Cell differentiation in a temperature-sensitive stalkless mutant of Dictyostelium discoideum.

A temperature-sensitive aggregateless and stalkless mutant was isolated from Dictyostelium discoideum NC-4. The mutant cells cannot aggregate at 27 degrees C, but aggregate and form normal fruiting bodies at 21 degrees C. When the temperature was shifted to 27 degrees C after aggregation at 21 degrees C, almost all of the cells in the aggregate differentiated into spores. Neither stalk cells nor stalk tubes formed at 27 degrees C. Inhibition of stalk formation was not lifted by addition of cyclic AMP. Nevertheless, the proportion of prespore to total cells within the mutant slugs was normal, at both 21 degrees C and 27 degrees C. At 27 degrees C, a slug was transformed into a spherical cell mass at the end of migration, within which pre-existing prespore cells differentiated into spores. The remaining prestalk cells were then converted to prespore cells which later became spores. As the cell-type conversion continued, formation of a spore mass resulted. The development of the mutant is thus consistent with the idea that the presumptive cell differentiation is directly related to the terminal cell differentiation. During migration at 27 degrees C, the number of prestalk cells decreased in the anterior part of the slug but instead increased at the foot or the rear part, whereas the prestalk-prespore pattern remained normal at 21 degrees C. The fact that a normal proportion of prespore cells was maintained in spite of their deranged distribution at 27 degrees C indicates that the regulation of proportion is independent of the formation of pattern.