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.