Competitions between matched pairs of diploid strains of Saccharomyces cerevisiae, one capable of
undergoing sexual recombination (MAT-heterozygous) and the
other not (MAT-homozygous),
have proved useful for measuring the effects of mitotic and meiotic recombination
and DNA
repair on competitive ability in this organism. Overall competitive
differences between the strains
can be enhanced by converting them to petites (aerobic respiration incompetent).
Here we report
the results of competitions between pairs of strains that also
differ in their ability to undergo
mismatch repair. In petite strains, the growth rates of mismatch-repair
defective strains declined
over time regardless of their MAT genotype. Mismatch-repair
proficient MAT-heterozygous strains
did not show a decline, while repair-deficient MAT-homozygous
strains did. The decline appears to
be due to the accumulation of deleterious mutations of small effect, which
can be corrected by
MAT-heterozygous strains having intact mismatch repair.
The relative competitive abilities of
MAT-heterozygous and MAT-homozygous strains diverged
during the course of the competitions,
and the variance of this divergence increased significantly when mismatch
repair was defective.
This large stochastic component indicates that a relatively small
number of deleterious mutations
may be involved. The accumulation of deleterious mutations and their subsequent
repair may have
a bearing on the origin of sex in this organism.