MICROTUBULE‐MICROFILAMENT‐CONTROLLED NUCLEAR MIGRATION IN THE DESMID EUASTRUM OBLONGUM 1

Postmitotic nuclear migration in Euastrum oblongnum Ralfs ex Ralfs starts about 80 min after septum formation with the nucleus leaving its central position in the isthmus and moving into the growing semicell. Nuclear migration is influenced by the chloroplast, which expands into the growing half‐cell and pushes the nucleus toward one side of the cell. The nucleus occupies its farthest position from the isthmus when located in the middle of the growing semicell directly under the central depression of the cell surface. It remains in this position during the subsequent stages of cell development and moves back toward the isthmus within a chloroplast groove about 12 h after completion of cell shape formation. Bundles of microtubules (MTs) emanating from a microtubule center surround the nucleus during its motion. They reach far into the growing half‐cell as long as the nucleus is moving but vanish when the nucleus stays in the growing semicell. MT‐disrupting agents inhibit the backward movement of the nucleus toward the isthmus indicating that MTs are involved in this motion too. Because both MT inhibitors and cytochalasin B influence nuclear motion in Euastrum , an interaction of MTs and microfilaments is thought to function as the motive force for nuclear migration.