Patterns of oscillation during mitosis in plasmodia ofPhysarum polycephalum

Abstract

The rhythmic contraction pattern in plasmodia ofPhysarum polycephalum was studied to determine whether characteristic changes occur during the synchronized nuclear division. An electrical method that measures the contraction rhythm in situ during several cell cycles was used. Biopsies of the plasmodia were taken at 17 min intervals for precise determination of the cell cycle stages and were correlated with the simultaneously measured contraction rhythm. All measurements were performed in a temperature controlled environment (27 °C) at 100% relative humidity with the plasmodia (less than 24 h old) growing on a semi-defined agar medium. A total of 14 different plasmodia have been examined, and on one occasion the plasmodium was followed through 3 subsequent mitoses. The mitotic stages were identified with aceto-orcein coloring techniques and by fluorescence methods. Except for a few cases where a mitotic asynchrony of 2–3 min was observed, the mitotic events occurred simultaneously in the nuclei within a single plasmodium. Both the occurrence of the first mitosis after inoculation and the intermitotic times were highly variable. Our study indicates that the contraction rhythm in plasmodia ofPhysarum is unperturbed during the synchronized nuclear division. However, in 5 of the 17 examined mitoses an amplitude decay was observed. We discuss possible explanations for the obtained results with emphasis on the applied techniques, interpretation of the oscillation patterns, and possible restrictions in the cell itself.