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Laboratory studies of competition in two species of cellular slime molds : Dictyostelium discoideum and Polysphondylium pallidum McQueen, Donald James


The mechanics of a short term interspecific competitive situation and some of the consequences of long term interspecific competition were studied in the laboratory using two species of cellular slime mold; Dictyosteliuin discoideum and Polysphondylium pallidum. The mechanics of competition were studied using a components method in which the process was divided into component parts. These were assessed experimentally, modelled mathematically, and linked together to form a computer model, the predictions of which were tested in the laboratory. Five major components contributed to the competitive situation. These were: exploitation, toxic interference, the effect of physical factors or external forces, the availability of resources, and the number of potential competitors engaged in exploitation and interference. The exploitation component depended upon all of the sub-components which contributed to the life cycle of the cellular slime mold species. These were: the time required for spore germination, the rate and form of amoeba colony expansion, the time required for fruiting body production, and the rate and form of fruiting body colony expansion. Both species interfered with the other's ability to form fruiting bodies. In mixed cultures, D. discoideum amoebae divided and consumed food between 9° and 27°C but D. discoideum fruiting did not occur above about 24°C. In mixed cultures, P. pallidum amoebae divided and consumed food between 18° and 37°C but P. pallidum fruiting bodies did not form below about 24°C. In both cases interference was mediated by temperature and competitor numbers. Temperature, the representative external force, altered the parameter values of all the sub-components contributing to exploitation and interference. When all of the components were assessed they were incorporated into a computer model which was used to predict the area occupied by the fruiting bodies of both species. The simulation was tested 324 times and was accurate in 90.1% of the cases. The long term experimental studies of the consequences of continued competition revealed that after a period of continued competition P. pallidum overcame the effects of D. discoideum inhibition and fruited in the presence of D. discoideum. When grown alone P. pallidum fruited from 18° to 37°C and D. discoideum fruited 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 24°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. Apparently P. pallidum converged towards D. discoideum, and at the same time D. discoideum increased its rate of resource use, and diverged away from P. pallidum. The data suggested that interference was related to the production of chemicals during the aggregation stage. It is possible that acrasin, the chemical which attracts amoebae to aggregation centers is involved. Experimental evidence also suggested that the change experienced by P. pallidum might have resulted from para-sexuality. This entails the production of diploid spores, the recombination of alleles, and chromosome loss, all of which tend to protect recessive and less fit characteristics.

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