the CENII-LYS2 interval. lys2-1/lys2-1 mitotic segregants of a control LYS2/lys2-1 diploid consist similarly of 18% of lys2-1/lys2-1 diploids that appear to have arisen by gene conversion of LYS2 to lys2-1 and 82% of lys2-1/lys2-1 diploids that appear to have arisen by mitotic recombination in the CENII-LYS2 interval. The methods described can be used to simultaneously monitor the effects of yeast gene mutations and carcinogens on the principal parameters affecting the genomic stability of diploid mitotic cells: mutation, gene conversion, intergenic recombination, and chromosomal loss or rearrangement.