Gode B. Calleja

Cell division in yeasts: III. The biased, asymmetric location of the septum in the fission yeast cell, Schizosaccharomyces pombe

Abstract Living, dividing, log-phase fission yeast cells (178 pairs) were photographed by fluorescence microscopy of their fluorochromed walls. Analysis of the lengths, volumes, and fission scar distributions of these cells led to the following conclusions: the new septum is sited asymmetrically at division by length parameters, and the asymmetric site is biased toward the newer end (that end generated by the previous cell division) of the dividing cells. The volumes of the resultant sibs, however, are equal. Some rather simple models for siting of the septum are presumed untenable on the basis of the evidence.

Cell Division: Key to Cellular Morphogenesis in the Fission Yeast, Schizosaccharomyces1

Publisher Summary This chapter discusses the cell division in the fission yeast, Schizosaccharomyces. Cell division by the fission yeast is a two-phase process involving elaboration and splitting of a transverse septum. The septum elaboration is also another two-phase process including (1) the centripetal growth of a primary septum, and (2) deposition of secondary septa on both sides of the primary septum. Polymer synthesis; and elaboration of walls having species-specific character and impressive beauty-morphogenesis occur at an extracytoplasmic site remote from the control center. The cell division is a process having long-lived morphological effects that become obvious after examination of only a few cellular processes, cellular structures, or a few morphometric analyses. Although cell division in the fission yeast seems simple on comparison with the division of most eukaryotes, it still has many complex consequences when considered as a key to morphogenesis.

Analyses of fission scars as permanent records of cell division in Schizosaccharomyces pombe.

Cell division in fission yeast is recorded on the walls of the progeny as fission scars and fuscannels. Theoretical analyses of scars in a population of cells allow us to deduce the total number of scars, the average number of scars per cell, and the distribution of scars in the population. The number of scars in the population is twice the number of cells; the average number of scars per cell is two. The predicted distribution of scars in the population is as follows: one-scar class, 33·33%, two-scar class, 43·50%; three-scar class, 15·96%; four-scar class, 5·08%; five-scar class, 1·52%; six-scar class, 0·44%; seven-scar class, 0·12%. The other scar classes are very rare. The predicted distribution fits the observations quite well. In the analyses, we assume that the cell lays down its septum with equal probability to the left or to the right of an end-most scar (which at fission time approximates the middle of the cell). We show the validity of this assumption and describe the possible modes of segregation of the scars in one generation. Given a cell with a certain number of scars, we deduce in terms of scar number its most likely progeny as well as its most likely progenitor. Finally, we rationalize the rarity of the multi-scar cell, demonstrate the improbability of its further ageing, and describe its mechanism for rejuvenation.

Conjugation in Chemostat Cultures of Schizosaccharomyces pombe

Summary: In chemostat cultures, Schizosaccharomyces pombe (NCYC 132 and derivative strains) formed conjugants in glucose-limited but not in ammonia-limited synthetic medium. Conjugation occurred at a low growth rate (doubling time, t d approx. 11 h), low aeration rate (0.1 fermenter vol. air min-1) and 32°C, but not at an increased growth rate (t d approx. 4.5 h) or at lower temperatures (20-25 °C). Conjugation of S. pombe 968h90 in chemostat culture was greater under glucose-limitation than ammonia-limitation. Batch cultures of S. pombe 968h90 in synthetic medium produced conjugants when the glucose supply was limited, but when glucose was in excess the amount of nitrogen influenced the degree of conjugation. Conjugation of strain 968h90 was inhibited in batch cultures in synthetic medium under conditions such that a large quantity of glucose was metabolized, and in cultures grown in synthetic medium containing added ethanol (0.5%). Inhibition of conjugation also occurred in batch cultures of S. pombe NCYC 132 and 968h90 in malt extract broth containing added ethanol.

The mating system of a homothallic fission yeast

SummaryExposed to iodine vapors, colonies of a homothallic strain of Schizosaccharomyces pombe were of two classes: P, with many black streaks, and d, with scarcely any. Contiguous P and d colonies, but not contiguous P colonies nor contiguous d colonies, gave the iodine junction reaction, a black line along the common boundary of two colonies. Neither class could be purified. On replating, a P colony gave rise to a P plate, which contained mostly P but also d colonies; a d colony gave rise to a d plate, which contained mostly d but also P colonies. The P/d colony ratio of a fresh isolate (if isolated as a P colony) was very high or (if isolated as a d conoly) very low. It fell, if initially high, or rose, if initially low, on subsequent replatings of the same isolate. Maintained for many generations, an isolate attained a fairly constant P/d colony ratio that was less than unity. Tetrad analysis showed 2:2 segregation of the classes. We conclude that a homothallic clone is a mixture of two types of cells: P, which gives rise to a P colony, and d, to a d colony. The two types are sexually complementary and interconvertible. The rate of intercoversion of P to d exceeds that of d to P by a factor of about 2.

EXTRACELLULAR AMYLASES OF STARCH-FERMENTING YEAST: pH EFFECT ON EXPORT AND RESIDENCE TIME IN THE PERIPLASM

Aerobic cultures of S. alluvius in Wickerhams yeast-nitrogen-base medium with starch as sole carbon source become strongly acidic and contain no detectable extra-cellular amylolytic activity during stationary phase, when the activity in buffered cultures is maximal. The extracellular amylases are irreversibly inactivated at the low pH value (less than 3.5) attained by the cultures. When adequately buffered, the medium yields maximal extracellular amylolytic activity. About 0.2 M phosphate buffer is adequate for substrate concentrations of up to 0.5% starch; higher starch concentrations require more buffer. Unbuffered cultures that are adjusted once with alkali to pH 5.5 also allow maximal extracellular amylolytic activity, provided the adjustment is made prior to the end of exponential growth. Automatic pH control allows use of high starch concentrations of up to 4%. Export is optimal at pH values higher than the optima for enzyme activity and stability and for population growth. The need for pH adjustment prior to the appearance of amylolytic activity in the medium suggests pH dependence of the export process itself and/or acid inactivation of enzymes transiently resident in the periplasm. (Refs. 23).

RUPTURED FISSION YEAST WALLS : STRUCTURAL DISCONTINUITIES RELATED TO THE CELL CYCLE

Distributions of rupture sites of fission yeast cells ruptured by glass beads have been related to a new morphometric analysis. As shown previously (Johnson et al.,Cell Biophysics, 1995), ruptures were not randomly distributed nor was their distribution dictated by geometry, rather, ruptures at the extensile end were related to cell length just as the rate of extension is related to cell length. The extension patterns of early log, mid-log, late log, and stationary phase cells from suspension cultures were found to approximate the linear growth patterns of Kubitschek and Clay (1986). The median length of cells was found to decline through the log phase in an unbalanced manner.

SMASHED FISSION YEAST WALLS: STRUCTURAL DISCONTINUITIES RELATED TO WALL GROWTH

Twenty-three samples of fission yeast cells (Schizosaccharomyces pombe) were smashed by shaking them with glass beads. The samples represented all phases of the culture cycle, with the lag and log phases emphasized. Ruptured walls of the smashed cells were observed by phase-contrast and electron microscopy. Ruptures were tabulated with respect to their magnitudes and locations. Ruptures occurred not at random, nor at sites directed by geometry, but predominated in certain definable wall regions. These discontinuities were correlated with morphogenetic activities of the cell. Thus, the extensile end was found to be most fragile through most of the culture cycle. Also fragile was the nonextensile end, its edge more than its middle. Further, the data were applied to the testing of predictions from extant models (Johnson endohydrolytic softening model and Wessels presoftened-posthardened and crosslinking model) for hyphal tip extension. The frequency of rupture at the extensile (old) end of the cell was qualitatively predicted by both models; the frequency at the nonextensile (new) end was not predictable by either. Rupture frequencies and characteristics at other regions conformed to predictions by one or the other model, but rarely by both.

Mating-type gene switching in a homothallic fission yeast

When single cells of a homothallic strain of the fission yeast ( Schizosaccharomyces pombe 968h 90 ) are plated upon sporulation agar, a couple of cell divisions yield four preconjugal cells ordered in a line. Within a line, conjugation occurs either between sibs (the pair at either end of the line) or between cousins (the central pair of cells) or not at all. Miyata & Miyata (1981) have shown that sib matings are favored over cousin matings, the ratio of sib:cousin:sterile being 96:23:10. To have mating within these 4-cell clones means that the mating types of some of the cells have switched. In a further analysis of their data, we come to a series of deductions, one of which is that switching of mating-type genes in the fission yeast must be asymmetrical. We propose a random model and a deterministic model based upon asymmetrical switching. Either model could generate the ratios provided, but the models are sufficiently different that experimental tests should be able to discriminate between the two.

Temporal asymmetry of sex interconversion in a strain of the homothallic fission yeastSchizosaccharomyces pombe

In the homothallic P/d sex interconversion system in a strain of a fission yeast (Schizosaccharomyces pombe), P→d is apparently twice as frequent as d→P. This is interpreted to mean that P→d occurs before DNA replication, whereas d→P occurs after. But the probabilities of their occurrence within a cell cycle are about the same (1 in 27 cell divisions).