The ultrastructure of mitosis inCricosphaera roscoffensis var.haptonemofera (Prymnesiophyceae)

Abstract

Cell division of a haptophyte,Cricosphaera roscoffensis var.haptonemofera Inouye et Chihara, was examined by electron microscopy. Onset of cell division is characterized by many kinds of cell organelle behaviour; migration of the nucleus toward the cell anterior, replication of basal bodies and haptonema, chloroplast division and proliferation of rough-ER. Flagellar roots of a complex structure do not duplicate during cell division. At prophase the nuclear envelope is vesiculized and the cytoplasmic microtubules, which probably are transformed from the flagellar roots, invade the nucleoplasm through gaps in the nuclear envelope. Condensed chromosomes in prophase form a typical equatorial plate, but later individuality of chromosomes becomes obscure. The spindle microtubules converge on opposite poles. Basal bodies or centrioles lie at the lateral sides of the poles. Microtubules converge in the rough-ER-mitochondria complex occupying the polar area. The equatorial plate is conspicuously surrounded by the membraneous precursor of the daughter nuclear envelope. In anaphase the equatorial plate divides into 2 plates and each moves toward the opposite poles. During the separation of the daughter chromatin plates toward the ends of the cell, the interzonal spindle microtubules proliferate conspicuously and extend between the two daughter plates. These events are accompanied by the development of the nuclear envelope, formed from the fusion of vesicles present around the equatorial plate. The immature daughter nuclear envelope invaginates to enclose the chromatin, pinching off a bundle of the interzonal microtubules. The interzonal spindle is cut at the equatorial level by the ingrowing of peripheral-ER, leading to cytokinesis. Cytokinesis is accomplished by furrowing of plasmalemma. After cytokinesis and nuclear division, the interzonal spindle halves remain persistent, not detached from the daughter nuclei. They will become the flagellar roots in the vegetative cell.