Role of cell shape in determination of the division plane in Schizosaccharomyces pombe: Random orientation of septa in spherical cells

The establishment of growth polarity in Schizosaccharomyces pombe cells is a combined function of the cytoplasmic cytoskeleton and the shape of the ce ll wall inherited from the mother cell. The septum that divides the cylindr ical cell into two siblings is formed midway between the growing poles and perpendicularly to the axis that connects them. Since the daughter cells al so extend at their ends and form their septa at right angles to the longitu dinal axis, their septal (division) planes lie parallel to those of the mot her cell. To gain a better understanding of how this regularity is ensured, we investigated septation in spherical cells that do not inherit morpholog ically predetermined cell ends to establish poles for growth. We studied fo ur mutants (defining four novel genes), over 95% of whose cells displayed a completely spherical morphology and a deficiency in mating and showed a ra ndom distribution of cytoplasmic microtubules, Tea1p, and F-actin, indicati ng that the cytoplasmic cytoskeleton was poorly polarized or apolar. Septum positioning was examined by visualizing septa and division scars by calcof luor staining and by the analysis of electron microscopic images. Freeze-su bstitution, freeze-etching, and scanning electron microscopy were used. We found that the elongated bipolar shape is not essential for the determinati on of a division plane that can separate the postmitotic nuclei. However, i t seems to be necessary for the maintenance of the parallel orientation of septa over the generations. In the spherical cells, the division scars and septa usually lie at angles to each other on the cell surface. We hypothesi ze that the shape of the cell indirectly affects the positioning of the sep tum by directing the extension of the spindle.