Ultrastructure of Mitosis in the EndoparasiteOlpidiopsis Varians

Abstract

Fine structural aspects of mitosis in the endoparasitic oomycete Olpidiopsis varians are described and compared with other oomycetous fungi. Mitotic nuclei within developing parasite thalli divide in synchrony so at any point during the duration of mitosis all the nuclei within a thallus exhibit the same mitotic features. Mitosis in O. varians is characterized by an intact nuclear envelope with polar fenestrae, persistent nucleolar material, and an intranuclear spindle. During metaphase the polar fenestrae are apparent and there is no alignment of chromatin at the equatorial plate. Anaphase is marked by the elongation of the spindle and an increase of endoplasmic reticulum at the interpolar regions. During telophase the spindle elongates further and the daughter nuclei are separated by an interzonal bridge. The interzonal bridge consists of a constricted portion ofthe nucleus surrounded by the nuclear envelope and contains microtubules and nuclear material. The pertinence of the mitotic characteristics of O. varians to fungal phylogeny is discussed.