Ammonia and the induction of microcyst differentiation in wild-type and mutant strains of the cellular slime mold Polysphondylium pallidum

Although low levels ($\text{0.16 μ}\text{mole}\text{10}{}^7\text{cells, ml}$) of ammonia are present in vegetative, germinating, and young encystment cultures of wild-type (WS-320) cells, a 12-fold increase in extracellular ammonia occurs during microcyst differentiation in the cellular slime mold Polysphondylium pallidum. When WS-320 amoebae were placed in conditioned medium from other wild-type encystment cultures, microcyst formation was initiated earlier than in control (120 mM KCl) cultures. Isoosmotic solutions containing NH₄Cl and KCl also caused WS-320 cells to encyst earlier. Similar results were obtained with mutant strains. In 120 mM KCl, strains mic-1 and mic-2 produce 21 and 64% microcysts, respectively, while strains PN582 and PN651 do not encyst. The mutant mic-1 secretes wild-type levels of ammonia, while mic-2, PN582, and PN651 all secrete much lower amounts. Conditioned medium and solutions containing NH₄Cl and KCl increased the rate of development and the number of microcysts produced by mic-2 and induced microcyst development in the cystless mutant PN582. Taken together these data indicate that ammonia acts as an inducer of microcyst differentiation.