Virus-induced Abnormalities of Mitosis in Certain Solanaceous Species

Mitotic abnormalities were observed in a range of Solanaceousspecies infected respectively with tobacco mosaic virus andaspermy virus. These abnormalities ranged from persistence ofthe nucleolar material as far as early anaphase, in all casesobserved, to a complete amitosis encountered in stem-enationsof one species examined, viz. Petunia violacea. These findingsare thought to support the view that virus particles, multiplyingwithin dividing cells, compete with the nuclear-DNA for thesupply of RNA located in the nucleolus.     

Induced genetic deficiency of the nucleolar organizer in rat kangaroo cells (PTK1) by ultraviolet laser microirradiation

An ultraviolet laser microbeam was used to irradiate one of the two nucleolar organizer regions of PTK1 cells in early prophase. The directed nucleolar deficiency induced by ultraviolet laser irradiation was maintained in the daughter cells through subsequent cell generations. However, the frequent occurrence of spontaneous cell fusion in low density cells following the cloning procedure facilitated a recovery of cells to two or more nucleoli.     

An antibody against a sequence of human topoisomerase II gives different immunofluorescence patterns in quiescent and proliferating nuclei of Pisum sativum L.

Immunofluorescence with an antibody against a C-terminal sequenceof human topoisomerase II has been performed on nuclei releasedfrom different tissues of Pisum sativum L. All the nuclei labelledand preincubation of the antibody with the corresponding immunogenpeptide strongly decreased the fluorescence. The labelling pattern(particularly nucleolar) was different in quiescent and proliferatingnuclei and changed during germination. In prophase nuclei, thelabelling was at the periphery of the condensing chromosomes,and in metaphase chromosomes, a characteristic labelling atpericentromeric regions was found. A computer search indicatedthat, apart from mammalian topoisomerase II, the immunogen peptidedid not match any other sequenced protein which could reasonablybe present in plant nuclei. The possible relation between theantigen recognized and topoisomerase II is discussed. Key words: Topoisomerase II, seed germination, Pisum sativum, immunofluorescence, nuclear proteins     

The condensin complex is essential for amitotic segregation of bulk chromosomes, but not nucleoli, in the ciliate Tetrahymena thermophila

The macronucleus of the binucleate ciliate Tetrahymena thermophila contains fragmented and amplified chromosomes that do not have centromeres, eliminating the possibility of mitotic nuclear division. Instead, the macronucleus divides by amitosis with random segregation of these chromosomes without detectable chromatin condensation. This amitotic division provides a special opportunity for studying the roles of mitotic proteins in segregating acentric chromatin. The Smc4 protein is a core component of the condensin complex that plays a role in chromatin condensation and has also been associated with nucleolar segregation, DNA repair, and maintenance of the chromatin scaffold. Mutants of Tetrahymena SMC4 have remarkable characteristics during amitosis. They do not form microtubules inside the macronucleus as normal cells do, and there is little or no bulk DNA segregation during cell division. Nevertheless, segregation of nucleoli to daughter cells still occurs, indicating the independence of this process and bulk DNA segregation in ciliate amitosis.     

BEHAVIOR OF NUCLEOLI AND CONTRACTING NUCLEOLAR VACUOLES IN TOBACCO CELLS GROWING IN MICROCULTURE

The ability to observe for extended periods of time individual tobacco cells growing in microculture has made it possible to describe the behavior of their nucleoli and contracting nucleolar vacuoles. Nucleoli typically disappeared in prophase and reappeared in telophase. If several nucleoli were present in telophase they generally fused to form only one or two during interphase. In one instance a nucleolus was seen to separate into two nucleoli prior to disappearance in late prophase. In aging and senescent cells the number of nucleoli or bodies similar to normal nucleoli often increased, and occasionally fragmentation of nucleoli was noted prior to death of cells. Budding of solid material from the nucleolus was also observed. The amount of nucleolar material decreased rapidly prior to death of tobacco cells. Nucleolar vacuoles were found to be a general and consistent component of tobacco cells in microculture. Nucleolar vacuoles typically formed and contracted repeatedly in interphase nuclei and apparently released a fluid material into the nucleus. Associated with the contraction of the nucleolar vacuoles was a corresponding decrease in diameter of the nucleolus. Nucleolar vacuoles were observed to occur in about 70% of the actively growing cells examined, whereas they were present in only 33% of the senescent or weakened cells. These data indicate a relationship between nucleolar vacuoles and the morphogenic status of the cells. Since it has been shown by others that the nucleolus is an active site of RNA metabolism, it is suggested that the contracting nucleolar vacuoles may be involved in the controlled release of a soluble product associated with RNA metabolism.     

Nuclear behaviour in callus cells: Morphology and division

The cells of some pea, tobacco and haplopappus callus strains reveal considerable variability in nuclear morphology (polymorphous nuclei, differences in nucleolar size and number, enlarged chromocentres) and chromosome counts. The specific features in the nuclear morphology of callus cells are related with some pecularities in the reproduction activity of these cells (amplification, amitosis, fragmentation, various deviations from normal mitosis) under cultural conditions including both the definite action of the culture system and the absence of the regulatory control by the intact organism.     

蚕豆染色体周边RNP形成过程的电镜研究

本文运用Bernhard染色方法研究了蚕豆根端分生组织细胞中染色体周边RNP的超微结构以及这种周边RNP在有丝分裂前期到中期的形成过程。我们观察到,在前期核仁解体过程中,来自核仁的RNP物质结合于染色体表面,形成染色体周边RNP。前期末时,大量核仁RNP颗粒向周围扩散并进一步结合于染色体表面,使染色体周边RNP有所增加。中期染色体的周边RNP明显多于前期,由直径15-20 nm的RNP颗粒构成。RNP物质在染色体周边的分布是不均匀的。姊妹染色单体之间往往有较多的RNP物质存在。本文观察结果表明染色体周边RNP来源于核仁RNP。     

The Effect of Thermal Shock On the Division Cycle of Meristematic Cells

Abstract. The dynamic adaptation of a meristem to a thermal change from 15 to 35°C has been studied. A gradual drop occurs in the mitotic index during the first 6 hr after the change, while the percentage of prophases drops less steeply. At the same time the phenomenon of nucleolar segregation is observed.
By making use of a cell population morphologically differentiated as binucleate (by 'labelling' with caffeine), it was possible (a) to prove that prophase is specifically prolonged and (b) to locate a highly thermosensitive segment of interphase, situated in the middle of the S period, and apparently coinciding with the sub-period S₂, described in the same material as characterized by a decrease in the rate of DNA synthesis.
The lengthening of prophase and nucleolar segregation resemble the effects produced by drugs which inhibit RNA synthesis.
Intermittent thermal shocks are suggested as a possible means for synchronizing the division cycle of plant cells.     

Organization of argyrophilic nucleolar material throughout the division cycle of meristematic cells

Summary The silver impregnation of nucleolar material facilitated the study of the morphological changes which take place in the nucleolus throughout the division cycle in root tip cells ofAllium cepa. The nucleolus appears to undergo no morphological changes throughout the interphase. It undergoes disorganization during the prophase, while in the telophase it appears uniformly on the chromatin as condensing into prenucleolar bodies.The appearance of the prenucleolar bodies is unaffected by puromycin, cordycepin, or ethidium bromide. This suggests that the argyrophilic material does not undergo synthesis during the telophase, nor require RNA or protein synthesis to effect the aggregation into prenucleolar bodies. However, the organization of nucleoli from prenucleolar bodies is inhibited by both cordycepin and ethidium bromide, suggesting that RNA synthesis is involved in this proccess.In aneuploid nuclei induced by treatment with colchicine we observed the appearance of prenucleolar bodies during the telophase even in the absence of the nucleolar organizer, but in this case the formation of nucleoli fails to take place. The nucleolar organizers proved to be capable of acting only in the nucleus to which they belong, but not on other nuclei within the same cytoplasm belonging to multinucleate cells.It seems logical to assume that one of the roles of the nucleolar organizer is related with the above-mentioned RNA synthesis, which is required to the aggregation of prenucleolar bodies into nucleoli.The work reported in the paper was undertaken during the tenure of a Research Training Fellowship awarded by the International Agency for Research on Cancer.     

LOCALIZATION OF RIBOSOMAL DNA WITHIN OOCYTES OF THE HOUSE CRICKET, ACHETA DOMESTICUS (ORTHOPTERA: GRYLLIDAE)

A large DNA-containing body is present in addition to the chromosomes in oocytes of the house cricket Acheta domesticus. Large masses of nucleolar material accumulate at the periphery of the DNA body during the diplotene stage of meiotic prophase I. RNA-DNA hybridization analysis demonstrates that the genes which code for 18S and 28S ribosomal RNA are amplified in the ovary. In situ hybridization indicates that the amplified genes are localized within the DNA body of early prophase cells. As the cells proceed through diplotene the DNA which hybridizes with ribosomal RNA is gradually incorporated into the developing nucleolar mass.     

Establishment of nucleolar deficient sublines of PtK2 (Potorous tridactylis) by ultraviolet laser microirradiation

One of the two nucleoli of tetraploid PtK2 WA cells in early prophase was irradiated with an ultraviolet (UV) laser microbeam. The daughter cells that maintained the nucleolar deficiency were isolated and cloned. Five nucleolar deficient sublines of PtK2 WA were established, thus providing an experimental system to study the ribosomal gene-nucleolar organizer complex.     

Pattern of Nucleolar Growth in Differentiating Cotton Fibres (Gossypium hirsutum L.)

The nucleolar size in cotton fibres has been examined at earlystages of growth by light microscopy. The volumes of the nucleoliwere computed from the perimeters measured by means of a projectionmicroscope. The frequency distributions were normal at eachstage. The mean nucleolar volume increased rapidly, reacheda maximum around 10 d after anthesis and declined to a constantvalue around 20 d after anthesis. The nucleolar growth patternsof some prediction models have been compared with the in situobservations. This comparison suggests a simultaneous increasein nucleolar size in all fibres within a population. Therefore,the hypothesis that the size of the fibres is correlated withthe size of resident nucleoli, is favoured. Gossypium hirsutum, cotton fibre, nucleolus, cell differentiation     

Amitosis and Endocytogenesis in the Fruit of Malus sylvestris

Cyto-histological investigations on the initiation and developmentof a non-pathogenic physiological ‘cork spot’ necrosisin the outer cortex of the fruit of Malus sylvestris Mill. ‘YorkImperial’ revealed two distinct aberrancies, namely, amitoticnuclear division and intracellular or endogenous proliferationswithout hyperplasia per se The incipient necrotic conditionbecomes evident internally about 3 weeks after fruit set asminute isolated, discoloured, amorphous spots of disorganizedruptured cells within otherwise healthy cortical tissue Thedisorganization continues slowly with adjoining cells becomingsimilarly necrotic After the lysis of the initial cork spotcells, about 1 month later, sporadic cellular changes occurin various healthy vacuolated cortical cells contiguous to andencompassing the necrotic tissues. The nuclei increase in sizeand volume as they assume distinctive positions in preparationfor amitotic nuclear divisions. The enlarged nucleus or macronucleus,containing one or several nucleoli, divides by a distinct cleavagedeveloping from a constriction perpendicular to its longitudinalaxis The division results in the amitotic formation of two daughtermicronuclei that usually become separated by the formation ofa cell wall. No evidence of cell plate formation was observedand the method of cell wall formation could not be determined.Repeated amitotic divisions of the micronuclei result in anintracellular or endocytogenetic proliferation of parenchymatouscells that are invariably confined within the original mothercell until its wall ruptures The endogenous proliferations arereleased into lacunae or intercellular spaces, eventually becomedisorganized, and disintegrate, with the accumulated residualincrements resulting in an overall ‘cork spot’ appearance. Malus sylvestris Mill., apple, amitosis, endocytogenesis, multinucleate cells, macronuclei, micronuclei     

Localization of histones and their synthesis by ammoniacal silver reaction in meristematic root tip cells of Allium cepa

Summary The ammoniacal silver reaction was used for localization of histones in meristematic root tip cells of Allium cepa. Electron microscopic observations showed that yellow or brown colour of interphase and prophase nuclei and brown nucleolar colour produced in the reaction coincides with the appearance of silver grains, about 400 Å in diameter, in the interphase and prophase chromatin and nucleoli. This together with the complete absence of staining reaction and silver grains in the cytoplasm could mean quite a specific reaction with histones and might suggest also that in these cells the site of histone synthesis is in the nucleolus.