Effect of heat shock on the different phases of the mitotic cycle in Physarum polycephalum

In view of the heat-labile nature of the mitosis-triggering substance, heat shock (from 24°C to 37°C for 30 min) was given to synchronously mitotic plasmodia of the myxomycete Physarum polycephalum at short intervals during the course of one mitotic cycle. The S phase was found to be least sensitive to heat shock, whereas delay increased throughout G2 till the transition point with respect to heat shock. This point was at about 30 min before metaphase. Heat shock given between transition point and onset of metaphase did not produce any mitotic delay, but resulted in abortive mitosis. Least-squares fitting of the delay curve has been carried out. The delay pattern for late G2 in our study differs from that of an earlier study and this aspect is discussed.