"Saccharomyces cerevisiae" is a yeast of the phylum "Ascomycota". During vegetative growth that ordinarily occurs when nutrients are abundant, "S. cerevisiae" reproduces by mitosis as diploid cells. However, when starved, these cells undergo meiosis to form haploid spores. Mating occurs when haploid cells of opposite mating types MATa and MATα come into contact. Ruderfer et al. pointed out that, in nature, such contacts are frequent between closely related yeast cells for two reasons. The first is that cells of opposite mating type are present together in the same acus, the sac that contains the cells directly produced by a single meiosis, and these cells can mate with each other. The second reason is that haploid cells of one mating type, upon cell division, often produce cells of the opposite mating type. An analysis of the ancestry of natural "S. cerevisiae" strains concluded that outcrossing occurs very infrequently (only about once every 50,000 cell divisions). The relative rarity in nature of meiotic events that result from outcrossing suggests that the possible long-term benefits of outcrossing (e.g. generation of diversity) are unlikely to be sufficient for generally maintaining sex from one generation to the next. Rather, a short term benefit, such as meiotic recombinational repair of DNA damages caused by stressful conditions (such as starvation) may be the key to the maintenance of sex in "S. cerevisiae".