Yeast can reproduce both sexually and asexually. The two mating types of haploid yeast are MATa and MATα. Each mating type produces unique pheromones that facilitate mating by allowing cells to sense nearby mating partners and grow toward them. Each of the haploid mating types are capable of being maintained indefinitely if kept seperate. If the two separate mating types encounter each other they may fuse, and produce a diploid cell through a process called conjugation. Conjugation is the sexual process by which haploid cells of the opposite mating type form a diploid zygote. Once the diploid state is formed, it can also be maintained indefinitely, given sufficient nutrients. If the conditions are not adequate the diploid cell will undergo meiosis, or sporulate, and produce four haploid spores known collectively as an ascus.
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