Bong Y. Yoo

Ultrastructural changes of the fission yeast (Schizosaccharomyces pombe) during ascospore formation

SummaryThe fission yeast, Schizosaccharomyces pombe, a homothallic haploid strain NCYC 132, was induced to flocculate and conjugate to facilitate the study of the ascosporogenesis. It is found that two nuclei are formed during meiosis I and these nuclei divide again during meiosis II. The forespore membrane emerges at the beginning of meiosis II, elongates without fragmentation to enclose the nucleus and other cytoplasmic organelles, including mitochondria and ER. Meiosis occurs without fragmentation of the nuclear membrane. Immediately after the enclosure of the nucleus, the forespore membrane is resolved into two separate membranes. The inner one appears to become the spore plasmalemma and the outer one, if it persists, becomes a limiting membrane of the spore wall. In some cases, the outer membrane is seen to be ruptured. Spore wall materials deposite in the space between the inner and outer membranes.

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.

Histones from exponential and stationary L-cells: Evidence for differential binding of lysine-rich and arginine-rich fractions in chromatin☆☆☆

Abstract Histones from exponential and stationary-phase mouse L-cells were quantitated after acrylamide gel electrophoresis in order to investigate cell cycle-dependent changes in the mode of binding of the various fractions in chromatin. By introducing various concentrations of citrate and divalent cations in the medium used for cell lysis and isolation of nuclei prior to histone extraction it was possible to demonstrate that certain histone fractions are preferentially retained in either exponential or stationary-phase nuclei. Differential retention of lysine-rich F 1 was most evident when the lysing medium contained 1 m m Mg 2+ and Ca 2+ and 5 m m citrate (pH 2.75). In these conditions twice as much F 1 is retained in stationary as in exponential nuclei. Differential retention of arginine-rich histones was most evident when the lysing medium contained 10 m m Mg 2+ and Ca 2+ and no citrate. In these conditions more F 2a 1 is retained in exponential than in stationary nuclei while the opposite is true for F 3 . However, the total amount of arginine-rich fractions (F 2a 1 + F 3 ) retained was found to be the same in both cell phases. The results are discussed in relation to known structural features of the histones.

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.

Ultrastructure of the ascospore walls of Schizosaccharomyces pombe during germination

At onset of germination, spores of S. pombe show localized swelling, the first vestige of germ-tube extension by the spore wall. As germination progresses, the original spore wall becomes thinned at the site of extension and is medially replaced by a new germinative layer. Germ tube growth is not erumpent. Cytology of germination of other spores is compared.