The ultrastructure of mitosis in Plasmodiophora brassicae (Plasmodiophorales)

Mitosis was examined in plasmodia of Plasmodiophora brassicae within artifically inoculated cabbage roots, using light- and electron microscopy. Mitotic nuclear divisions are characterized by a persistent nucleolus, bipolar centrioles paired end-to-end, densely staining chromatin, and a complex array of membranes that surround and ramify through the spindle. Chromatin begins to condense in prophase, and is aligned at metaphase in a reticulate plate on the nuclear equator. The chromatin is not resolvable into distinct chromosomes at metaphase, and a chromosome count is not possible. Large amounts of membrane cisternae within the spindle are most clearly visible at metaphase, and apparently represent the remains of the nuclear envelope. The nuclear envelope is disrupted during prometaphase and may become entangled in the spindle when centriolar microtubules enter the nucleus. Several concentric sheets of perinuclear endoplasmic reticulum surround the spindle and give the mitotic nucleus the superficial appearance of having an intact nuclear envelope. This interpretation of the identity of nucleus-associated membranes differs from those previously reported for other protists, including members of the Plasmodiophorales.