A CELL-CYCLE PHASE-ASSOCIATED CELL-TYPE CHOICE MECHANISM MONITORS THECELL-CYCLE RATHER THAN USING AN INDEPENDENT TIMER

Upon starvation, cells of the simple eukaryote Dictyostelium discoideu m aggregate and differentiate into several cell types. Two main cell t ypes are prestalk and prespore, which later usually become stalk and s pore cells. The differentiation is plastic, and several factors can al ter cell-type ratios. Two mechanisms have been proposed to regulate th e initial differentiation. One hypothesis is that different levels of extracellular factors within the aggregate determine initial cell type . We and others have proposed that cell type is initially determined b y cell-cycle phase at the time of starvation: prestalk cells are deriv ed from cells which, at the time of starvation, happen to be in a roug hly 2-hr-long sector of the cell cycle which overlaps S and early G2 a nd that certain extracellular factors are then used to maintain the pr oper prestalk: prespore ratio and to control later stages of developme nt such as the prestalk-to-stalk conversion. To examine the relationsh ip between initial cell-type choice and the cell cycle, and how this 2 -hr-long sector is generated, we increased the length of S phase by mi ld treatments of cells with DNA-synthesis inhibitors. When the fractio n of the cell cycle occupied by S phase is increased and the cells are then starved, the prestalk:prespore ratio increases. This increase wa s observed using two markers for prestalk cells, CP2 and ecmA::lacZ. I n addition, there is a close correlation between the fraction of the c ell cycle occupied by S phase and the prestalk:prespore ratio, irrespe ctive of total cell-cycle length. These results validate the hypothesi s that the initial choice of cell type is determined by cell-cycle pha se at the time of starvation, and indicate that the cell-type choice m echanism monitors the cell cycle rather than using an independent 2-hr -long timer started at the beginning of S phase. (C) 1996 Academic Pre ss, Inc.