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Saccharomyces cerevisiae

Saccharomyces cerevisiae is a heterothallic species and require the presence of two mating strains of yeast cells that have been designated as "a" and "a". Fusion of the two mating strains will produce the zygote. 

Unlike other species of fungi, the life cycle of this species is not zygotic. The diploid cell does not undergo meiosis and will assimilate food and reproduce asexually for a time. Thus, there is a true alternation of generations and this species has a sporic life cycle. 

The life cycle is completed when meiosis occurs in the diploid yeast cells and each of the four nuclei becomes an ascospore

Two will be of the "a" and the other two will be of the "a" mating strains. The original cell wall of the diploid yeast cell is the ascus.