THE DICTYOSTELIUM CELL-CYCLE AND ITS RELATIONSHIP TO DIFFERENTIATION

The Dictyostelium vegetative cell cycle is charcaterized by a short mitotic period followed immediately by a short S-phase (less than 30 min) and a long and variable G2 phase. The cell cycle continues during differentiation despite a decrease in cell mass: DNA replication and mitosis occur early in development and also at the tipped aggregate stage. Cells that are in mitosis, S-phase or early G2, when starved differentiate into prestalk cells and cells that are in the middle of G2 differentiate into prespore cells. We postulate that there is a restriction point late in the G2 phase, about 1-2 h before mitosis, where the cells can be arrested either by starvation and the initiation of development, by growing into stationary phase, or by prolonged incubation at low temperature. During development, this block persists to the tipped aggregate stage, where it is specifically released in prespore cells, and these cells then go through one more round of cell division. Genes encoding components of the cell cycle machinery have recently been isolated and attemps to specifically block the cell cycle by reverse genetics to study the effects on differentiation have been initiated.