Prespore cell fate bias in G1 phase of the cell cycle in Dictyostelium discoideum

By generating a population of Dictyostelium cells that are in the G1 phase of the cell cycle we have examined the influence of cell cycle status on cell fate specification, cell type proportioning and its regulation, and terminal differentiation. The lack of observable mitosis during the development of these cells and the quantification of their cellular DNA content suggests that they remain in G1 throughout development. Furthermore, chromosomal DNA synthesis was not detectable these cells, indicating that no synthesis phase had occurred, although substantial mitochondrial DNA synthesis did occur in prespore cells. The G1-phase cells underwent normal morphological development and sporulation but displayed an elevated prespore/prestalk ratio of 5.7 compared to the 3.0 (or 3:1) ratio normally observed in populations dominated by G2-phase cells. When migrating slugs produced by G1-phase cells were bisected, each half could reestablish the 5.7 (or 5.7:1) prespore/prestalk ratio. These results demonstrate that Dictyostelium cells can carry out the entire developmental cycle in the G1 phase of the cell cycle and that passage from G2 into G1 phase is not required for sporulation. Our results also suggest that the population asymmetry provided by the distribution of cells around the cell cycle at the time of starvation is not strictly required for cell type proportioning. Finally, when developed together with G2-phase cells, G1-phase cells preferentially become prespore cells and exclude G2-phase cells from the prespore-spore cell population, suggesting that G1-phase cells have an advantage over G2-phase cells in executing the spore cell differentiation pathway.