Mitosis in Phlyctochytrium irregulare

Abstract

Mitosis in zoosporangia of the chytrid Phlyctochytrium irregulare is described from electron microscope observations and also from light microscope observations of both living and haematoxylin-stained thalli. At the onset of prophase the centriole complex replicates, and the complexes migrate to polar positions. The semi-persistent nucleolus is appressed to the nuclear envelope as the nuclear pockets invaginate, finally rupturing to create polar fenestrae, through which spindle microtubules penetrate the nucleus from the region of the centrioles at prometaphase. Metaphase chromosomes form an equatorial plate. Initial separation at anaphase seems to be accomplished mainly by shortening of chromosome-to-pole microtubules; additional anaphase and telophase separation is accomplished by elongation of the nucleus. A system of perinuclear endoplasmic reticulum is formed during prophase and is completed by metaphase. It persists during all division stages after its formation. Features of this mitotic apparatus are discussed with reference to earlier light microscope studies of chytrid mitosis. The ultrastructure of P. irregulare's mitotic apparatus is similar to that of certain unicellular green algae.