The cell cycle during the vegetative stage of Dictyostelium discoideum and its response to temperature change.

The cell cycle in amoebae of Dictyostelium discoideum has been analysed in cells growing asynchronously in axenic medium. For cells growing at the optimum growth temperature of 22 degrees C with a culture doubling time of 8 h the average times for the cell cycle phases are as follows: G1, 1.5 h; S, 2.1 h; G2, 4.4 h; M, 15.2 min. When amoebae are grown at temperatures below 22 degrees C, culture doubling time increases and the cell cycle phases are altered in ways characteristic for each phase. G2 is the most variable period and may occupy up to 70% of the total cell cycle time; S and G1 are the least affected, increasing by only 20% when the cell generation time is doubled. When cells which have reached the stationary phase of growth in liquid medium are washed and reinoculated into fresh medium they divide synchronously after a lag period of 5 h. By following cell number increase and nuclear DNA synthesis in these cultures we have shown that stationary phase cells are arrested in the G2 phase of the cell cycle. Finally, although more than 97% of amoebae grown on a bacterial food source are uninucleate, when grown axenically up to 35% of the cell population may become multinucleate. Our results suggest that these cells probably arise through the failure of cytokinesis to follow karyokinesis. Multinucleate cells appear to have a slightly longer G2 period than mononucleate cells.