Intracellular pH and the control of cell differentiation in Dictyostelium discoideum

During development in the cellular slime mould Dictyostelium discoideum starved amoebae aggregate to form multicellular structures that display a simple antero-posterior pattern: prestalk cells occupy the front 20% of the aggregate, and prespore cells occupy the remainder. We have attempted to elucidate the nature of the mechanism regulating the proportions of the two cell types by examining the factors that influence the pathway of differentiation of amoebae in vitro. Amoebae of D. discoideum strain V12 M2 form stalk cells efficiently in appropriate conditions and 'sporogenous' derivatives produce spores as well as stalk cells. Mature spores are formed in a medium containing only cyclic AMP and salts, whereas formation of stalk cells requires, in addition, a low molecular weight hydrophobic factor (DIF). Recent observations have led us to propose that DIF is a morphogen responsible for activating stalk cell differentiation. Here we present evidence that ammonia is a second morphogen, that acts antagonistically to DIF, and that the choice of differentiation pathway is mediated by intracellular pH.