[Effects of cytochalasin B upon mitosis of Physarum polycephalum]

Cytochalasin B, known as a functional inhibitor of actin, was microinjected into naturally synchronous plasmodia of Physarum polycephalum, and the mitotic behaviours of both CB-treated specimens and the control were examined with light and electron microscopy. Mitosis in the CB-treated specimens began about 20 to 60 minutes later than that of the control. It was delayed 35 minutes in the specimens treated with CB in the S phase of the cell cycle, and the delayed time was 20 minutes and 45 minutes, respectively. In the specimens treated with CB in early and middle G2 phase, the longest delay was 60 minutes found in the specimens treated in late G2 phase, indicating that mitosis was affected in Physarum polycephalum when the function of actin was inhibited by CB treatment. The CB-treated specimens and the control showed similarities in the process of mitosis and dynamic changes of nuclear structures, suggesting that the main effect of CB treatment upon mitosis may be to delay the triggering of the mitosis.