Nuclei may anchor at specific locations during nuclear determination in Paramecium caudatum

Abstract

Summary During the conjugation of Paramecium caudatum, the fates of postzygotic nuclear division products are closely associated with their anteroposterior locations at a critical time. Soon after the third division of the synkaryon, one of the four anterior nuclei becomes a micronucleus and the other three degenerate, while all the posterior ones differentiate into macronuclear anlagen. Recently, we reported that heat shock induced abnormal nuclear determination. To understand the mechanism of this phenomenon, the behaviour of spindles in postzygotic cells was studied using an anti-alpha tubulin antibody. In control cells without the heat shock, the spindle lengthened in a fairly straight axial line at first and then curved at the two poles of the cells, elongating continuously but no longer moving the nuclei. The eight nuclei kept their 4 to 4 anteroposterior localization, though the nuclei themselves were rotated by the attached spindles during the third telophase. In heat-treated or emetine-treated cells, spindles also elongated, but they U-turned or wound in the center region of the cells; as a result, all eight nuclei seemed to be passively moved by the spindles and lost anteroposterior localization. All of these results strongly suggest a nuclear anchoring mechanism that controls the anteroposterior nuclear localization and plays an essential role in normal nuclear determination.