Mitosis and the phylogeny of the genus Neocallimastix

Ultrastructural details of mitosis in species of the anaerobic chytridiomycete rumen fungal genus Neocallimastix are described in an attempt to clarify the phylogeny of the genus. Mitosis and spindle formation are entirely intranuclear and start with a group of kinetochores subtending a differentiated region of the nuclear envelope adjacent to extranuclear nucleus-associated organelles. Following duplication of the nucleus-associated organelles, a bipolar spindle develops among the kinetochores and elongates as the nucleus-associated organelles separate. At metaphase the kinetochores are spread along the spindle; heterochromatin is associated with them and also lies throughout the nucleoplasm remote from the spindle. During anaphase the kinetochore microtubules shorten and the two interdigitating arrays of nonkinetochore microtubules slide apart. Karyokinesis involves medial constriction of the nucleus and the persistent nucleolus. Mitotic nuclei in young thalli bear discoidal nucleus-associated organelles which are replaced by a variable number of centrioles in older thalli. These features are dissimilar from other described chytridiomycete mitoses and indicate either a distant relationship between the Neocallimasticaceae and other chytridiomycetes or substantial changes induced in the mitotic apparatus by the specialized environment of the organisms.