Evolution in Saccharomyces cerevisiae: identification of mutations increasing fitness in laboratory populations.

Since the publication of the complete sequence of the genome of Saccharomyces cerevisiae, a number of comprehensive investigations have been initiated to gain insight into cellular function. The focus of these studies has been to identify genes essential for survival in specific environments or those that when mutated cause gross phenotypic defects in growth. Here we describe Ty1-based mutational approaches designed to identify genes, which when mutated generate evolutionarily significant phenotypes causing small but positive increments on fitness. As expected, Ty1 mutations with a positive fitness effect were in the minority. However, mutations in two loci, one inactivating FAR3 and one upstream of CYR1, identified in evolving populations, were shown to have small but significantly positive fitness effects.