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

By generating a population of Dietyostelium cells that are in the G, 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 G, 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 G(1)-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 G(2)-phase cells. When migrating slugs produced by G(1)-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 G, phase of the cell cycle and that passage from G, into G, 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 G(2)-phase cells, G(1)-phase cells preferentially become prespore cells and exclude G(1)-phase cells from the prespore-spore cell population, suggesting that G(1)-phase cells have an advantage over G(1)-phase cells in executing the spore cell differentiation pathway.