Transition of starving Dictyostelium cells to differentiation phase at a particular position of the cell cycle

Abstract

Abstract The relationship between proliferation and differentiation in Dictyostelium discoideum Ax-2 was analyzed with reference to the cell-cycle position at the onset of starvation, using cells synchronized by temperature shift (11.5°C-22.0°C). To examine how far Ax-2 cells at any particular phase of the cell cycle are able to progress through the cycle in response to nutritional deprivation, we measured temporal changes in cell number and nuclearity after starvation. Nuclear DNA synthesis in synchronously developing cells was also monitored by pulse-labeling with [methyl- 3H]thymidine. Increase in cell number and subsequent DNA synthesis occurred in cells just before mitosis (referred to as TO.5 cells and T1 cells; 0.5 h and 1 h after the shift-up from 11.5° C to 22.0° C respectively), but not in T3, T5, or T7 cells. When T1 cells were incubated for 6 h in the absence of external nutrients, they (T1 + 6 cells) exhibited developmental features similar to T7 cells, which most rapidly acquired chemotactic sensitivity to 3′,5′-cyclic adenosine monophosphate (cAMP) and EDTA-resistant cohesiveness after starvation. Thus, it is quite likely that Ax-2 cells may progress through the cell cycle to a particular point (possibly the cell-cycle position of T7 cells), irrespective of the presence or absence of nutrients, and enter the differentiation phase from this point under conditions of nutritional deprivation. There was no difference in the ratio of prestalk to prespore cells in migratory pseudoplasmodia derived from cells that had been starved at other cell-cycle positions. The possible significance of cell-cycle-dependent variations in the timing of differentiation is discussed in relation to pattern formation and cell-type proportioning.