Although yeasts do not produce true branching mycelial growths, they are usually included among the fungi that produce asci and ascospores. However, in contrast to fungi such as Neurospora, yeast may exists as either haploid or diploid unicellular organisms that have the distinction of multiplying by 'budding'. In Saccharomyces cerevisiae, for example, most cultures consists of diploid oval shaped cells that grow by extruding small cytoplasmic buds from the cell wall. When the cell nucleus divides, the bud receives one of the daughter nuclei and eventually separates as a distinct cell. This kind of vegetative reproduction can continue until unfavorable conditions occur which cause the cell wall to thicken and form an ascus. In this process the diploid cell undergoes two meiotic divisions, forming four spherical haploid products known as ascospores. When favorable conditions return, the haploid ascospores swell in size and can then form either gametes or colonies of spherical haploid vegetative cells. Both events usually lead to formation of diploid zygotes through fusion between two of the products of the haploid ascospores. Occasionally the nucleus of an individual haploid ascospore can also duplicate itself without cytoplasmic division and then combine with its product to form a diploid cell. As in Neurospora, some yeasts have mating types which do not permit indiscriminate conjugation between haploid ascospore or their products; only opposite mating types will produce diploid zygotes. Single ascospores, and their clones isolated from such yeasts, will therefore maintain the haploid condition until brought into the presence of cells from the opposite mating type.
