Physarum plasmodia do contain cytoplasmic microtubules!

It has been claimed that the plasmodium of the myxomycete Physarum polycephalum constitutes a very unusual syncytium, devoid of cytoplasmic microtubules. In contrast, we have observed a cytoplasmic microtubule network, by both electron microscopy and immunofluorescence in standard synchronous plasmodia, either in semi-thin sections or in smears, and in thin plasmodia, used as a convenient model. Cytoplasmic microtubules could be seen after immunofluorescent staining with three different monospecific monoclonal anti-tubulin antibodies. The immunolabelling was strictly restricted to typical microtubules as shown by electron microscopy. These cytoplasmic microtubules were entirely and reversibly disassembled by cold treatment and by either of two microtubule poisons: methyl benzimidazole carbamate and griseofulvin. The microtubule network, present in all strains that have been studied, contains single microtubules and microtubule bundles composed of two to eight microtubules. Cytoplasmic microtubules form a dense and complex three-dimensional network, distinct from the microfilamentous domains and from the nuclei. The orientation of the microtubule network varies according to the plasmodial domain examined. Generally microtubules show no special orientation except in plasmodial veins where they are oriented parallel to the long axis of the veins. Differences between our observations and those of previous workers who failed to find cytoplasmic microtubules in plasmodia are discussed. We propose that they reflect difficulties of observation mainly due to the fluorescent background. In contrast with the previous view, the discovery of a microtubule cytoplasmic cytoskeleton in Physarum plasmodia raises several questions concerning its relationships with other cellular organelles and its dynamics during different cell cycle events.