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Environmental Factors Influencing the Growth and Fructification of Dictyostelium polycephalum

William F. Whittingham and Kenneth B. Raper
American Journal of Botany
Vol. 44, No. 7 (Jul., 1957), pp. 619-627
Stable URL: http://www.jstor.org/stable/2438936
Page Count: 9
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Environmental Factors Influencing the Growth and Fructification of Dictyostelium polycephalum
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Abstract

The relative humidity within the culture vessel, or the gradient between the atmosphere of the vessel and the external atmosphere, was of primary importance in obtaining optimum fruiting of Dictyostelium polycephalum. Proper control of this environmental factor permitted the slime mold to complete its life cycle with optimum formation of sorocarps in the absence of a third cultural associate, Dematium nigrum, which had previously been required. Apparently, the relative humidity required to induce the formation of sorocarps is slightly lower for D. polycephalum than for other members of the genus. In addition, proper manipulation of the culture atmosphere reduced the time required for maximum sorocarp production from 10-21 days to 5-6 days. Optimum fruiting was also dependent upon the pH of the substrate, the minimum being 5.6, the maximum 8.7, and the optimum 6.5. Therefore, the range within which sorocarps will differentiate appears to be shifted toward the basic side when compared to the ranges reported for most of the other Acrasieae. The optimum temperature for sorocarp formation and growth was found to be 30⚬C. while the minimum temperature for fruiting, but not growth, was 10-15⚬C. The nutrition of the bacterial associate was also instrumental in affecting the fruiting process of the slime mold; e.g., it was shown that the acidic and basic end-products in the metabolism of Escherichia coli interfered with sorocarp formation. This inhibition was not caused by the products per se, but rather resulted from the adverse pH which they induced in the substrate. If the bacterial growth was restricted, the pH of the substrate remained within a favorable range, and near the optimum, for fruiting. These results indicate that the primary beneficial function of Dematium in the culture of D. polycephalum, as previously reported by Raper, was the reduction of the humidity within the culture vessel which stimulated the differentiation of pseudoplasmodia into mature sorocarps. A second function of the Dematium could have been the removal, or neutralization, of acidic or basic end-products resulting from the metabolism of the bacterial associate.

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