Starvation spurs yeast to begin meiosis. But the process is complicated and irreversible, and cells that start too early or too late are at a disadvantage. To determine how yeast decide when to enter meiosis, the researchers cut rations for about 4,000 cells and tracked their progress.
Although the cells were genetically identical, they required anywhere between 7 and 17 hours to reach the first meiotic division. Much of this variability stemmed from the protein Ime1, which activates other meiosis-promoting proteins such as Dmc1. Ime1 production commenced shortly after the beginning of starvation and built up at a constant rate within an individual cell. But the protein's accumulation varied from cell to cell, explaining why cells entered meiosis on different schedules.
A yeast's position in the cell cycle had no effect on meiotic timing. Cell size had an influence, but further analysis indicated that size acted through Ime1. The work suggests that Ime1 is a meiosis timer. Like the countdown to a rocket launch, Ime1's count-up might allow cells to abort entry into meiosis if conditions change.