Among the spheroidal fission-yeast (Schizosaccharomyces pombe) cells resulting from aculeacin A treatment were found cells whose putative growth axis and polarity differed from those of their progenitor, that is they were changed in the orientation of their septum. The ratio of cell length (measured perpendicular to the septum plane) to septum diameter of these cells equalled or exceeded unity without exception, whether the septum orientation changed or not and whether the cellular shape was spherical, spheroidal or cylindrical. From these results we conclude that the septum is always oriented perpendicular to the plane including the longest axis of the cell even if the morphology is irregular or the new septum has become perpendicular to the previous septum. There is no cellular region forbidden to new septa.
I N T R O D U C T I O NThe septum at division not only divides a cell into two cells but also indicates an axis of the cell. The position and orientation of the septum, therefore, are of fundamental morphogenetic importance for subsequent cellular growth, especially for differentiation and morphogenesis in plants. Cells of the sausage-shaped fission yeast Schizosaccharomyces pombe usually grow cylindrically and form their transverse septa near the middle of the major axis. However, questions of how the septum is oriented or whether some rules exist related to axial orientation of the cell remain unanswered, especially if the cellular morphology is altered.Fission-yeast cells grow spherically in the presence of low doses of aculeacin A (Acu), an antifungal antibiotic (Miyata et al., 1985). When the temperature-sensitive mutant cdc 10-121 hwas cultured at the restrictive temperature in the presence of Acu, the cellular morphology changed to 'round-bottomed flask (RBF) like' (see Fig. 2h). When these RBF-like cells were transferred to fresh medium without Acu and cultured at the permissive temperature, the cells began to divide after recovery growth for about 3 hthe 'recovery system'. The first division of an RBF-like cell showed the remarkable feature that its septum was formed without exception at the boundary plane between the spheroidal and the cylindrical region: the RBF-like cell divided into a spheroidal and a cylindrical sib. The second division divided the spheroidal and the cylindrical progeny into two spheroidal and two cylindrical sibs, respectively. The alterations of morphology caused by Acu were simple and reversible; the wall components were changed, resulting in the alteration of cellular morphology, but the growth of the protoplasm of the RBFlike cells seemed to equal that of the controls (Miyata et al., 1985, 1986). RBF-like cells in the recovery system are therefore convenient for investigating the relationship between septation and cellular morphology. Using the recovery system, we found a correlation between the Abbreviations: Acu, aculeacin A; RBF, round-bottomed flask; CL, cell length; SD, septum diameter. 0001-3241 0 1986 Government of Canada
