Highly genomic instability of super-polyploid strains of Saccharomyces cerevisiae

Polyploid (2n, 3n, and 4n) genomes are known to be unstable in Saccharomyces cerevisiae. Here, we attempted construction of super-polypoid strains (defined as having higher ploidy than tetraploidy) up to 32n by using the matα2-PBT method that we newly developed and investigated their genomic stability. It is known that cell size increases as ploidy increases up to tetraploid. However, unexpectedly, there was no change in the average cell size of the super-polyploid strains compared with tetraploid or pentaploid strains. Smaller sized cells were observed at a rather higher frequency in super-polyploid cell populations compared with those of diploid, triploid and tetraploid strains, suggesting that ploidy reduction in super-polyploid strains occurs quickly at a relatively high frequency. Assuming that ploidy reduction occurs through chromosome loss (or non-disjunction) during mitotic growth, we also estimated the frequency of chromosome loss (or non-disjunction) in various polyploid strains. Our results indicated that the frequency of chromosome loss (or non-disjunction) is drastically increased (10^-2-10^-3/cells plated) in super-polyploid strains compared with that (10^-4-10^-5/cells plated) of conventional polyploid (2n-4n) strains. This is the first attempt of construction of super-polyploid strains and investigation of their genomic stability in S. cerevisiae. We believe that the matα2-PBT method will be an invaluable tool for investigating a variety of interesting issues regarding polyploidy and their genomic characterization in eukaryotes.