Byron F. Johnson

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.

Morphometric analysis of yeast cells: IV. Increase of the cylindrical diameter of Schizosaccharomyces pombe during the cell cycle☆

Abstract The maximum cylindrical diameter of fussion yeast cells was measured using an image-shearing eyepiece with an optical microscope. The mean diameters were 3.4, 3.8, 4.2, 4.4 and 4.8 μm for cells bearing one to five fission scars, respectively. These increments amount to about 10% per generation, and correspond to 10% increases in cellular surface area during expansion. This result was found wholly comparable with areal increases of Saccharomyces adult cells calculated from length and width data in the literature). Extant notions about growth of yeast cell walls are inadequate to explain these size increases.

Cell Division: a Separable Cellular Sub-cycle in the Fission Yeast Schizosaccharomyces pombe

Summary: Carbon-limited growth of Schizosaccharomyces pombe in chemostats under certain conditions of dissolved oxygen concentration and temperature gave rise to multiseptate and branched hyphal cells. On the basis of these observations it is suggested that fission can be uncoupled from growth, nuclear processes and septation.

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.

Sexual co-flocculation by heterothallic cells of the fission yeast Schizosaccharomyces pombe modulated by medium constituents.

Novel simple synthetic media for inducing sexual co-flocculation in a short time after mixing heterothallic fission-yeast (Schizosaccharomyces pombe) cells of h- and h+ were devised; The most effective of these, mannose synthetic medium (MSM), contains 0.4% mannose as a carbon source in addition to galactose, KH2PO4 (pH4.0) and 4 vitamins. The addition of galactose to the medium suppressed the asexual self-flocculation but rather promoted the sexual co-flocculation. By transferring and mixing h- and h+ cells grown in malt-extract broth plus galactose into MSM, these heterothallic strains were revealed to be sexually ready through a long period of the log to stationary phases. Furthermore, a variety of C sources and NH4Cl at various concentrations in various media were examined for their effects upon sexual co-flocculation, conjugation and sporulation; it was found that the sugar concentration strictly affected the progress of the sequence of sexual reproduction at 26°C but not 30°C and that sexual co-flocculation of the heterothallic strains was induced only under lower concentrations of C and N source than that for the homothallic one.

A kinetic analysis of spontaneous ρ− mutations in yeast

Summary Spontaneous mutation to the petite state at the level of the individual cell was studied in a haploid strain of yeast by the technique of pedigree analysis. Results indicated that (1) the mutability of ρ+ cells within a population in log phase is variable; (2) ρ+ mitotic buds are, on the average, about 50% more mutable than the ρ+ cells from which they arose; (3) the mutability of a ρ+ cell tends to decrease as it produces consecutive buds: (4) the probability that a mother cell will become ρ− at or immediately subsequent to cell division is, on the average, one third the probability that its bud will be ρ−; (5) most, if not all spontaneous ρ− mutant cells contain mitochondrial DNA as judged from suppressiveness measurements. The data indicate that the spontaneous production of a mutant cell is a multi-step process. Neither a replicative advantage of defective mitochondrial DNA nor the existence of a “master” mitochondrial genome provides a satisfactory explanation of the process. Either selective dispensation of defective mitochondria to the bud at cytokinesis or normal retention by the mother cell of factors influencing the amplification or rate of induction of defective mitochondrial DNA could be involved.

Asymmetric Location of the Septum in Morphologically Altered Cells of the Fission Yeast Schizosaccharomyces pombe

Summary: Cells of the fission yeast Schizosaccharomyces pombe, normally sausage-shaped, changed to a round-bottomed flask (RBF)-like morphology during growth in the presence of aculeacin A (Acu), an antifungal antibiotic. The volume of RBF-like cells was comparable to that of the control cells. After being transferred to normal conditions (without Acu at 25°C), the RBF-like cells continued to grow at the cylindrical and/or spherical end(s) and then the septum at the subsequent division of the cells was formed without exception at the boundary plane between the spheroidal and the cylindrical region; it is at this boundary that the nucleus was located before mitosis. Hence the RBF-like cell divided into a spheroidal and a cylindrical sib at the first cell division. At the end of the second cell cycle, the spheroidal and the cylindrical progeny divided into two spheroidal and two cylindrical sibs respectively. The values of the mean length (long/short) and volume (big/small) ratios of paired sibs were larger in order of (a) cylindrical normal, with both mean ratios 1·06; (b) cylindrical control; (c) cylindrical progeny of RBF-like cell; (d) spheroidal progeny of RBF-like cell; and (e) RBF-like cell, whose mean length ratio was 1·25 but whose mean volume ratio was 1·94. That is, the more the morphology deviated from the cylindrical form, the greater was the degree of asymmetry. There was no rule relating the biases to the growth pole in these asymmetries.

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.

Temperature sensitivity of flocculation induction, conjugation and sporulation in fission yeast.

Homothallic cultures of Schizosaccharomyces pombe, anaerobically grown to stationary phase in broth at 32°C, were induced by aeration to flocculate. Flocculation was followed by copulation, conjugation, zygote formation, meiosis and sporulation. Cultures grown to stationary phase at 32°C and then aerated at 37°C did not sporulate. Grown to stationary phase at 37°C, cultures were not immediately inducible when aerated at 32°C. To identify which events in the developmental sequence were thermosensitive, we grew and induced cultures at 32°C and then shifted them at various times to 37°C. We observed the following events to be thermosensitive: development of respiratory sufficiency, readiness (inducibility of a culture within 1 h), flocculation induction, copulation, conjugation and early sporulation (including meiosis). Respiration, flocculation and spore maturation were thermoresistant. Conjugation-induced lysis and post-developmental deflocculation were enhanced at 37°C.