Mitosis of the free-living flagellate Bodo saltans strain Ps+ (Kinetoplastidea, Bodonida)

Mitosis of the free-living flagellate Bodo saltans of the Ps+ strain characterized by the presence of prokaryotic cytobionts in the perinuclear space was studied. Division of B. saltans Ps+ nuclei occurs by the closed intranuclear type of mitosis without condensation of chromosomes. At the initial stages of nuclear division, consecutive anlage of two spatially separated microtubular spindles begins. The spindle containing about 20 microtubules appears first, then, at an angle of 30–40° to it, the second spindle containing half as many microtubules is formed. The microtubules of the first spindle are associated with 4 pairs of kinetochores, the microtubules of the second one—with 2 pairs. The kinetochores of B. saltans Ps+ have a pronounced laminar structure. Both spindles rest with their ends directly on the internal membrane of the nuclear envelope and form 4 well-pronounced poles. The equatorial phase of mitosis in B. saltans Ps+ is not revealed. The divergence of sister kinetochores towards the poles occurs independently in each spindle. At the elongation phase of mitosis, the poles of both spindles are united in pairs to form a single bipolar structure composed of two loose bundles of microtubules. At this stage of nuclear division, the kinetochores reach the poles of the subspindles and cease to be visible. At subsequent nuclear division stages the nucleus acquires a dumbbell shape. During the reorganization phase the sister nuclei are separated. In the perinuclear space of the interphase nuclei of B. saltans Ps+, 1–2 prokaryotic cytobionts are present. In the course of mitosis, these organisms divide intensively, such that their number can reach 20 and more per nucleus. During separation of sister nuclei, the “excessive” cytobionts are released into the cytoplasmic vacuoles formed by external membranes of the nuclear envelope.     

ENDOMITOSIS IN TAPETAL CELLS OF EREMURUS (LILIACEAE)

True endomitosis in the anther tapetum of the liliaceous plant Eremurus is described. The nuclear membrane does not disappear, but during metaphase the chromosomes are condensed, often considerably more than in normal mitosis. When the pollen mother cells (PMCs) go through the last premeiotic mitosis, the tapetal cells have one diploid nucleus which divides while the cell remains undivided. The two diploid nuclei may undergo an endomitosis and the resulting tetraploid nuclei a second endomitosis. An alternative pathway is an ordinary mitosis—again without cell division—instead of one of the endomitotic cycles. The cytological picture in the tapetum is further complicated by restitution in anaphase and fusion of metaphase and anaphase groups during mitosis, processes which could give rise to cells with one, two, or three nuclei, instead of the expected two or four. No sign of the so-called “inhibited” mitosis is seen in these tapetal cells. When the PMCs are in leptotene-zygotene, very few tapetal nuclei are in endomitosis. When the PMCs have reached diplotene, almost 100% of cells which are not in interphase show an endomitotic stage.     

The dissociation of nuclear and centrosomal division in gnu, a mutation causing giant nuclei in Drosophila

We describe a recessive, maternal-effect lethal mutation of Drosophila, gnu. gnu uncouples nuclear division from many cytoplasmic events of mitosis in the Drosophila embryo. Embryos from homozygous females are defective in nuclear division, but not in DNA replication, and therefore develop a small number of giant nuclei. Centrosomes divide independently of nuclear division and migrate to the surface of the syncytial blastoderm. There they nucleate microtubules into asters, which appear normal at first but become very large. Only later, when the giant nuclei begin to break down, are spindles sometimes formed. The cortical actin of these embryos develops into a characteristic network.     

MONOPLASTIDIC CELL DIVISION IN LOWER LAND PLANTS

In many bryophytes and vascular cryptogams mitosis and/or meiosis takes place in cells containing a single plastid. In monoplastidic cell division plastid polarity assures that nuclear and plastid division are infallibly coordinated. The two major components of plastid polarity are morphogenetic plastid migration and microtubule organization at the plastids. Before nuclear division the plastid migrates to a position intersecting the future division plane. This morphogenetic migration is a reliable marker of division polarity in cells with and without a preprophase band of microtubules (PPB). The PPB, which predicts the future division plane before mitosis, is a characteristic feature of land plants and its insertion into the cytokinetic apparatus marks the evolution of a cortical microtubule system and a commitment to meristematic growth. Microtubule systems associated with plastid division, the axial microtubule system (AMS) in mitosis and the quadripolar microtubule system (QMS) in meiosis, contribute to predictive positioning of plastids and participate directly in spindle ontogeny. Division polarity in monoplastidic sporocytes is remarkable in that division sites are selected prior to the two successive nuclear divisions of meiosis. Plastid arrangement prior to meiosis determines the future spore domains in monoplastidic sporocytes, whereas in polyplastidic sporocytes the spore nuclei play a major role in claiming cytoplasmic domains. It is hypothesized that predivision microtubule systems associated with monoplastidic cell division are early forming components of the mitotic apparatus that serve to orient the spindle and insure equal apportionment of nucleus and plastids. “Can it be supposed that cytoplasm would be intrusted with so important a task as the preparation of a chloroplast for each of the four nuclei that are later to preside over the spores before there is any indication that such nuclear division is to take place?” Bradley Moore Davis, 1899     

Formation and division of binucleated cells in kidney cell cultures of microtus agrestis

Summary Epithelial kidney cell cultures of Microtus agrestis contain 10 to 25% binucleated cells. Observations of living cells under the phase contrast microscope showed that binucleated cells can arise by nuclear mitosis without cytoplasmic division. When binucleated cells divide the two nuclei are highly synchronized as they enter mitosis. In mitosis the chromosomes of both nuclei combine to a common metaphase plate leading to polyploid cells. In one case a tripolar spindle was seen after formation of a metaphase by the chromosomes of the two nuclei of a binucleated cell. This tripolar mitosis resulted in one binucleated and one mononucleated cell. The DNA-content (Feulgen photometry) and the distribution of heterochromatic bodies of the nuclei were corresponding to a tetraploid, a triploid and a haploid chromosome set. This suggests the possibility of somatic segregation of complete haploid sets.Supported by the Deutsche Forschungsgemeinschaft.     

daughterless-abo-like, a Drosophila maternal-effect mutation that exhibits abnormal centrosome separation during the late blastoderm divisions

daughterless-abo-like (dal) is a maternal-effect semilethal mutation in Drosophila. The nuclear divisions of embryos derived from homozygous dal females are normal through nuclear cycle 10. However, during nuclear cycles 11, 12 and 13, a total of about half of the nuclei in each embryo either fail to divide or fuse with a neighboring nucleus during telophase. These abnormal nuclei eventually sink into the interior of the embryo, leaving their centrosomes behind on the surface. The loss of about one-half of the peripheral nuclei into the interior of the embryo results in these embryos cellularizing during nuclear cycle 14 with about one-half the normal number of cells. Surprisingly, many of these embryos develop a nearly normal larval cuticle and 8% develop to adulthood. Observations of live embryos doubly injected with tubulin and histones that have been fluorescently labeled allows nuclear and centrosomal behavior to be directly followed as the embryo develops. We find that the abnormal nuclei arise from nuclei whose centrosomes have failed to separate normally in the previous interphase. These incompletely separated centrosomes can cause a non-functional spindle to form, leading to a nuclear division failure. Alternatively, they can form an abnormal spindle with a centrosome from a neighboring nucleus, causing two nuclei to share a common spindle pole. Such nuclei with a shared centrosome will undergo telophase fusions, unequal divisions, or division failures later in mitosis. These findings have helped us to understand the function of the centrosome in the Drosophila embryo.     

Involvement of an “all-or-none event” in the triggering of chromosome segregation and of S phase in procaryotic and eucaryotic cells

A structural model of the triggering of mitosis is described. It is proposed that the number of structural effectors varies discontinuously both just before mitosis through an “all-or-none event” and during the mitotic process itself. The effector persists from mitosis to mitosis, but is not active in G2 due to the polymerization of soluble monomers. The “all-or-none event” which triggers mitosis is postulated to involve the doubling of the number of the structural effectors which are then temporarily in an active state, thus initiating mitosis. The subsequent segregation during mitosis of the active structural effectors to the two dividing nuclei allows the initiation of S phase as soon as the chromosomal replicative machinery is ready.     

An electron microscope study of nuclear and cell division in a dinoflagellate

Summary Light microscopical observations on the cell division of the small dinoflagellate Woloszynskia micra are correlated for the first time with an electron microscopical study. In prophase, whilst the nucleus enlarges and becomes pearshaped, the chromosomes divide to give pairs of chromatids. This process starts at one end and works to the other giving Y- and V-shaped chromosomes as it occurs. Cytoplasmic invaginations pass through the nucleus and by the end of prophase these are seen to contain a number of microtubules of about 180 Å diameter. There is no connection between the microtubules in the nuclear in vagination and either the flagellar bases or the chromosomes. At anaphase the nucleus expands laterally and the sister chromatids move towards opposite ends. The cell hypocone is now partially divided and the two longitudinal flagella well separate. The nucleus completes its division into two daughter nuclei and for a time portions of the cytoplasmic invaginations remain visible. Cell cleavage is completed by the division of the epicone. The nuclear membrane remains intact throughout division and the nucleolus does not break down.The mitotic division in this organism, which is unusual in comparison with the mitosis of higher organisms, is discussed in the light of other types of mitosis which have been reported and of earlier light microscopical observations on dinoflagellates.     

KARYOLOGY DURING CONIDIOGENESIS IN GLIOMASTIX MURORUM: LIGHT MICROSCOPY

Light microscopic observations of nuclear behavior (karyology) during conidiogenesis in the long, narrow phialides of Gliomastix murorum (Corda) Hughes are presented and discussed. Nuclei were observed mostly in a submedian position in phialides. The onset of mitosis was signalled by an increase in the size of nuclei and by the appearance of numerous chromatinic granules (chromosomes?). The number of chromatinic granules appeared to decrease, while the chromatin was arranged in the characteristic “double track” associated with somatic nuclear divisions in hyphomycetes. Transverse separation of the “double track” arrangement produced two daughter chromatinic masses which stained intensely, were small, and moved apart. Separation of daughter chromatinic masses (nuclei) appeared to be largely a function of migration of distal daughter nuclei several micrometers toward phialidic apices; the submedian position of proximal daughter nuclei was maintained. Upon movement of migrating distal daughter nuclei into conidial initials, conidia were delimited septally. Conidial nuclei remained condensed, while daughter nuclei remaining in phialides decondensed (i.e., enlarged and stained less intensely), thereby entering interphase. Repeated, single nuclear divisions and migrations were correlated with repeated conidial development. Karyological events described herein were compared with other published studies of both phialidic and non-phialidic species, and a “phialidic pattern” of nuclear behavior was suggested as a possibility. But, apparently a non-phialidic pattern cannot yet be suggested.     

Formation of a micronuclei-like structure induced by colchicine treatment in Saccharomyces cerevisiae

Minute nuclei named “smaller nuclei” were generated when the cells of Saccharomyces cerevisiae were treated with colchicine. The formation of “smaller nuclei” seemed to be related to nuclear division because those nuclei were only produced under conditions suitable for nuclear division. The fact that the average DNA content of “smaller nuclei” was almost one tenth of that of the isolated normal diploid nuclei showed that the “smaller nuclei” are not condensed nuclei but aneuploid nuclei like micronuclei in animal cells. It appeared therefore likely that a micronuclei-like structure could be produced by colchicine treatment in S. cerevisiae.     

ULTRASTRUCTURE OF MITOSIS AND CYTOKINESIS IN THE MULTINUCLEATE GREEN ALGA ACROSIPHONIA

The processes of mitosis and cytokinesis in the multinucleate green alga Acrosiphonia have been examined in the light and electron microscopes. The course of events in division includes thickening of the chloroplast and migration of numerous nuclei and other cytoplasmic incusions to form a band in which mitosis occurs, while other nuclei in the same cell but not in the band do not divide. Centrioles and microtubules are associated with migrated and dividing nuclei but not with nonmigrated, nondividing nuclei. Cytokinesis is accomplished in the region of the band, by means of an annular furrow which is preceded by a hoop of microtubules. No other microtubules are associated with the furrow. Characteristics of nuclear and cell division in Acrosiphonia are compared with those of other multinucleate cells and with those of other green algae.     

Mitosis in the free-living flagellate Bodo saltans strain PS+ (Kinetoplastidea, Bodonida)

The mitosis in the free-living flagellate Bodo saltans Ps+ with prokaryotic cytobionts in perinuclear space has been studied. The nuclear division in B. saltans Ps+ occurs by closed mitosis type without condensation of chromosomes. Two spatially separated mitotic spindles begin to form consistently at the initial stages of nuclear division. The spindle including about 20 microtubules appears first and later the second spindle with half the number of microtubules comes at the angle of 30-40 degrees. Both spindles rest their ends against the inner nuclear membrane and form 4 distinct poles. The microtubules of the first spindle are associated with 4 pairs of kinetochores, the microtubules of the second one are associated with 2 pairs of kinetochores. The divergence of the kinetochores towards the poles occurs independently in each spindle. The equatorial phase is not revealed in B. saltans Ps+. The poles of both spindles unite in pairs at the elongation phase of mitosis and form the integrated bipolar structure. At this stage of the nuclear division, the kinetochores reach the poles of subspindles and become indistinguishable. Then the nucleus takes the shape of a dumbbell. The inner nuclear membranes of just formed nuclei have layers of condensed chromatin characteristic of the interphase nuclei of kinetoplastidea. The daughter nuclei separate at the phase of reorganization. There are 1-2 prokaryotic endocytobionts in the perinuclear space of the interphase nuclei in B. saltans Ps+. The symbionts multiply during mitosis and their number reaches more than 20 specimens par nucleus.     

A NOVEL PATTERN OF APICAL CELL POLYPLOIDY,SEQUENTIAL POLYPLOIDY REDUCTION AND INTERCELLULAR NUCLEAR TRANSFER IN THE RED ALGA POLYSIPHONIA

Nearly a century ago, Rosenvinge published a now-classic paper reporting nuclear transfer between cells of Polysiphonia during secondary pit connection (SPC) formation. While reinvestigating this phenomenon, we discovered that the uninucleate apical cell, which is the progenitor of all cells in the plant, has many times (ca. 64–128 ×) the level of nuclear DNA characteristic of nuclei of gametes or mature pericentral cells. Via a regular sequence of cell divisions, the polyploid apical cell gives rise to tiers of cells, each composed of a number of pericentral cells which surround a single central cell. A large proportion of the nuclear divisions are not accompanied by DNA replication. Thus, as the number of nuclei within elongating pericentral cells increases, the DNA level of nuclei in these cells “cascades” down to the DNA level expected for the particular life history generation (i.e., gametophyte or tetrasporophyte). In mature pericentral cells, the number of nuclei is proportional to the volume of the cell. The pattern of nuclear division, reduction in ploidy level and the timing of intercellular nuclear transfer via SPC formation is regular and characteristic of a species. Nuclei transferred from one cell to an adjacent cell participate in the further nuclear divisions of the recipient cell. The degree of polyploidy in apical cells may determine the number of cells in a “determinant” branch or even the number of cells in “indeterminant” axes. In addition, the highly polyploid state of the germinating spore and its pattern of development may provide for the rapid initial growth so characteristic of this taxon.     

How nuclei of Giardia pass through cell differentiation: semi-open mitosis followed by nuclear interconnection

Differentiation into infectious cysts (encystation) and multiplication of pathogenic trophozoites after hatching from the cyst (excystation) are fundamental processes in the life cycle of the human intestinal parasite Giardia intestinalis. During encystation, a bi-nucleated trophozoite transforms to a dormant tetra-nucleated cyst enveloped by a protective cyst wall. Nuclear division during encystation is not followed by cytokinesis. In contrast to the well-studied mechanism of cyst wall formation, information on nuclei behavior is incomplete and basic cytological data are lacking. Here we present evidence that (1) the nuclei divide by semi-open mitosis during early encystment; (2) the daughter nuclei coming from different parent nuclei are always arranged in pairs; (3) in both pairs, the nuclei are interconnected via bridges formed by fusion of their nuclear envelopes; (4) each interconnected nuclear pair is associated with one basal body tetrad of the undivided diplomonad mastigont; and (5) the interconnection between nuclei persists through the cyst stage being a characteristic feature of encysted Giardia. Based on the presented results, a model of nuclei behavior during Giardia differentiation is proposed.     

ULTRASTRUCTURE OF MEIOSIS IN DASYA BAILLOUVIANA (RHODOPHYTA). II. PROMETAPHASE I—TELOPHASE II AND POST-DIVISION NUCLEAR BEHAVIOR1

This is the second of two papers which together are the first comprehensive ultrastructural report of meiosis in a red alga. Many details of the meiotic process in Dasya baillouviana (Gmelin) Montagne are the same as those reported previously for mitotic cells in ceramialian red algae, but several characteristics seem unique to meiotic cells. The nucleus and nucleolus of meiotic cells are larger than those of mitotic cells and large accumulations of smooth ER are often found at the division poles during meiosis 1. The function of the ER accumulations is unknown. Importantly, both interkinesis and a simultaneous division of two separate nuclei during meiosis II was demonstrated. These new observations fail to support earlier speculation on higher red algae for a “uninuclear” meiosis (both nuclear divisions within the same nuclear envelope). However, following meiosis II the four nuclei migrate centripetally and possibly fuse in the center of the tetrasporangium. This post-division nuclear maneuvering is not understood, but our interpretation accounts for the earlier and erroneous impression of “uninuclear” meiosis. Perhaps the most important aspect of meiosis observed in Dasya is its basic adherence to the pattern commonly seen in higher plants and animals. This conservatism of the meiotic process lends further skepticism to the belief that red algae are extremely “primitive” organisms, although they undoubtedly represent a very “ancient” group of eukaryotic plants.     

Bicarbonate enhances synchronous division of the giant nuclei of sporophytes in<Emphasis Type="Italic"> Bryopsis plumosa</Emphasis>

A mature sporophyte of Bryopsis plumosa (Hudson) C. Agardh forms a huge number of zoospores in its cell continuum. Zoospore formation starts with the division of a single giant nucleus and subsequent repeated mitosis. We found that an elevation of photosynthetic activity triggered the division of a mature giant nucleus. Transfer to short-day conditions was not necessary. Giant nuclei did not divide in darkness or in the presence of 1 μM DCMU. Giant nuclei of as many as 90% of sporophtyes started to divide following the addition of 5 mM NaHCO₃ to the growth medium under continuous white light (6–12 W m⁻²). Frequency of nuclear division increased with increased light intensity. By combining those parameters that promoted the division of giant nuclei, we developed the "two-step culture method" which is composed of preliminary and main cultures. This new method guarantees that giant nuclei of more than 90% of all sporophytes synchronously divide between 72 and 96 h after the transfer to the main culture (continuous white light of 12 W m⁻² in PES medium supplemented with 5 mM NaHCO₃).     

Macroautophagy is repressed during mitosis – seeing is believing

ABSTRACT

For the last two decades there has been wide ranging debate about the status of macroautophagy during mitosis. Because metazoan cells undergo an “open” mitosis in which the nuclear envelope breaks down, it has been proposed that macroautophagy must be inhibited to maintain genome integrity. While many studies have agreed that the number of autophagosomes is greatly reduced in cells undergoing mitosis, there has been no consensus on whether this reflects decreased autophagosome synthesis or increased autophagosome degradation. Reviewing the literature we were concerned that many studies relied too heavily on autophagy assays that were simply not appropriate for a relatively brief event such as mitosis. Using highly dynamic omegasome markers we have recently shown unequivocally that autophagosome synthesis is repressed at the onset of mitosis and is restored once cell division is complete. This is accomplished by CDK1, the master regulator of mitosis, taking over the function of MTORC1, to ensure autophagy is repressed during mitosis.     

Time-course analysis of nuclear events during conjugation in the marine ciliate Euplotes vannus and comparison with other ciliates (Protozoa,Ciliophora)

Ciliates represent a morphologically and genetically distinct group of single-celled eukaryotes that segregate germline and somatic functions into two types of nuclei and exhibit complex cytogenetic events during the sexual process of conjugation, which is under the control of the so-called “mating type systems”. Studying conjugation in ciliates may provide insight into our understanding of the origins and evolution of sex and fertilization. In the present work, we studied in detail the sexual process of conjugation using the model species Euplotes vannus, and compared these nuclear events with those occurring in other ciliates. Our results indicate that in E. vannus: 1) conjugation requires about 75 hours to complete: the longest step is the development of the new macronucleus (ca. 64h), followed by the nuclear division of meiosis I (5h); the mitotic divisions usually take only 2h; 2) there are three prezygotic divisions (mitosis and meiosis I and II), and two of the eight resulting nuclei become pronuclei; 3) after the exchange and fusion of the pronuclei, two postzygotic divisions occur; two of the four products differentiate into the new micronucleus and macronucleus, respectively, and the parental macronucleus degenerates completely; 4) comparison of the nuclear events during conjugation in different ciliates reveals that there are generally three prezygotic divisions while the number of postzygotic divisions is highly variable. These results can serve as reference to investigate the mating type system operating in this species and to analyze genes involved in the different steps of the sexual process.     

Dynamics of the nuclear envelope and of nuclear pore complexes during mitosis in the Drosophila embryo

Early embryonic development in Drosophila melanogaster is marked by a series of thirteen very rapid (10-15 min) and highly synchronous nuclear divisions, the last four of which occur just beneath the embryo surface. A total of some 6000 blastoderm nuclei result, which are subsequently enclosed by furrow membranes to form the cellular blastoderm. We have examined the fine structure of nuclear division in late syncytial embryos. The mitotic spindle forms adjacent to the nuclear envelope on the side facing the embryo surface. During prophase, astral microtubules deform the nuclear envelope which then ruptures at the poles at the onset of prometaphase. The nuclear envelope remains essentially intact elsewhere throughout mitosis. A second envelope begins to form around the nuclear envelope in prometaphase and is completed by metaphase; the entire double layered structure, referred to as the spindle envelope, persists through early in the ensuing interphase. Pole cell spindles are enclosed by identical spindle envelopes. Interphase and prophase nuclei contain nuclear pore complexes (PCs) of standard dimensions and morphology. In prometaphase PCs become much less electron-dense, although they retain their former size and shape. By metaphase, no semblance of PC structure remains, and instead, both layers of the spindle envelope are interrupted by numerous irregular fenestrae. PCs are presumably disassembled into their component parts during mitosis, and reassembled subsequently. Yolk nuclei remain among the central yolk mass when most nuclei migrate to the surface, cease to divide, yet become polyploid. These nuclei nonetheless lose and regain PCs in synchrony with the dividing blastoderm nuclei. In addition, they gain and lose a second fenestrated membrane layer with the same timing. Cytoplasmic membranes containing PCs (annulate lamellae) also lose and regain pores in synchrony with the two classes of nuclear envelopes. The factors that affect the integrity of PCs in dividing blastoderm nuclei appear to affect those in other membrane systems to an equivalent degree and with identical timing.     

The ultrastructure of mitosis in myxamoebae and plasmodia of Physarum flavicomum

Electron microscopy of glutaraldehyde-osmium-fixed samples of haploid myxamoebae and diploid plasmodia of the myxomycete Physarum flavicomum Berk. reveal dissimilar spindle apparatus during mitosis in the two cell types. Myxamoebae exhibit an astral type of mitosis with centrioles at the poles and nuclear envelope breakdown during prophase. Plasmodial nuclei lack centrioles at mitosis and have an intranuclear spindle, with nuclear envelope persisting during the entire division. Coated vesicles are noted during prophase and telophase in myxamoebae and their role in spindle formation and dispersion is suggested.