Bk. Mable et Sp. Otto, Masking and purging mutations following EMS treatment in haploid, diploid and tetraploid yeast (Saccharomyces cerevisiae), GENET RES, 77(1), 2001, pp. 9-26
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.