The changing distribution of actin and nuclear behavior during the cell cycle of the mite-pathogenic fungus Neozygites sp.

We have analyzed the behavior of nuclei and actin during the cell cycle of Neozygites sp. with mithramycin and rhodamine-labeled phalloidin. This fungus is an entomophagous zygomycete which grows as a rod-shaped fission yeast containing 2 to 12, mostly 3 to 4, nuclei per cell. The cell cycle is regulated such that there is not a constant nucleus-to-cytoplasmic volume ratio, and mitosis is initiated slightly asynchronously from one end of the cell. During interphase, detected actin occurs exclusively as peripheral plaques, which are most abundant at growing cell tips, and as perinuclear shells. Because the shells disperse and reform concomitantly with the formation and breakdown of a new septum-associated actin array, we infer that they are a novel form of actin storage. Intranuclear mitosis occurs in the absence of detectable spindle actin which suggests that actin is not a universal feature of mitotic systems and may be a cytoplasmic contaminant in open spindles of plant cells. Actin is involved in septum synthesis in previously unreported ways. Prior to morphologically detectable septum initiation, a peripheral equatorial band of longitudinal actin filaments assembles and then shortens to a transverse belt at the future site of septum synthesis. We suggest that this actin array recruits and organizes cell wall synthetic complexes for subsequent septum growth. During detectable septum synthesis, the invaginating plasmalemma bears plaques at a similar concentration to those at growing cellular tips.