Effects of continuous competition in two species of cellular slime mold: Dictyostelium discoideum and Polysphondylium pallidum

Competitively induced population changes were studied in the laboratory. The experimental organisms chosen for the work were two cellular slime mold species: Dictyostelium discoideum and Polysphondylium pallidum. Culturing these organisms on a temperature gradient of 15 °C to 30 °C induces long-term continuous competition for food and space. The organisms respond by changing their rates of resource exploitation and their susceptibility to interference.Before competition, D. discoideum interfered with P. pallidum fruiting, body formation. This effect was not observed after competition. Specifically, when grown alone before competition, P. pallidum fruited from 18° to 37 °C and D. discoideum from 9° to 27 °C. In mixed cultures, before competition. P. pallidum fruited from about 24° to 37 °C and D. discoideum from about 9° to 27 °C. In mixed cultures, after continued competition, P. pallidum fruited from about 20° to 37 °C and D. discoideum from about 9° to 24 °C. There is evidence to suggest that the change is genetic and related to parasexuality in P. pallidum.Rates of resource exploitation (which depend upon spore germination times and amoeba colony expansion rates) also changed. Germination times were unaltered but amoeba colony expansion rates increased.Apparently continued competition resulted in convergence and divergence with respect to resource use. Because P. pallidum gained the ability to fruit in the presence of D. discoideum it converged with respect to resource use. At the same time the two species diverged by increasing rates of resource use over different segments of the temperature range.