Supernumerary chromosomes and their behaviour in meiosis of the holocentric Cuscuta babylonica Ghoisy

A supernumerary chromosome and its behaviour during meiosis is described in the parasitic herb Cuscuta babylonica Choisy (2n = 8 + 1B). This species is characterized by holocentric chromosome behaviour, and the occurrence of a supernumerary here is considered noteworthy.     

Cytokinesis in plant male meiosis

In somatic cell division, cytokinesis is the final step of the cell cycle and physically divides the mother cytoplasm into two daughter cells. In the meiotic cell division, however, pollen mother cells (PMCs) undergo two successive nuclear divisions without an intervening S-phase and consequently generate four haploid daughter nuclei out of one parental cell. In line with this, the physical separation of meiotic nuclei does not follow the conventional cytokinesis pathway, but instead is mediated by alternative processes, including polar-based phragmoplast outgrowth and RMA-mediated cell wall positioning. In this review, we outline the different cytological mechanisms of cell plate formation operating in different types of PMCs and additionally focus on some important features associated with male meiotic cytokinesis, including cytoskeletal dynamics and callose deposition. We also provide an up-to-date overview of the main molecular actors involved in PMC wall formation and additionally highlight some recent advances on the effect of cold stress on meiotic cytokinesis in plants.     

Polyarchal spindles in higher plant meiosis

Morphogenesis and functioning of polyarchal spindles in higher plant meiosis were studied. A mechanism of anastral spindle formation in discussed.     

Alterations in the baseline sister-chromatid exchange frequency in human lymphocyte culture following a number of cell divisions

A significant decrease in the baseline of sister-chromatid exchanges (SCEs) was observed in cultured human lymphocytes, if 5-bromodeoxyuridine (BrdU) was added after 60 h of culture, and the cells were harvested at least 24–30 h after BrdU exposure. This decrease is supposed to occur if at least one cell division takes place before the addition of BrdU. For cytogenetic monitoring of mutagenic environmental factors, using human lymphocyte cultures, it is assumed that two time periods are sufficient for comparison.     

The ultrastructure of an intraspindle membrane system in meiosis of spider spermatocytes

An extensive system of membranes was found in the spindles of spermatocytes of two wolf spider species, Lycosa georgicola and L. rabida. Serial section reconstructions of this membrane system revealed that each meiotic bivalent is encased in a tube of membrane, which encloses both kinetochore microtubule bundles and approaches to within a few microns of the centriolar complex. The membrane tube is open at the polar ends. The membrane composing the tube is doubled and resembles smooth endoplasmic reticulum (ER). No evidence of nuclear pore complexes has been found in the intraspindle membrane system, but typical pores are present on the nuclear envelope of prophase cells. The membrane tubes are fenestrated and microtubules sometimes penetrate these fenestrae. Besides its possible function in the regulation of chromosome movement, the intraspindle membrane system may participate in the nonrandom segregation of the sex chromosomes at meiosis in these spiders.     

Hypothesis: cell volume limits cell divisions

Mammalian somatic cells and also cells of the yeast Saccharomyces cerevisiae are capable of undergoing a limited number of divisions. Reaching the division limit is referred to, apparently not very fortunately, as replicative aging. A common feature of S. cerevisiae cells and fibroblasts approaching the limit of cell divisions in vitro is attaining giant volumes. In yeast cells this phenomenon is an inevitable consequence of budding so it is not causally related to aging. Therefore, reaching a critically large cell volume may underlie the limit of cell divisions. A similar phenomenon may limit the number of cell divisions of cultured mammalian cells. The term replicative (generative) aging may be therefore illegitimate.     

Mutant genes affecting higher plant meiosis

Summary That meiosis is conditioned by a large number of genes majority of which are present in a dominant state, is evidenced by the detection of numerous monogenic recessive mutant genes which affect the premeiotic, meiotic and post-meiotic course of events. These genes are site- and stage-specific, and a few are sex specific. Of these, the most prevalent are the mutant genes affecting male meiosis and causing male sterility (ms genes) and those inhibiting synapsis and chiasma formation (synaptic genes) and leading to gametic sterility. Majority of the mutant genes affect the entire chromosomal complement but a few influence only specific chromosomes of a complement so that the chromosomes behave differentially within a genome of the same species. Some mutant genes alter chromosome form and function, others modify integrity, degree of spiralization, movement and migration of chromosomes. Their cytogenetic behaviour, genetic significance and breeding utility are described and discussed.Dedicated to Professor Dr. Werner Gottschalk, Director, Institute of Genetics, University of Bonn, Federal Republic of Germany, on his 65th birthday for his excellent scientific contribution and humane nature     

Synchronous cell divisions in Anacystis nidulans Richter

Summary The blue-green alga, Anacystis nidulans was successfully synchronized in continuous light (15,000 lux) by a 14 hr cycle, consisting of 8 hrs at 26°C, and 6 hrs at 32°C, coupled with periodic dilution of the algal suspension to a constant cell number at the end of each cycle. The alga continues to grow at the lower temperature, and a division burst begins 2 hrs after transfer to the higher temperature.

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Zusammenfassung Die Cyanophycee Anacystis nidulans wurde in Dauerlicht von 15 000 Lux vollständig synchronisiert. Die Cyclen bestehen aus 8 Std bei 26° C und 6 Std bei 32° C, an ihrem Ende wurde jeweils mit frischer Nährlösung auf die Ausgangszellzahl von 3·10⁷ Zellen/ml verdünnt. Bei beiden Temperaturen kann sich die Alge vermehren, unter den synchronisierenden Bedingungen des Temperaturwechsels beginnt der Teilungsschub (Verdopplung der Zellzahl) bei 32° C nach 2 Std.

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Pringsheim (1968) renamed this alga as Lauterbornia nidulans.     

Orientation of vascular cell divisions in Arabidopsis

Orientation of cell division is essential for plant development as the direction of growth is determined by the direction of cell expansion and orientation of cell division. We have demonstrated that cell division orientation in vascular tissue is regulated by the interactions between a receptor kinase (PXY) expressed in dividing cells and its peptide ligand (CLE41) that is localized to adjacent phloem cells. Given that other receptor kinases have been identified as orienting the cell division plane in several developmental processes, we suggest that localized signaling from adjacent cells may be a general mechanism for defining the plane of cell division.Key words: xylem, phloem, cell division orientation, procambium, cambiumThroughout the life of plants, new organs are generated from meristems which contain stem cells at their center. Meristematic cells divide in regulated processes resulting in displacement of daughter cells to the periphery of the meristem where they differentiate, taking on new cell identities.¹ Vascular meristems (cambium and procambium) are responsible for radial growth and are the main source of plant biomass.² Their regulation has a come under increasing scrutiny as biomass is likely to play an increasing role in generation of renewable energy.³Arabidopsis vascular tissue is organized into discrete collateral bundles in stems,⁴ whereas in hypocotyls, vasculature forms in a continuous ring, much like that of trees.⁵ In both cases spatially separated xylem and phloem are formed along the stem mediolateral axis and are populated with cells derived from the procambium or cambium (Fig. 1). Vascular initials displaced from the meristematic zone towards the center of the stem differentiate into xylem whereas those displaced towards the outside of the stem differentiate into phloem. This organization occurs because vascular meristematic cells are long and thin and divide periclinally down their long axis, perpendicular to the mediolateral axis. Because these are highly ordered divisions, vascular tissue is characterized by long files of cells. Until recently regulatory factors which influence the highly ordered nature of these divisions—and therefore plant vascular tissue organization were entirely unknown.Open in a separate windowFigure 1Arabidopsis vascular tissue at the base of inflorescence stems (A) and hypocotyls (B). The mediolateral axes are marked with arrows, x is xylem, ph is phloem, pc is procambium, c is cambium. Scale bars are 50 µm.     

Oriented cell divisions in the extending germband of Drosophila

Tissue elongation is a general feature of morphogenesis. One example is the extension of the germband, which occurs during early embryogenesis in Drosophila. In the anterior part of the embryo, elongation follows from a process of cell intercalation. In this study, we follow cell behaviour at the posterior of the extending germband. We find that, in this region, cell divisions are mostly oriented longitudinally during the fast phase of elongation. Inhibiting cell divisions prevents longitudinal deformation of the posterior region and leads to an overall reduction in the rate and extent of elongation. Thus, as in zebrafish embryos, cell intercalation and oriented cell division together contribute to tissue elongation. We also show that the proportion of longitudinal divisions is reduced when segmental patterning is compromised, as, for example, in even skipped (eve) mutants. Because polarised cell intercalation at the anterior germband also requires segmental patterning, a common polarising cue might be used for both processes. Even though, in fish embryos, both mechanisms require the classical planar cell polarity (PCP) pathway, germband extension and oriented cell divisions proceed normally in embryos lacking dishevelled (dsh), a key component of the PCP pathway. An alternative means of planar polarisation must therefore be at work in the embryonic epidermis.     

The ndhF chloroplast gene detected in all vascular plant divisions

A 335-bp segment of the NADH dehydrogenase F (ndhF) gene from a representative of each nonflowering vascular plant division (Coniferophyta, Filicophyta, Ginkgophyta, Gnetophyta, Lycophyta, Psilophyta, Sphenophyta) has been sequenced and aligned with those of rice, tobacco, an orchid and a liverwort. Because ndhF is apparently absent in the genus Pinus L. (Coniferophyta), it has been speculated that this gene may be absent in the gymnosperms. However, this study suggests that the absence of the ndhF gene in Pinus may be unique and is not a general characteristic of the gymnosperms.Abbreviations ndhF NADH dehydrogenase F - PCR polymerase chain reactionThis research was supported by grants from NSF LASER/EPS-CoR 92-96-ADP-02, LEGSF RD-A-13 (1991–1994), and by the Department of Plant Biology, Louisiana State University.     

Cortical development and asymmetric cell divisions

The development of the mammalian neocortex involves rounds of symmetric and asymmetric cell division of neural progenitors to fulfill needs of both self-renewal of progenitors and production of differe...     

Oriented cell divisions and muscle satellite cell heterogeneity

Satellite cells are crucial for maintaining muscle homeostasis and for regeneration following injury. In this issue, Kuang et al. (2007) reveal that muscle satellite cells are a heterogeneous mixture of stem cells and committed myogenic progenitors. They show that asymmetric division of stem cells in the satellite cell niche is a mechanism for generating these two populations.     

中枢神经系统发育的不对称细胞分裂机制

无论在无脊椎动物还是脊椎动物中,组成中枢神经系统（CNS）的大多数细胞都是由极性神经祖细胞不对称分裂而来。通过简要综述果蝇（Drosophila melanogaste）成神经母细胞（NB）不对称分裂机制,并与近年来在脊椎动物不对称细胞分裂上取得的研究成果相比较,尝试找出两个系统的相似性和相异性。     

Microtubule patterns during meiosis in two higher plant species

Summary A monoclonal antibody to higher plant tubulin was used to trace microtubule (MT) structures by immunofluorescence throughout mitosis and meiosis in two angiosperms,Lycopersicon esculentum andOrnithogalum virens. Root tip cells showed stage specific MT patterns typical of higher plant cells. These included parallel cortical interphase arrays oriented perpendicular to the long axis of the cell, preprophase band MTs in late interphase through prophase, barrelshaped spindles, and finally phragmoplasts. Pollen mother cell divisions exhibited randomly oriented cortical MT arrays in prophase I, pointed spindles during karyokinesis, and elongate phragmoplasts. A preprophase band was not observed in either meiotic division. MT initiation sites were seen as broad zones associated with the nuclear envelope.