Regulation of the anterior-like cell state by ammonia in Dictyostelium discoideum.

Ammonia appears to be an important regulatory signal for several aspects of the Dictyostelium life cycle. The postulated role of ammonia in the determination of the prespore pathway in cells of the slug stage has led us to examine the effect of ammonia on the prestalk/prespore ratio of migrating slugs. In the presence of 10(-3) M ammonium chloride, the volume of the prestalk region decreases by 40.8%. The kinetics of the process make it unlikely that this is due to a shift in the differentiation pathway. A test of the hypothesis that the decrease in volume of the prestalk region is due to the conversion of prestalk cells to anterior-like cells shows that the percent of anterior-like cells in the posterior region increases by the amount predicted by the hypothesis. This suggests that ammonia may be the molecular signal, produced by the tip, that prevents anterior-like cells from chemotactically migrating to the tip and thereby becoming anterior cells. The effect of enzymatic removal of ammonia from vitally stained migrating slugs is the appearance of a series of dark stripes beginning at the posterior end and progressing forward. We interpret this as a result of progressive removal of anterior-like cells from tip dominance and essentially as the formation of new potential tips. Indeed, in a few cases one or even two of the stripes separate from the posterior of the cell mass and form small fruiting bodies. We consider the phenomenon of stripe formation further evidence that the tip acts on anterior-like cells through ammonia.

Ammonia and thermotaxis: Further evidence for a central role of ammonia in the directed cell mass movements of Dictyostelium discoideum.

Evidence is presented to support the hypothesis that, in addition to its possible role in mediating chemo- and phototaxis, ammonia (NH(3)) is also the key substance responsible for directing thermotaxis of the migrating slugs of Dictyostelium discoideum. NH(3) is produced by the cells of the slug and we show that high and low concentrations of NH(3) decrease the speed of the amoebae while intermediate concentrations increase their speed. NH(3) production by amoebae is affected by temperature: the greater the temperature, the more NH(3) is produced. From these facts we speculate that both the positive and the negative thermotaxis found in slugs can be explained by temperature gradients stimulating regional differences in NH(3) production, and depending upon the temperature, the amount of NH(3) will either be in the range that stimulates or inhibits the rate of movement. If this explanation is correct, then minute localized differences in the production of NH(3) and their differential effect on cell speed could account for all the directed movements of the cell masses of these slime molds.

The possible role of ammonia in phototaxis of migrating slugs of Dictyostelium discoideum.

Previously we showed that the rising cell masses of cellular slime molds orient away from high concentrations of ammonia gas, presumably by speeding up the cells on one side. Here we show that in the same way NH(3) could also be involved in the highly sensitive phototaxis found in the migrating slugs of Dictyostelium discoideum. We have evidence that light increases their speed of migration and their production of NH(3). Since unilateral light is concentrated on the distal side of a cell mass by the "lens effect," this leads to the obvious hypothesis that the light stimulates the local production of NH(3), which, in turn, stimulates the cells in the illuminated region to move faster.

Successive asexual life cycles of starved amoebae in the cellular slime mould, Dictyostelium mucoroides var. stoloniferum.

Dense masses of spores of Dictyostelium mucoroides var. stoloniferum have the ability to germinate and aggregate rapidly in the absence of food. This is made possible by the presence of a dominant, self-produced spore germination activator. The germination-aggregation cycle can be repeated in as many as six successive generations. In each generation the spore size is reduced so that ultimately they are only a fraction of the size of those produced by the parental, bacteria-fed amoebae.

pH affects fruiting and slug orientation in Dictyostelium discoideum.

We have demonstrated two interesting facts about the transition from the migration stage to the final fruiting stage of Dictyostelium discoideum. One is that fruiting is favoured on acid substrata, and secondly, migrating slugs tend to migrate towards the acid side of a pH gradient. The suggestion is offered that these results can be interpreted in terms of the effects of NH3. It appears to be an additional mechanism (besides phototaxis and thermotaxis) to assure that the final fruiting takes place in a favourable environment.