Differentiation response of Physarum polycephalum to macrocysts at various times in nuclear cycle.

Macrocyst (spherule) formation was induced in synchronized suspension cultures of microplasmodia of Physarum polycephalum under conditions where DNA synthesis was inhibited. Plasmodia in early G2 phase of nuclear cycle were able to differentiate to spherules in the presence of an inhibitor of DNA synthesis, whereas those in late G2 phase required another round of DNA replication before they could enter into the spherulation process. These facts suggest that commitment to DNA synthesis occurred about halfway through G2 phase. The idea was also supported by the results of autoradiographic study in which spherulating plasmodia were fed with radioactive thymidine and labelled plasmodia were scored at the terminal differentiation stage.