Calcium uptake during mitosis in the myxomycete Physarum polycephalum

Abstract

THE notion that Ca²⁺ is involved in the regulation of the kinetic and contractile events associated with mitosis in eukaryotic cells is attractive. The contractile proteins, actin and myosin, have been identified in nuclei of Physarum polycephalum¹ and their interaction is apparently regulated by Ca²⁺ (ref. 2). The formation of spindle microtubules may also be sensitive to Ca²⁺ (ref. 3), although as yet there is little evidence of Ca²⁺ fluctuations during mitosis. Macroplasmodia of the myxomycete P. polycephalum are particularly suited to the analysis of events during the cell cycle and mitosis. Synchronous mitoses of large numbers of nuclei proceed against a lowered background of contractile events in the surrounding cytoplasm as cytokinesis does not occur and cyclosis or cytoplasmic streaming slows or ceases during the mitotic period⁴. Extranuclear filaments form in telophase⁵, presumably to participate in nuclear division, but earlier nuclear mitotic events should be amplified. We present evidence here that cyclic uptake and release of Ca²⁺ from the extracellular medium occurs during mitosis in P. polycephalum, and that these fluctuations correlate with specific structural and kinetic events in the mitotic nuclei.