Cell integrity limits ploidy in budding yeast

Evidence suggests that increases in ploidy have occurred frequently in the evolutionary history of organisms, and can serve adaptive functions to specialized, somatic cells in multicellular organisms (Edgar & Orr-Weaver, 2001; Orr-Weaver, 2015; Van De Peer et al., 2017). However, the sudden multiplication of all chromosome content may present physiological challenges to the cells in which it occurs. Experimental studies have associated increases in ploidy with reduced cell survival and proliferation (Andalis et al., 2004; Fujiwara et al., 2005). To understand the physiological challenges that suddenly-increased chromosome content imposes on cells, we used S. cerevisiae cells to ask how much chromosomal DNA cells may contain, and what determines this limit. We generated polyploid cells using two distinct methods causing cells to undergo endoreplication and identified the maximum ploidy of these cells, 32-64C. We found that physical determinants that alleviate or exacerbate cell surface stress increase and decrease the limit to ploidy, respectively. We also used these cells to investigate gene expression changes associated with increased ploidy, and identified the repression of genes involved in ergosterol biosynthesis. We propose that ploidy is inherently limited by the impacts of growth in size, which accompany whole genome duplication, to cell surface integrity. Overall design: Gene expression profiling of two yeast strains that double in ploidy with each cell cycle. Genome duplication was induced in mad1? bub2? (MB) and cdc24-1 swe1? (CS) cells and gene expression was profiled in three replicate samples of each strain, every four hours, for 16 hours total. Reference samples, of constant DNA content, were included to control for effects of each strain's genotype on gene expression with experimental time (MB cells not treated with nocodazole; CS cells grown at 25C). Additional reference samples, consisting of cell cycle-arrested cells, were included to control for effects of enlarged cell size on gene expression with experimental time (WT cells treated with nocodazole; cdc24-1 cdc20-1 swe1? cells grown at 37C). Mean DNA content was measured for each sample and samples were used, in two tests, to identify changes in gene expression relative to DNA content.