INITIAL CELL-TYPE CHOICE IN A SIMPLE EUKARYOTE - CELL-AUTONOMOUS OR MORPHOGEN-GRADIENT DEPENDENT

Dictyostelium discoideum is a simple eukaryote that lives as an amoeba until starvation triggers aggregation. The aggregate forms a slug whi ch then develops into a fruiting body with two main cell types, stalk and spore cells. Two mechanisms have been proposed to explain cell-typ e differentiation. Studies using expression of the ecmA gene as a pres talk cell marker indicated that gradients of morphogens determine cell fate in the slug. However, studies using dyes or the cysteine protein ase 2 (CP2) gene product as a prestalk cell marker indicated that cell autonomous factors such as cell-cycle phase at the time of starvation cause an initial choice of cell fate. To help resolve these differenc es, we have used transformed cells containing the promoter of the pres talk gene ecmA fused to beta-galactosidase (Jermyn and Williams, 1991) to study the differentiation of Dictyostelium cells at low cell densi ty, at which cell-to-cell interactions and morphogen gradients are min imal. We find that under all conditions of low cell density in which e xpression of the ecmA fusion gene occurs, it is invariably detected in less than 25% of the cells from a clonal population. This suggests th at a cell-autonomous mechanism is involved in ecmA expression. We then used double-labeled immunofluorescence to examine the ontogeny of the CP2-positive and the ecmA-positive cells. In developing aggregates, 5 , to 12% of the cells are CP2-positive from 12 to 24 hr of development . The ecmA-positive cells are first detected at 16 hr as a subset of t he CP2-positive cells and then increase in number. At approximately 20 hr, the CP2-positive cells and the ecmA-positive cells are almost com pletely overlapping sets. By late development, all of the CP2-positive cells are ecmA-positive and an additional 10% of the CP2-negative cel ls are also ecmA-positive. This indicates that up to 20 hr of developm ent, ecmA is expressed only in CPP-positive cells. The data thus sugge st that cell-cycle phase at the time of starvation causes an initial c hoice of cell type and that during later development other factors inf luence cell fate. (C) 1995 Academic Press, Inc.