Figure 1

The life cycle of a unicellular yeast. The budding yeast Saccharomyces cerevisiae is an example of a sexual unicellular yeast, with haploid cells of two mating types, a and α, that mate when pheromones produced by each cell is recognized by pheromone receptors expressed on the opposite cell type: this stimulates fusion of the two cells to produce an a/α diploid cell that grows vegetatively when nutrients are plentiful, but upon starvation undergoes meiosis to produce four haploid spores that can remain dormant until conditions improve. Reproduced with permission from Figure 2-5 of Morgan DO, The Cell Cycle, Oxford University Press (2007).