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Genomic convergence towards diploidy in Saccharomyces cerevisiae : patterns and mechanisms Gerstein, Aleeza Cara

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 isogenic lines of the yeast Saccharomyces cerevisiae. Over the course of ~1800 generations of mitotic division, we observed convergence towards diploid DNA content in all replicate lines. Comparative genomic hybridization with microarrays revealed nearly euploid DNA content by the end of the experiment. As the vegetative life cycle of Saccharomyces 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. It is not surprising to us that mutants diploids would be able to out-compete tetraploid individuals; diploid growth rates are significantly higher than tetraploids (data not shown) and it is known that tetraploid individuals are quite ’sick’. To determine the selective benefit that would allow mutant diploids to out-compete haploids, we measured a number of fitness and cell size parameters. Neither growth rate nor biomass production differed between ploidy levels. At the phenotypic level we found that cell size increased dramatically over the 1800 generations of evolution. We are thus left with the hypothesis that batch culture evolution selects on larger cell size, and that diploids, which are larger than haploids, have an advantage. To investigate further the mechanism that allows tetraploid individuals to lose two entire sets of chromosomes, a second batch culture experiment was conducted starting with triploid-sized individuals. This experiment demonstrated that a transition toward diploid genomic content was indeed possible for triploids. This result suggests that a chromosome distributive system may be present in Saccharomyces cerevisiae that allows chromosomes to be properly segregated (i.e., in euploid or near euploid sets) to daughter cells through a mitotic process. Selection on genome size remains poorly understood. This thesis provides evidence that historical size may act as a constraint, and that multiple mechanisms are at play to allow haploids and tetraploids to rapidly change genome size towards diploidy, the historical ploidy of Saccharomyces cerevisiae.

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