Abstract
Saccharomyces cerevisiae is an exceptional genetic system, with genetic crosses facilitated by its ability to be maintained in haploid and diploid forms. Such crosses are straightforward as long as the mating type and ploidy of the strains are known. Haploid S. cerevisiae cells are either MATa or MATα mating type. Several techniques can be used to determine mating type (or ploidy), but are typically time-consuming, require specialized components, and/or the results are inconsistent and transient. Here we validated a simple, cheap and robust method to enable rapid identification of S. cerevisiae mating types. When cells of opposite mating type are mixed in liquid media, they creep up culture vessel sides, a phenotype that can easily be detected visually. In contrast, mixtures of cells of the same mating type or with a diploid strain(s) simply settle out. The method does not require specialized equipment, and is robust to different media, cell densities, temperatures and strains. It can be performed in 96-well plates, and the phenotype is observable for several days. The simplicity and robustness of this method makes it ideal for routine verification of S. cerevisiae mating type, and it could be used to screen for genes underlying the creeping phenotype.