Cell cycle-dependent assembly and disassembly of cytoplasmic microtubules in the plasmodium of the myxomycete Physarum polycephalum.

The giant syncytium of Physarum plasmodia possesses a complex cytoplasmic microtubule network except during the occurrence of the intranuclear mitosis. In early prophase stages, intranuclear spindles assemble concomitantly as the cytoplasmic microtubule network disassembles. No cytoplasmic microtubules are present in metaphase. They begin to reassemble in telophase. The complex cytoplasmic microtubule network reappears in early reconstruction stages. The assembly of cytoplasmic microtubules occurs on cytoplasmic foci, both in telophase stage and during rewarming after cold microtubule disassembly. These foci, independent of the nuclei, correspond to the foci observed in the cytoplasm during interphase, both by immunofluorescence and electron microscopy. As cytoplasmic and intranuclear microtubule-organizing centers are spatially distinct, plasmodial syncytia offer the possibility to study the effects of cell regulatory pathways on two types of microtubule-organizing centers that differ in their nucleating activity during the cell cycle.