Genomic Convergence toward Diploidy in Saccharomyces cerevisiae

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Abstract
Genome size, a fundamental aspect of any organism, is subject to a variety of mutational and selection pressures. We investigated genome size evolution in haploid, diploid, and tetraploid initially isogenic lines of the yeast Saccharomyces cerevisiae. Over the course of ~1,800 generations of mitotic division, we observed convergence toward diploid DNA content in all replicate lines. This convergence was observed in both unstressful and stressful environments, although the rate of convergence was dependent on initial ploidy and evolutionary environment. Comparative genomic hybridization with microarrays revealed nearly euploid DNA content by the end of the experiment. As the vegetative life cycle of S. cerevisiae is predominantly diploid, this experiment provides evidence that genome size evolution is constrained, with selection favouring the genomic content typical of the yeast's evolutionary past. Genome size is a fundamental aspect of all species and has the potential to influence a number of individual characteristics such as cell size, generation time, ecological tolerances, and reproductive traits. Although genome sizes range widely among species, the forces shaping the evolution of genome size are only poorly known. Here we provide the results of an ~1,800 generation evolution experiment using lines of the budding yeast S. cerevisiae with either one, two, or four copies of their genome (haploid, diploid and tetraploid, respectively). We found, surprisingly, that all haploid and tetraploid lines converged toward diploidy, the historical state of S. cerevisiae, by the end of the experiment. Further experiments suggest that entire sets of chromosomes were lost as genome size changed from tetraploid to diploid. Our results suggest that genome size is constrained by selection acting against changes from the historical genome size.