Differentiation of Dictyostelium discoideum in monolayer cultures and its modification by ionic conditions.

We have compared the pattern of enzyme expression in cyclic AMP-induced monolayer cultures of Dictyostelium discoideum with that found during normal development. We find that both the temporal and quantitative pattern of enzyme expression are initially similar in the two situations, although the developmental sequence is more protracted and terminal cell differentiation is delayed in the monolayer situation. We describe differentiation conditions that permit the expression of only one terminal phenotype, which may be useful for further biochemical studies. Enzyme accumulation patterns under these conditions indicate that UDP gal transferase is not required for stalk cell differentiation (i.e., it is a prespore enzyme). We have shown that, when cell monolayers are incubated with cAMP, the presence of a weak acid at low extracellular pH favors stalk-cell differentiation, while a weak base at high extracellular pH favors spore differentiation. Finally, we show that variations in the monovalent cation content of the buffer, or the addition of an ion transport inhibitor (scillaren), or an ionophore (valinomycin) all affect the ratio of stalk cells to spores. Taken together, these results suggest that intracellular H+ and/or other cations may play an important role in regulating differentiation of specific cell types in D. discoideum.