Nuclear and cell division inSorodiplophrys stercorea,a labyrinthulid-like protist

Summary Nuclear division in the labyrinthulid-like protist,Sorodiplophrys stercorea was studied and compared to the mitotic processes reported in other protistan forms.S. stercorea was found to have centrioles with nine peripheral singlet microtubules and a tubular core 300–400 Å in diameter. The nuclear envelope remains intact throughout division, with microtubules passing through nuclear pores. The nucleolus disappears during nuclear division. After intranuclear division, cells cleave by introgressive cleavage while pronucleoli appear in the nuclei and fuse to form the single nucleolus characteristic of the interphase nucleus.     

NUCLEAR BEHAVIOR IN THE VEGETATIVE HYPHAE OF CERATOCYSTIS FAGACEARUM

Vegetative nuclear division in Ceratocystis fagacearum (Bretz) Hunt was found to differ from classical mitosis in that: (1) division always occurs perpendicular to the longitudinal axis of the cell, (2) anaphase movement is unilateral and unsynchronized, (3) a spindle occurs only between separating chromatids. Interphase and prophase nuclei and nucleoli are morphologically similar to those in higher plants. At metaphase the associated chromosomes form a bar of chromatin and lie against the hyphal wall. Spindle fibers appear between separating chromatids, perhaps pushing them apart. When nuclear division is complete the nuclei become attenuated and migrate. Vegetative nuclear division in C. fagacearum may be an evolutionary form of classical mitosis.     

Control of the timing of cell division in fission yeast : Cell size mutants reveal a second control pathway

Strains of Schizosaccharomyces pombe carrying the wee 1 mutation divide at a reduced cell size compared with the wild-type. In this paper, we investigate the mechanism which determines the time of division and cell size at division in wee 1 strains, using three experimental approaches. The evidence suggests that the wild-type control (a cell size control over entry into nuclear division) is absent in wee 1 strains. Instead, a mechanism operates which comprises a cell size control over the initiation of S phase plus a minimum incompressible period in G2 (“timer”) from S phase to nuclear division. The elements of this second control mechanism exist in wild-type cells, though the control is not normally expressed. In particular, the G2 interval in wild-type cells is normally longer than that in wee 1 cells, but can be reduced to this minimum value by delaying S phase. Thus there are two independent controls over entry into nuclear division, one of which operates in wild-type, and the other in wee 1 cells.     

Genetic control of the cell division cycle in the fission yeast Schizosaccharomyces pombe

Summary Twenty seven recessive temperature sensitive mutants have been isolated in Schizosaccharomyces pombe which are unable to complete the cell division cycle at the restrictive temperature. These mutants define 14 unlinked genes which are involved in DNA synthesis, nuclear division and cell plate formation. The products from most of these genes complete their function just before the cell cycle event in which they are involved. Physiological characterisation of the mutants has shown that DNA synthesis and nuclear division form a cycle of mutually dependent events which can operate in the absence of cell plate formation. Cell plate formation itself is usually dependent upon the completion of nuclear division.     

Control of cell size at division in fission yeast by a growth-modulated size control over nuclear division

In the fission yeast Schizosaccharomyces pombe, nutritional reduction of growth rate by supplying poor nitrogen, carbon or phosphate sources causes a decrease in cell size. The effect on cell division following three different nutritional shifts-up has been investigated. In all cases, about 20% of the cells divide at the original cell length, and then cell division stops for a period. Cell division then resumes at the new faster rate, cell length at division being characteristic of the new medium. Further investigation reveals that the first effect of the shift is to inhibit nuclear division rapidly and completely. These results are strongly suggestive of the operation of a cell size requirement for entry into nuclear division. The cell size necessary for nuclear division is set, or modulated, by the prevailing growth conditions. This model is confirmed by a nutritional shift-down, where nuclear division and cell division are stimulated after the shift. Cell length at division falls rapidly until the new shorter length is attained, when a new steady state is assumed at a slower growth rate. The control system is compared with that in bacteria, and its implications for various models proposed for the control of timing of mitosis are discussed.     

Fine structure of mycota 12. Karyochorisis — somatic nuclear division — in Cordyceps militaris

Summary The literature on somatic nuclear division in the fungi consistently suggests that, with but one present exception, the process is non-mitotic. The correlation of previous electron microscopic studies of yeasts with the present study of Cordyceps militaris supports this interpretation and provides a possible mechanism, identified as karyochorisis (nuclear sundrance). During karyochorisis there are two successive invaginations of the inner and outer elements of the perinuclear cisterna. The invagination of the inner nuclear membrane divides the nucleoplasm into two or more subunits described by the new term karyome. The subsequent invagination of the outer nuclear membrane separates the karyomes into daughter nuclei. It is suggested that, in contrast to continued sterile efforts to prove that mitosis is the general mode of fungal somatic nuclear division, the hypothesis of karyochorisis raises new questions that offer new areas for future research. Four that are put forth are 1. the possibility of a correlation between nuclear size and the mode of division; 2. the nature of the mechanism of chromosome replication and separation; 3. the cause of nuclear elongation and the possibility that its initiation depends on a critical RNA volume; and 4. the possibility that colchicine produces polyploidy by blocking the invaginations of the nuclear envelope. In a concluding section the suggestion is made that the proposed hypothetical mode of fungal nuclear division, karyochorisis, may represent only a special example of a general phenomenon of division of double membrane organelles in primitive cells.It is a pleasure to acknowledge the technical assistance of Mrs. Barbara Raymond, Messrs. Lloyd Thibodeau and Philip Spencer of our Laboratory and to thank Professor R. Emerson and Mr. Robert Berman of the Botany Department for making available material of Blastocladiella.This investigation was supported by Postdoctoral Fellowships 9197-C2 and 2F2 AI 9197-04 from the United States National Institute of Allergy and Infectious Diseases, Dr. James H. McAlear, sponsor; and partly by grant AI-05514-01 from the same Institute.     

Division of the generative nucleus in cultured pollen tubes of the Bromeliaceae

Pollen germination, division of the generative nucleus and position of the generative nucleus in the pollen tube during in vitro germination were examined for six bromeliad cultivars. The influence of mixed amino acids (casein hydrolysate) and individual amino acids (Arg, Asn, Asp, Glu, Gly, Met, Phe, Orn, Tyr) were tested. Aechmea fasciata and A. chantinii pollen tubes showed more generative nuclear division in cultured pollen tubes than the other four cultivars tested. Casein hydrolysate did not stimulate generative nuclear division. In general arginine (1 mM) improved division of the Aechmea generative nucleus and to a lesser extent this of Vriesea `Christiane', Guzmania lingulata and Tillandsia cyanea. A concentration of 2 mM arginine reduced pollen tube growth of Aechmea. The vegetative nucleus was ahead of the generative nucleus in approximately 50% of the pollen tubes of all cultivars studied. In about 25% of the pollen tubes, the generative nucleus was ahead and in ±25% pollen tubes the vegetative and generative nuclei were joined together. The distance between the two generative nuclei and the distance from the generative nuclei to the pollen tube tip differed significantly for Aechmea fasciata and A. chantinii. The influence of different amino acids for Aechmea fasciata and A. chantinii varied with respect to pollen germination and generative nuclear division. Arg and Met improved nuclear division of both Aechmea cultivars. Pollen germination and sperm cell production were not linked. This information is important to ameliorate in vitro pollination methods used to overcome fertilization barriers in Bromeliaceae and other higher plants.     

Cell division and the production of cells lacking nuclear bodies in a mutant of Salmonella typhimurium

Summary There are two distinct division phases when the temperature-sensitive DNA synthesis initiation mutant Salmonella typhimurium strain 11G is shifted from 25° to 38°. The first phase appears to represent segregation of the nuclear bodies formed at 38°. Division in this phase takes place at the normal size and produces mainly organisms with one nuclear body. It is dependent on the termination of the rounds of replication in operation at the time of the temperature shift and sensitive to low doses of penicillin. This division phase continues for 60–75 min and then after a short lag division restarts. At first the cells undergoing the second division phase are only slightly larger than normal but they soon grow into short filaments which bud off cells at both ends even if only one nuclear body is present. The cells budded off in this division phase are about 3 long on both broth and M M. They lack nuclear bodies but have a small amount of DNA which may be exclusively in the form of a large plasmid. This second division phase is also dependent on rounds of replication being allowed to terminate at 38° and is sensitive to low levels of penicillin. It is 80–90 min after the temperature shift before the second division phase starts and this lag is maintained even if rounds of replication have been completed prior to the temperature shift by amino acid starvation at 25°. The occurrence of this lag and the demonstration (using penicillin) of potential division sites at regular intervals along 11G filaments suggest that division is initiated some time before the actual division event.     

Pattern of DNA synthesis in nuclei of feeding and starving Amoeba proteus : A cytofluorometric study

The DNA content in isolated nuclei of Amoeba proteus was determined for each of the three groups of synchronized amoebae over different intervals after division. Several nuclei of each amoeba group were fixed 1 h after division, before the amoebae were fed. About h after division, some amoebae in each group were given food (Tetrahymena pyriformis), while the rest were left starving. Samples of the nuclei of fed and starved amoebae were fixed 24 h and (in different groups) 42–55 h after division. In each group from 22 to 48% of the fed amoebae had divided prior to the last nuclei fixation. Starved amoebae did not undergo division. In all three amoeba groups the nuclear DNA content of fed cells by the end of interphase had increased to 280–300% the value for 1 h amoebae. The nuclear DNA content of starved amoebae of all three groups was also increased, and in two groups it exceeded the initial level more than two-fold. However, in all three groups, it was lower than that of fed amoebae. In all the groups the nuclear DNA content in fed amoebae grew after 24 h, i.e. during the second half of interphase, the increase accounting for from 11 to 48% of the total increase. The hypothesis is put forward that the increase in the nuclear DNA content during the cell cycle of Amoeba proteus is the result of two processes: (1) one-time replication of the DNA of the whole genome; and (2) repeated replication of some part of the DNA. In amoebae the relation of the pattern of nuclear DNA synthesis to the diet is considered.     

The ultrastructure of mitosis during sporangiogenesis inWoronina pythii (Plasmodiophoromycetes)

Summary Mitotic divisions during sporangiogenous plasmodial cleavage inWoronina pythii were studied with transmission electron microscopy. We conclude that these nuclear divisions (e.g., transitional nuclear division, and sporangial mitoses) share basic similarities with the cruciform nuclear divisions inW. pythii and other plasmo-diophoraceous taxa. The major distinction appeared to be the absence of nucleoli during sporangial mitosis and the presence of nucleoli during cruciform nuclear division. The similarities were especially evident with regard to nuclear envelope breakdown and reformation. The mitotic divisions during formation of sporangia were centric, and closed with polar fenestrae, and characterized by the formation of intranuclear membranous vesicles. During metaphase, anaphase, and telophase, these vesicles appeard to bleb from the inner membrane of the original nuclear envelope and appeared to coalesce on the surface of the separating chromatin masses. By late telophase, the formation of new daughter nuclear envelopes was complete, and original nuclear envelope was fragmented. New observation pertinent to the mechanisms of mitosis in thePlasmodiophoromycetes include a evidence for the incorporation of membrane fragments of the original nuclear envelope into new daughter nuclear envelopes, and b the change in orientation of paired centrioles during sporangial mitosis.     

COLONY DEVELOPMENT IN EUDORINA ELEGANS (CHLOROPHYTA,VOLVOCALES)1

A detailed ultrastructure study was made of cell division and colony development in Eudorina elegans Ehrenberg. At the onset of cell division and prior to nuclear division the nucleus moved from the cell center to the cell surface. During nuclear division the nuclear membrane remained intact, except for openings occurring at the nuclear poles. The spindle microtubules appeared to arise from a MTOC-like (microtubule organizing centers) structure, while centrioles were absent from the nuclear poles. Following telophase, daughter nuclei formed which were separated by several distinct bands of endoplasmic reticulum. Cytokinesis occurred with formation of a cleavage furrow, associated with a typical phycoplast band of microtubules. However, cytokinesis was incomplete, resulting in formation of cytoplasmic bridges between the plakeal cells. Upon completion of up to five successive cell divisions, the plakea underwent inversion, which appeared to involve the production of colonial envelope material and rearrangement of cytoplasmic bridges. A new hypothesis concerning inversion is postulated based on these observations.     

Centriolar plaque and spindle microtubules in the ascomyceteSordaria humana

Centriolar plaque and spindle microtubules in young asci of an ascomycete,Sordaria humana were studied by electron microscopy. Centriolar plaque is electron opaque and has an amorphous structure. Two dispositions of centriolar plaque were observed, one entirely contiguous to the nuclear envelope in a meiotic division and the other partially joined to the envelope in a mitotic division following meiosis. The spindle was formed inside the nuclear envelope and spindle microtubules terminated at the polar protrusion of the nuclear envelope. Some spindle microtubules seem to connect directly with the centriolar plaque passing through perforations of the nuclear envelope.     

In situ and experimental evidence of the influence of turbulence on cell division processes of Peridinium cinctum forma westii (Lemm.) Lefèvre

Comparison of the diel variations of the wind intensity and the division rate (DR) of Peridinium cinctum forma westii (Lemm.) Lefèvre in situ reveals that the intensity of the wind blowing throughout the whole day does not affect the DR. On the other hand, a strong inhibitory effect is noticed when the wind episodes occur during the time period 18.00–02.00 h. Systematic hourly sampling and staining of Peridinium cells showed that nuclear division takes place between 23.00 and 02.00 h and is completed before cytokinesis begins. Thus the time period 18.00–02.00 h corresponds to the premitotic and mitotic phases of cell division, and the turbulence generated by the wind affects the process of nuclear division.

The relationship between water turbulence and the DR of Peridinium which was observed in Lake Kinneret (Israel) has been checked under experimental conditions. Peridinium was grown without shaking, with continuous rotary shaking and with intermittent shaking at 100 r min^-1. The specific growth rate (k), generation time (G) and mortality rate were followed and compared. The results obtained confirm the facts observed in situ and clarify some aspects. Intermittent shaking of 2 h day^-1 during the dark phase reveals the inhibitory effect of agitation on nuclear division. Continuous shaking causes a high rate of cell mortality. Shaking during the light period does not affect the division process.

The effect of turbulence on the DR of Peridinium explains why the timing of the bloom in Lake Kinneret is a function of the duration and intensity of the mixing period in the lake.     

VEGETATIVE NUCLEAR DIVISION IN NEUROSPORA

A re-examination of the mode of vegetative nuclear division in Neurospora crassa was facilitated by the availability of the mutant “clock” which produces definite growth bands. Since the growth rhythm is correlated with nuclear divisions, stained mycelial mats of this mutant prepared at intervals from the beginning of a growth period provided a sequence of stages of division. In a 28-hour period the following broad features of nuclear behavior were observed: In the early part of the period during rapid mycelial growth, dividing elongated nuclei predominated. At the end of the period the mycelium contained mostly rounded resting nuclei. In the middle of a growth period nuclear forms of various degrees of annularity occurred along with elongated and rounded nuclei. Elongated and rounded nuclei completed division cycles without change in form, although the corresponding stages of the two types were similar. Elongated nuclei assumed a spiral form at the beginning of division. As division proceeded, relaxation of the nuclear gyres was accompanied by a visible duplication of the chromatin thread and the appearance of chromomere-like bodies on the daughter threads. One of the chromomere-like bodies became displaced and was interpreted to be a chromosome or a segment of a chromosome that acts as a mitotic center. All the chromosomes were found to be interconnected and to act as a unit throughout the division cycle. Only after the separation of the daughter chromatin threads could seven chromosomes be counted. Electron microscopic studies complemented the observations with the light microscope. On the basis of the evidence it was concluded that the vegetative nuclear division in Neurospora differs from the classical mitotic pattern in the following respects: (1) absence of visible centrioles, (2) the presence of interconnected chromosomes, (3) the comparatively late appearance of countable chromosomes, and (4) the frequent presence of interzonal connections between separating chromatin threads.     

Coordination of division events in theChlamydomonas cell cycle

Summary At concentrations that did not affect growth, hydroxyurea and 2¹-deoxyadenosine inhibited DNA synthesis inChlamydomonas. Evidence that initiation of mitosis is dependent upon completion of DNA replication was provided by the arrest of inhibited cells with undivided nuclei containing undispersed nucleoli. Initiation of cytokinesis is not dependent upon progress of nuclear division since, in arrested cells, cleavage microtubules became deployed in a phycoplast and a cleavage furrow developed fully, until obstructed by the undivided nucleus. Chloroplast constriction and division also continued independently of nuclear division. It is concluded that nuclear division, cytoplasmic cleavage and chloroplast division are in separate sequences of dependent events. This is supported by flexibility of their relative timing in successive divisions, since after the first commitment to divide nuclear division is followed by initiation of cleavage and then chloroplast division, whereas following subsequent commitments these events occur in reverse time order. This flexibility of order indicates changing rates of progress through separate sequences of events.Deposition of wall material was dependent upon the completion of cytokinesis, but this inhibition of wall deposition by incomplete cytokinesis did not extend to other daughters within the same mother cell.These observations are correlated with our earlier data concerning the rate-limiting control points for division and a model for the coordination of division events is presented. The relationships between different plant cell cycles is discussed in view of the findings presented.     

Development of the Mycetozoan Echinosteliopsis oligospora*

SYNOPSIS. The mycetozoan genus Echinosteliopsis, resembling the myxomycete Echinostelium in some of its features, is described. The single species, E. oligospora Reinhardt & Olive, forms small sporocarps which consist of a basal disk, stalk and a sporangium with only 1–8 spores. Spores form progressively, not simultaneously, by segmentation. The spores germinate to release non-flagellate amebae which, in liquid, assume a characteristic broad, fan shape. Each ameba has one or more nuclei. The nucleus is distinctive because of refractile, globular to elongate peripheral bodies which cytochemical tests indicate to be primarily RNA. At the time of nuclear division the characteristic RNA bodies disappear and, as observed with the phase microscope and in stained preparations, optically dense material accumulates in the middle area of the nucleus. Threads, either a spindle or actual chromatin, can be seen attached to the nuclear membrane. The threads separate to opposite poles as the nucleus elongates. During this division process the nuclear membrane apparently remains intact. Synchronous binucleate divisions, as well as a tripolar nuclear division, have been observed. Uninucleate and synchronous binucleate divisions may or may not be followed by cytokinesis. The absence of cell division after nuclear division leads to the production of cells with varying numbers of nuclei. Nuclear divisions in early sporangial stages and in spores have not been observed. The spores are uni- to multinucleate. In 8-spored sporangia and in most 4-spored sporangia there is a characteristic small “stalk spore” at the apex of the stalk. The stalk spore germinates slowly, if at all, but the larger spores germinate readily. No evidence of a sexual process has been found.     

Effects of microtubule-inhibitors on nuclear migration and rhizoid differentiation in germinating fern spores (Onoclea sensibilis)

Summary Germinating spores of the sensitive fern,Onoclea sensibilis L., undergo premitotic nuclear migration before a highly asymmetric cell division partitions each spore into a large protonemal cell and a small rhizoid initial. Nuclear movement and subsequent rhizoid formation were inhibited by the microtubule (MT) inhibitors, colchicine, isopropyl-N-3-chlorophenyl carbamate (CIPC) and griseofulvin. Colchicine prevented polar nuclear movement and cell division so that spores developed into enlarged, uninucleate single cells. CIPC and griseofulvin prevented nuclear migration, but not cell division, so that spores divided into daughter cells of approximately equal size. In colchicine-treated spores, MT were not observed at any time during germination. CIPC prevented MT formation at a time coincident with nuclear movement in the control and caused a disorientation of the spindle MT. Both colchicine and CIPC appeared to act at a time prior to the onset of normal nuclear movement. The effects of colchicine were reversible but those of CIPC were not. Cytochalasin b had no effect upon nuclear movement or rhizoid differentiation. These results suggests that MT mediate nuclear movement and that a highly asymmetric cell division is essential for rhizoid differentiation.     

红毛菜丝状体核分裂研究

选择具异型世代交替的福建人工栽培的红毛菜为研究材料,对红毛菜丝状体世代的丝状藻丝及孢子囊枝等阶段进行了较系统的核分裂观察研究,探讨红毛菜核分裂特征.结果显示:红毛菜营养藻丝和孢子囊枝细胞均为二倍体细胞,2n=8,其核分裂显示为有丝分裂的过程;同时,丝状体阶段细胞核分裂至前期末均会出现同源染色体配对现象,显示有丝分裂同源染色体配对行为是红毛菜丝状体核分裂的一个重要特征.     

ULTRASTRUCTURE OF CELL DIVISION AND REPRODUCTIVE DIFFERENTIATION OF MALE PLANTS IN THE FLORIDEOPHYCEAE (RHODOPHYTA): CELL DIVISION IN POLYSIPHONIA1

Cell division in the marine red algae Polysiphonia harveyi Bailey and P. denudata (Dillwyn) Kutzing was studied with the electron microscope. Cells comprising the compact spermatangial branches of male plants were used exclusively because of their small size, large numbers and the ease with which the division planes can be predetermined. Some features characterizing mitosis in Polysiphonia confirm earlier electron microscope observations in Membranoptera, the only other florideophycean algae in which mitosis has been studied in detail. Common to both genera are a closed, fenestrated spindle, perinuclear endoplasmic reticulum, a typical metaphase plate arrangement of chromosomes, conspicuous, layered kinetochores, chromosomal and non-chromosomal microtubules, and nucleus associated organelles (NAOs) known as polar rings (PRs) located singly in large ribosome-free zones of exclusion at division poles in late prophase. However, other features, unreported in Membranoptera, were observed consistently in Polysiphonia. These include the presence of PR pairs in interphase-early prophase cells, the attachment of PRs to the nuclear envelope during all mitotic stages, the migration of a single PR to establish the division axis, a prominent, nuclear envelope protrusion (NEP) at both division poles at late prophase, the prometaphase splitting of PRs into proximal and distal portions, and the reformation of post-mitotic nuclei by the separation of an elongated interzonal nuclear midpiece at telophase. During cytokinesis, cleavage furrows impinge upon a central vacuolar region located between the two nuclei and eventually pit connections are formed in a manner basically similar to that reported for other red algae. Diagrammatic sequences of proposed PR behavior during mitosis are presented which can account for events known to occur during cell division in Polysiphonia. Mitosis is compared with that reported in several other lower plants and it is suggested that features of cell division are useful criteria to aid in the assessment of phylogenetic relationships of red algae.     

The role of asymmetric cell division in pteridophyte cell differentiation. I. Localized metal accumulation and differentiation inVittaria gemmae andOnoclea prothallia

Summary In gemmae ofVittaria graminifolia and prothallia ofOnoclea sensibilis, cell differentiation is initiated by nuclear migration and geometrically asymmetric cell division. The small daughter cells inVittaria develop into antheridia in the presence of gibberellic acid or into rhizoids or new prothallia in its absence. Antheridial differentiation from asymmetric division is induced inOnoclea byPteridium antheridiogen, whereas rhizoid or vegetative cell formation occurs in its absence. Although asymmetric cytokinesis initiates differentiation, it does not in itself determine the developmental fate of the smaller cell. Several histochemical techniques demonstrate that prior to nuclear migration and cell division, Ca²⁺ accumulates in the cytoplasm and wall of the cell at the site where asymmetric division will occur, regardless of the developmental fate of the small cell. The cytoplasmic localization of Ca²⁺ appears to reflect a mobilization of Ca²⁺ from within the cell that eventually moves into the cell wall. We propose that this internal accumulation of Ca²⁺ leads to a localized decrease in cytosolic [Ca²⁺] which in turn may regulate developmental events such as nuclear migration.Publishing prior to 1984 as Alix R. Bassel.