The genome-wide rate and spectrum of spontaneous mutations differs between haploid and diploid yeast

Organisms vary in the number of genome copies per cell—ploidy. By altering how DNA is replicated and repaired, ploidy may determine the number and types of mutations that arise, affecting how evolution proceeds. We sequenced the genomes of >200 replicate lines of yeast (Saccharomyces cerevisiae) with one versus two genome copies (haploid versus diploid), after accumulation of thousands of new mutations. Haploids were more susceptible to single-nucleotide mutations, particularly for DNA replicated later in the cell cycle, whereas large changes to genome structure were more common in diploids. Haploid and diploid populations will therefore have access to distinct kinds of genetic variation, contributing to differences in their evolutionary potential.