Life cycle of the budding yeast Saccharomyces cerevisiae.

as a starting point for learning about S. cerevisiae and its cell types. This review is intended for people who are being exposed to the study of S. cerevisiae for the first time and also for specialists who work on other organisms such as filamentous fungi. There will be no attempt to be comprehensive, but considerable efforts to be comprehensible! The life cycle of S. cerevisiae and, in particular, its cell types have provided rich ground for studying molecular mechanisms governing gene expression and programmed genetic rearrangement and for addressing numerous problems in cell biology, including the workings of a receptor-signalling system. Several reviews that focus on these aspects have been written (48, 49, 51, 52, 76, 105, 139). The main characters of this review are the cell types of S. cerevisiae, with a focus on the life cycle and an accent on biological aspects of the subject. By "life cycle," I refer to two broad aspects of the life of S. cerevisiae. The first is cell proliferation, the process by which a cell of one type gives rise to two cells that are essentially identical (discussed briefly in the next section). The second aspect has to do with changes in the ploidy of the organism, i.e., transitions in the life cycle. Both haploid and diploid forms exist: haploids mate to form diploids, and diploids undergo meiosis to form haploids (discussed in the section, "Transitions: Mating and Sporulation"). These processes, mating (which involves cell fusion) and meiosis, are fundamental biological processes and also occur in multicellular organisms. The life cycle of S. cerevisiae has an additional aspect beyond proliferation, mating, and meiosis: haploid yeast cells can change their cellular type by a programmed deoxyribonucleic acid (DNA) rearrangement. This allows some S. cerevisiae strains to exhibit a homothallic life cycle (a life cycle in which a single haploid cell can give rise to diploid cells capable of meiosis and spore formation). Other strains of S. cerevisiae exhibit a heterothallic life cycle (a life cycle in which a single haploid cell is unable to produce diploid cells) (discussed in the section, "Homothallism and Heterothallism: Mating-Type Interconversion"). Understanding the homothallic and heterothallic life cycles is important for understanding many organisms (including filamentous fungi) and raises issues concerning how cellular diversity can be generated (discussed in sections, "Variations on the S. cerevisiae Life Cycle" and "A Theoretical Discussion of Homothallism"). To put S. cerevisiae into context and to call attention to interesting features of other organisms, I briefly review aspects of the life cycles of the fission yeast Schizosaccharomyces pombe and some other organisms. Throughout this article, I shall sometimes refer to S. cerevisiae as "yeast," realizing, of course, that other yeasts such as Schizosaccharomyces pombe do