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.

Ultrastructural changes in nuclear membranes and organelle associations during mitosis of the aquatic fungus Entophlyctis sp.

“…In these fungi, chromosomal microtubules terminate a t spherical to pyriform dense areas adjacent to the chromosomes. In Entophlyctis chromosomal microtubules apparently end at a single-layered short plate similar to the one-to several-layered plates observed in Phlyctochytrium (McNitt 1973). These layered structures may also function as kinetochores.…”

“…If both centrioles are preserved in replication and function as parent centrioles, the longer centriole must shorten before centriolar replication. Unequal centrioles have been found in other aquatic uniflagellate fungi during mitosis (Lessie and Lovett 1968;Olson and Fuller 1968) as well as at interphase (Ichida and Fuller 1968), but only equal centrioles have been shown adjacent to nuclei in others (Whisler and Travland 1973;McNitt 1973). The elongation of the primary centriole during mitosis may reflect preparation for the production of a flagellum.…”

“…The inner membrane of the original nuclear envelope appears to invaginate and proliferate inner membrane cisternae, which coalesce around the replicated chromatin and form the new daughter nuclear envelopes. The array of membranes surrounding the spindle after early metaphase in Entophlyctis bears a superficial resemblance to the organization of endoplasmic reticulum around nuclear envelopes during mitosis in some fungi (McNitt 1973) and algae (see reviews by Pickett-Heaps 1972 andStewart et al 1973). This similarity could lead Microbody-Nuclear Association to the speculation that the original nuclear One to several microbodies are interposed envelope fragments in Entophlyctis as endobetween a cisterna of endoplasmic reticulum plasmic reticulum accumulates around the and the nuclear envelope during prophase spindle and then begins to reform at metaphase.…”

“…Differences in the events occurring during mitosis in the chytrids are known primarily from light microscopic studies (Sparrow 1960). Although more recent electron microscopic investigations have provided new insights into the behavior of the nucleus during mitosis in three chytridiomycetes, Phlyctochytrium irregulare (McNitt 1973), Catenaria anguillulae (Ichida and Fuller 1968), and Harpochytrium hedinii (Whisler and Travland 1973), a wider spectrum of these fungi must be studied before the importance of variations can be assessed. The chytrids have been considered a monophyletic group by some taxonomists (Cantino and Turian 1959;Sparrow 1958), but much variety in form has already been suggested in such stages as the zoospores (Koch 1958(Koch , 1961 apparatus which question the homogeneity of this group.…”

Mechanics of chromosome separation during mitosis in Fusarium (Fungi imperfecti): new evidence from ultrastructural and laser microbeam experiments.

“…It would certainly seem to be a possibility for the numerous other situations where both astral microtubules and a distinct anaphase-telophase elongation of the central spindle occur . Included in this group are numerous fungi (10,16,17,(48)(49)(50)(51)(52)(53)(54)(55)(56)(57), several slime molds (58)(59)(60), some algae (61,62), certain protozoans (45,63), and higher animals (1, 2, 9, 11) . Girbardt's concept (10) of mitosis in the Basidiomycete, Trametes versicolor, is especially concordant .…”