The Ultrastructure of Nuclear Division in Monoblepharella sp.

Abstract

Mitosis in germlings of Monoblepharella sp. is intranuclear. At early prophase, diplosomes establish the poles of the mitotic apparatus. Numerous microtubules emanate from pericentriolar material and some of the microtubules impinge on the nuclear envelope. At late prophase, spindle microtubules penetrate the nucleus through fenestrae formed as a result of the rupture of the nuclear envelope at the spindle poles. During prophase the nucleolus persists and is appressed to one side of the nucleus. Metaphase chromosomes form an equatorial plate and the persistent nucleolus is sequestered in a pocket of nucleoplasm below and to one side of the metaphase plate. Initial separation of chromosomes at anaphase appears to be accomplished by shortening of chromosome-to-pole microtubules. During telophase the nucleus elongates and the nuclear envelope constricts around polar chromosome masses. The interzone is excluded from daughter nuclei. The parental nucleolus is released into the cytoplasm where it disperses. Dictyosomes are associated with the spindle poles at prophase and metaphase but are not detected at telophase. During all mitotic stages a single, large dictyosome is closely associated with the nucleus in the region of the persistent nucleolus. Perinuclear endoplasmic reticulum is lacking in Monoblepharella. Mitosis in Monoblepharella sp. is similar to that described for members of the Chytridiales and Harpochytriales but differs significantly from mitosis in blastocladialean fungi.