Independence of Propagation Ability and Developmental Processes in Irradiated Cellular Slime-moulds

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Abstract
When assayed for reproductive ability, the cellular slime mould Dictyostelium discoideum NC-4 is resistant to 60Co gamma-rays (D10 = 300 krads) whereas a derived mutant, γs-13, is much more sensitive (D10 = 4 krads). If ability to proceed through development to form normal-appearing spores (a process requiring neither DNA synthesis nor cell division) is assayed after irradiation of amoeboid cells, the γs-13 strain is more sensitive than NC-4, but the differential sensitivity is only about 4× at the 20 per cent level (270 krads for NC-4). When the reproductive ability of these spores is determined, those of γs-13 are mostly inviable at doses above 20 krads, while those of NC-4 are largely viable up to 200 krads. Quantitatively, the proportion of viable spores among those produced closely follows the proportion of viable cells in the irradiated population used to initiate development. The greatest diminution of spore production occurs for irradiation done at the beginning of aggregation; however, the viability proportions among the spores that do appear do not vary with stage of irradiation. We conclude: (1) cells incapable of further reproductive division can still differentiate into spores; the spores so produced retain the reproductive characteristics of the cells from which they developed; (2) there is little if any selection during development in favour of viable cells becoming spores; (3) a radiation-sensitive stage for development occurs at the beginning of aggregation; and (4) the somewhat greater developmental sensitivity of γs-13 supports the view that at least some transcription is necessary during development in this organism.

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