Masking and purging mutations following EMS treatment in haploid, diploid and tetraploid yeast (Saccharomyces cerevisiae)

The yeast, Saccharomyces cerevisiae, was used as a model to investigate the ories of ploidy evolution. Mutagenesis experiments using the alkylating age nt EMS (ethane methyl sulphonate) were conducted to assess the relative imp ortance that masking of deleterious mutations has on response to and recove ry from DNA damage. In particular, we tested whether cells with higher ploi dy levels have relatively higher fitnesses after mutagenesis, whether the a dvantages of masking are more pronounced in tetraploids than in diploids, a nd whether purging of mutations allows more rapid recovery of haploid cells than cells with higher ploidy levels. Separate experiments were performed on asexually propagating stationary phase cells using (1) prototrophic hapl oid (MAT alpha) and diploid (MATa/alpha) strains and (2) isogenic haploid, diploid and tetraploid strains lacking a functional mating type locus. In b oth sets of experiments, haploids showed a more pronounced decrease in appa rent growth rate than diploids, but both haploids and diploids appeared to recover very rapidly. Tetraploids did not show increased benefits of maskin g compared with diploids but volume measurements and FACScan analyses on th e auxotrophic strains indicated that all treated tetraploid strains decreas ed in ploidy level and that some of the treated haploid lines increased in ploidy level. Results from these experiments confirm that while masking del eterious mutations provides an immediate advantage to higher ploidy levels in the presence of mutagens, selection is extremely efficient at removing i nduced mutations, leading growth rates to increase rapidly over time at all ploidy levels. Furthermore, ploidy level is itself a mutable trait in the presence of EMS, with both haploids and tetraploids often evolving towards diploidy (the ancestral state of S. cerevisiae) during the course of the ex periment.