whole-genome resequencing of multicellular fast sedimenting Saccharomyces cerevisiae IMS0267
Saccharomyces cerevisiae
Evolved CEN.PK113-7D strain in anaerobic sequential batch cultures after 500 generations on a mixture of 20 g l-1 glucose and 20 g l1 galactose.    Laboratory evolution of the yeast Saccharomyces cerevisiae in bioreactor batch cultures has, under different selection pressures, yielded variants that grow as multicellular, fast-sedimenting clusters. Knowledge on the molecular basis of this phenomenon may contribute to the understanding of the natural evolution of multicellularity and to manipulating cell sedimentation in laboratory and industrial applications of S. cerevisiae. Multicellular, fast-sedimenting lineages obtained from two independent evolution experiments with a haploid S. cerevisiae strain were analyzed by whole-genome resequencing. The two independent cell lines showed different frameshift mutations in a stretch of eight adenosines in ACE2, which encodes a transcriptional regulator implicated in cell cycle control. Introduction of the two ace2 mutant alleles into the haploid parental strain led to slow-sedimenting cell clusters that consisted of just a few cells, thus representing only a partial reconstruction of the evolved phenotype. In addition to single-nucleotide mutations, a whole-genome duplication event had occurred in both evolved multicellular strains. Construction of a diploid reference strain with two mutant ace2 alleles led to complete reconstruction of the multicellular-fast sedimenting phenotype. This study shows that whole-genome duplication and a frameshift mutation in ACE2 are sufficient to generate a fast-sedimenting, multicellular phenotype in S. cerevisiae. The nature of the ace2 mutations and their occurrence in two independent evolution experiments encompassing fewer than 500 generations of selective growth suggest that switching between unicellular and multicellular phenotypes may be relevant for competitiveness of S. cerevisiae in natural environments.