Data from: Mutations in yeast are deleterious on average regardless of the degree of adaptation to the testing environment

The role of spontaneous mutations in evolution depends on the distribution
of their effects on fitness. Despite a general consensus that new
mutations are deleterious on average, a handful of mutation accumulation
experiments in diverse organisms instead suggest that of beneficial and
deleterious mutations can have comparable fitness impacts, i.e., the
product of their respective rates and effects can be roughly equal. We
currently lack a general framework for predicting when such a pattern will
occur. One idea is that beneficial mutations will be more evident in
genotypes that are not well adapted to the testing environment. We tested
this prediction experimentally in the laboratory yeast Saccharomyces
cerevisiae by allowing nine replicate populations to adapt to novel
environments with complex sets of stressors. After >1000 asexual
generations interspersed with 41 rounds of sexual reproduction, we
assessed the mean effect of induced mutations on yeast growth in both the
environment to which they had been adapting and the alternative novel
environment. The mutations were deleterious on average, with the severity
depending on the testing environment. However, we find no evidence that
the adaptive match between genotype and environment is predictive of
mutational fitness effects.