Invvestigations on Early Embryogenesis of Actinidia chinensis Planch var. chinensis

This paper deals with early embryogenesis of Actinidia chinensis var. chinensis. 1. Ovary superior consists of 34—45 carpels. Each carpel contains 11–45 ovules. The ovule is uni-integument and tenuinucellar. The ovule is anatropous. The archesporium is formed by a single cell, and directly develops into megaspore mother cell. Sometimes the archesporium consists of 2–3 cells, but only one of them develops into megaspore mother cell and the others are degenerated. 2. The mature pollen grain is two-celled and the embryo sac belongs to olygonum type. In most embryo sacs two polar nuclei are fused before fertilization. One of the synergids was destroyed as the pollen tube penetrated into embryo sac the other one disappeared after fertilization. In most cases the antipodal cells became degenerated in fertilization process, only some remained until the first division of primary endosperm nucleus. 3. In Beijing area the double fertilization of Actinidia chinensis occurred 30–72 hours after pollination. In the fertilization one sperm fused with egg nucleus and the other sperm fused with the secondary nucleus as usual. The fusion of the secondary nucleus with sperm was in advance of the fusion of the egg nudeus. 4. The endosperm is cellular type.     

Early cytological events after induction of cell division in egg cells and zygote development following in vitro fertilization with angiosperm gametes

Early events, such as formation of the cell wall, first nuclear division and first unequal division of the zygote, were examined following in vitro fusion of single egg and sperm protoplasts of maize ( Zea mays L.). The time course of these events was determined. The formation of cell wall components was observed 30 sec following egg—sperm fusion and proceeded continuously thereafter. Within 15 h after fusion most of the organelles became more densely grouped around the nucleus of the zygote. In the in vitro produced zygote the location of the cell organelles and of the dividing nucleus showed polarity. Two nucleoli were first observed 18 h after gamete fusion. The zygotic nucleus remained undivided for about 40 h. The first cell division was observed 40–60 h, generally 42–46 h, after egg—sperm fusion. The non-fused egg cell could be triggered to sporophytic development in vitro by pulses of high amounts of 2,4-D. Without such a treatment, cultured egg cells of different maize lines did not divide. Although nuclear fusion seemed to occur, fusion products of two egg cells also did not divide. Cell wall formation was incomplete and non-uniform, showing a polarity of cultured egg cells and fusion products of two egg protoplasts. Cell division was also induced after fusion of maize egg with sperms of genetically remote species, such as Coix, Sorghum, Hordeum or Triticum . These gametic heterologous fusion products developed to microcalli. Moreover, cell division occurred in fusion products of an egg and a diploid somatic cell-suspension protoplast from maize.     

AUTOGAMOUS AUXOSPORULATION IN PINNULARIA NODOSA (BACILLARIOPHYCEAE)1

Auxosporulation of the freshwater epipelic diatom Pinnularia nodosa (Ehrenb.) W. Sm. was studied in a clonal culture. Interphase cells possessed two chloroplasts with invaginated pyrenoids. The nucleus contained a single small body of heterochromatin at one end, also visible during most of meiotic prophase. During auxosporulation, induced by transfer of stationary‐phase cells to fresh medium and suppressed by high nitrogen (N), an unpaired mother cell produced a single auxospore. Although meiosis II and nuclear fusion were not observed, indirect evidence indicated that auxosporulation was autogamous (rarely reported in pennate diatoms), rather than apomictic; paedogamy was excluded. The protoplast produced after meiosis either (1) matured into a “pseudozygote,” via an asymmetrical contraction after meiosis I to form a single spherical cell at one end of the mother cell (pathway 1); or (2) constricted into two spherical cells (pathway 2). In pathway 2, the “pseudogametes” never fused and only one or none developed into a pseudozygote and then into an auxospore. Pathway 2 could be suppressed by continuous light. During metamorphosis of the spherical pseudozygote into an elongate young auxospore, a complete covering of thin siliceous incunabular strips was formed, separate from the organic wall formed around the pseudozygote when first formed and from the perizonium. Mature auxospores produced via pathway 2 had 60% of the volume as those produced via pathway 1 and had smaller chloroplasts (through loss of fragments during protoplast cleavage), but they achieved exactly the same lengths, suggesting that absolute length is monitored during expansion.     

Two male-sterile mutants of Zea Mays (Poaceae) with an extra cell division in the anther wall

Two recessive male-sterile mutants of maize with similar patterns of pollen abortion were studied. Genetic studies showed that one of the two mutations was allelic with a previously identified male-sterility locus (ms23) and the other mutation was in a newly identified male-sterility locus (ms32). Cytological characterization of homozygous mutants and fertile heterozygous control siblings was performed using brightfield, fluorescence, and electron microscopy. During normal anther development, the final anther wall periclinal division divides the secondary parietal anther wall layer into the middle layer and tapetum, forming an anther with four wall layers. This is followed by differentiation of the tapetal cells into protoplastic binucleate, secretory tissue. In both the ms23 and ms32 mutants, the prospective tapetal layer divided into two layers, termed t1 and t2, forming an anther with five wall layers. Neither the t1 nor the t2 layers differentiated normally into tapetal layers, as determined by examination of cell walls, nucleus number, and cytoplasmic organization. Pollen mother cells aborted after the onset of prophase I of meiosis, suggesting that an early developmental coordination may exist between tapetum and pollen mother cells.     

Ontogenesis of Tannin Coenocytes in Sambucus racemosa L. II. Mother Tannin Cells

Tannin coenocytes develop from mononucleate tannin mother cells.The process occurs within the whole of the first (youngest)internode and its development can be divided into three stages.In stage I the MTC is isodiametric and similar to the surroundingcells of the flank meristem, being present in the ninth celllayer from the apex surface. The nucleus becomes lanceolateand elongates, and large cytoplasmic vacuoles appear. A twofoldelongation of both the cell and nucleus continues in the secondstage, the cell-nucleus ratio indicating that it is due to theenlarged vacuole, which pushes a thin layer of cytoplasm closeto the cell wall. In this layer of cytoplasm dilated ER cisternaoccur together with small and large vacuoles, a fusion of thevacuoles increasing their volume. Such cells are diploid inspite of larger nuclear volume and rough structure of its chromatin. Sambucus racemosa L., tannin cells, development, ontogenesis     

Embryological Studies on Sagittaria guayanensis H. B. K. Subsp. lappula (D. Don) Bojin (Alismataceae)

This paper deals with the embryological characteristics of Sagittaria guayanensis H. B.K. subsp. lappula (D. Don) Bojin. The anther wall development follows the Monocotyledonous type. The cytokinesis of microspore mother cell in meiosis is of the Successive type. The tetrads of microspores show an isobilateral arrangement, and the mature pollen grains are 3-celled. The ovule is bitegminous, pseudo-crassinucellate and anatropous. The megaspore mother cell originates directly from a single archesporial cell. The mature embryo sac consists of 7 cells including 8 nuclei and conforms to the Allium type. The two polar nuclei do not fuse into a secondary nucleus before fertilization. Instead, one sperm fuses with the micropylar end polar nucleus first , and the fertilized polar nucleus then migrates to the chalazal end, where it fuses with the second polar nucleus, forming the primary endosperm nucleus. The embryo development conforms to the Caryophyllad type. The mature embryo is U-shaped and forms the embryonic shoot apex accompanied by two leaves. The endosperm development corresponds to the Helobial type. The primary endosperm nucleus (invariably lying in the chalazal part of the embryo sac) divides and forms two chambers:large micropylar one and small chalazal one. The chalazal endosperm chamber remains binucleate, while, in the micropylar chamber free nuclear divisions occur and then cellnlarization takes place. During the embryo formation the endosperm gradually degrades and can not be found in the mature seed. The subgenus Lophotocarpus is different from the subgenus Sagittaria in some embryological aspects, especially in the structure of mature embryo sac and the double fertilization process.     

冠果草的胚胎学研究

冠果草花药壁的发育为单子口十型,绒毡层为周原质团型。小孢子母细胞减数分裂为连续型,四分体呈左右对称式排列,成熟花粉为三细胞型。双珠被,假厚珠心,倒生胚珠。胚囊发育为葱型,成熟胚囊的特点是两个极核分别位于中央细胞两端,不融合成次生核。受精过程中,一个精于与卵核融合形成合子,另一精子先与珠孔端极核融合,之后受精极核再移动到合点端与另一极核融合,形成初生胚乳核。胚的发育为石竹型。成熟胚呈马蹄形,具有2片真叶。胚乳发育为沼生目型。随着胚的发育,胚乳细胞逐渐解体,成熟种子中无胚乳。     

Ultrastructure of stomatal development in Arabidopsis (Brassicaceae) leaves

Stomatal development was studied in wild-type Arabidopsis leaves using light and electron microscopy. Development involves three successive types of stomatal precursor cells: meristemoid mother cells, meristemoids, and guard mother cells (GMCs). The first two types divide asymmetrically, whereas GMCs divide symmetrically. Analysis of cell wall patterns indicates that meristemoids can divide asymmetrically a variable number of times. Before meristemoid division, the nucleus and a preprophase band of microtubules become located on one side of the cell, and the vacuole on the other. Meristemoids are often triangular in shape and have evenly thickened walls. GMCs can be detected by their roughly oval shape, increased starch accumulation, and wall thickenings on opposite ends of the cells. Because these features are also found in developing stomata, stomatal differentiation begins in GMCs. The wall thickenings mark the division site in the GMC since they overlie a preprophase band of microtubules and occur where the cell plate fuses with the parent cell wall. Stomatal differentiation in Arabidopsis resembles that of other genera with kidney-shaped guard cells. This identification of stages in stomatal development in wild-type Arabidopsis provides a foundation for the analysis of relevant genes and of mutants defective in stomatal patterning, cell specification, and differentiation.     

Electron microscopy of sporulation inSchwanniomyces alluvius

Sporulation inSchwanniomyces alluvius appeared to be preceded by fusion of a mother and a daughter cell. Meiosis probably occurred in the mother cell and one or two spores were formed in the latter. A study of thin sections showed that the spore wall developed from a prospore wall. The mature spore wall consisted of a broad light inner layer and a thinner dark outer layer including warts. An equatorial ledge was present. During germination in the ascus, a new light inner layer was formed and the old layers of the spore wall partly broke up. Ascospores in a strain ofS. persoonii had a different wall structure in that the dark layer had changed into light areas separated by dark material which formed bulges at the surface.     

Endosperm development after fusion of isolated, single maize sperm and central cells in vitro

We demonstrate here the possibility of endosperm development in vitro after the fusion of pairs of an isolated sperm and an isolated central cell of maize. The occurrence of karyogamy and the time course of the fusion of sperm and central cell nuclei are presented. The fusion of the sperm nucleus occurred either with one of the two polar nuclei or with the secondary nucleus and was completed within 2 hr after in vitro cell fusion. The in vitro study of early events after cell and nuclear fusion indicates that the resulting primary endosperm cell develops into a characteristic tissue capable of self-organization apart from the mother tissue. The technology presented here opens the way for new cellular and molecular studies, especially of early events after sperm and central cell fusion. These studies should lead to a better understanding of the processes of double fertilization and endosperm development.     

Ricerche Embriologiche su Senecio Vulgaris L. var. Thyrrenus Fiori

Abstract

Embryological researches on SENECIO VULGARIS L. var. THYRRENUS Fiori. — Male gametophyte, development of tapetal cells and female gametophyte have been studied in Senecio vulgaris L. var thyrrenus Fiori.

1) The development of male gametophyte is normal. Divisions of the microspore mother cells are of the simultaneous type. The division of the generative nucleus has never been observed till the pollen grain was in the anther.

2) The tapetal cells follow a very simple development. The nucleus of each cell divides only twice starting at the same time with the meiotic divisions of pollen mother cells but ending much earlier; subsequently, as usually happens with the Asteraceae, the ameboid involution of the tapetum begins. Endomitosis or any other process which leads to a polyploidy not due to nuclear fusion, has never been observed.

3) The female gametophyte is eight nucleate of the normal type (Polygonum). At maturity it shows only three antipodal cells whose nucleus undergoes at first, two or three divisions. Only later these new nuclei, always within the antipodal cell, may fuse in a polyvalent one.     

Asymmetric cell divisions in flowering plants - one mother, "two-many" daughters

Plant development shows a fascinating range of asymmetric cell divisions. Over the years, however, cellular differentiation has been interpreted mostly in terms of a mother cell dividing mitotically to produce two daughter cells of different fates. This popular view has masked the significance of an entirely different cell fate specification pathway, where the mother cell first becomes a coenocyte and then cellularizes to simultaneously produce more than two specialized daughter cells. The "one mother - two different daughters" pathways rely on spindle-assisted mechanisms, such as translocation of the nucleus/spindle to a specific cellular site and orientation of the spindle, which are coordinated with cell-specific allocation of cell fate determinants and cytokinesis. By contrast, during "coenocyte-cellularization" pathways, the spindle-assisted mechanisms are irrelevant since cell fate specification emerges only after the nuclear divisions are complete, and the number of specialized daughter cells produced depends on the developmental context. The key events, such as the formation of a coenocyte and migration of the nuclei to specific cellular locations, are coordinated with cellularization by unique types of cell wall formation. Both one mother - two different daughters and the coenocyte-cellularization pathways are used by higher plants in precise spatial and time windows during development. In both the pathways, epigenetic regulation of gene expression is crucial not only for cell fate specification but also for its maintenance through cell lineage. In this review, the focus is on the coenocyte-cellularization pathways in the context of our current understanding of the asymmetric cell divisions. Instances where cell differentiation does not involve an asymmetric division are also discussed to provide a comprehensive account of cell differentiation.     

The cytoskeleton in the unique cell reproduction by conidiogenesis of the long-neck yeast Fellomyces (Sterigmatomyces) fuzhouensis

Summary. The morphology of conidiogenesis and associated changes in microtubules, actin distribution and ultrastructure were studied in the basidiomycetous yeast Fellomyces fuzhouensis by phase-contrast, fluorescence, and electron microscopy. The interphase cell showed a central nucleus with randomly distributed bundles of microtubules and actin, and actin patches in the cortex. The conidiogenous mother cell developed a slender projection, or stalk, that contained cytoplasmic microtubules and actin cables stretched parallel to the longitudinal axis and actin patches accumulated in the tip. The conidium was produced on this stalk. It contained dispersed cytoplasmic microtubules, actin cables, and patches concentrated in the cortex. Before mitosis, the nucleus migrated through the stalk into the conidium and cytoplasmic microtubules were replaced by a spindle. Mitosis started in the conidium, and one daughter nucleus then returned to the mother via an eccentrically elongated spindle. The cytoplasmic microtubules reappeared after mitosis. A strong fluorescence indicating accumulated actin appeared at the base of the conidium, where the cytoplasm cleaved eccentrically. Actin patches then moved from the stalk together with the retracting cytoplasm to the mother and conidium. No septum was detected in the long neck by electron microscopy, only a small amount of fine “wall material” between the conidium and mother cell. Both cells developed a new wall layer, separating them from the empty neck. The mature conidium disconnected from the empty neck at the end-break, which remained on the mother as a tubular outgrowth. Asexual reproduction by conidiogenesis in the long-neck yeast F. fuzhouensis has unique features distinguishing it from known asexual forms of reproduction in the budding and fission yeasts. Fellomyces fuzhouensis develops a unique long and narrow neck during conidiogenesis, through which the nucleus must migrate into the conidium for eccentric mitosis. This is followed by eccentric cytokinesis. We found neither an actin cytokinetic ring nor a septum in the long neck, from which cytoplasm retracted back to mother cell after cytokinesis. Both the conidium and mother were separated from the empty neck by the development of a new lateral wall (initiated as a wall plug). The cytoskeleton is clearly involved in all these processes. Correspondence and reprints: Department of Biology, Faculty of Medicine, Masaryk University, Tomešova 12, 602 00 Brno, Czech Republic.     

A re-examination of the morphology and reproduction of Nothocladus lindaueri (Batrachospermales, Rhodophyta)

The vegetative morphology and reproduction of the freshwater rhodophyte Nothocladus lindaueri Skuja [=Batrachospermum lindaueri (Skuja) Necchi et Entwisle] were examined by light and electron microscopy. It was confirmed that this alga has a typical batrachospermalean pit plug with two cap layers, the outer one of which is domed. During elongation of hair cells, the primary wall is broken, forming a basal collar. Hair cells have a single nucleus and abundant Golgi bodies, en-doplasmic reticula (ER) and vesicles. Dividing apical cells of the fascicles have a nucleus with art adjacent zone of exclusion, the latter containing a single polar ring. Branched trichogynes and fertilized carpogonia are shown for the first time in this species. Carpogonial branch and involucral cells contain a prominent axial nucleus, proplastids, ER and vesicles. The pit plugs disintegrate among these cells leaving open pit connections. Carpogonia have plentiful mitochondria and vesicles. The wall at the trichogyne apex is thickened and densely stained. The carposporophyte centre consists of a mass of fusion cells with open pit connections, and indeterminate gonimoblast filaments arise from this mass. The combination of a symmetrical carpogonial base, a carposporophyte centre consisting of a mass of fusion cells, and exclusively indeterminate gonimoblast filaments appears to be unique among the members of the Batrachospermaceae. The specimen of N. lindaueri contains epiphytic filaments of Audouinella meiospora producing both spermatangia and monosporangia. Spermatium formation in N. lindaueri remains unknown.     

A study of the fine structure of ascosporogenesis in Saccobolus kerverni

Summary Observations of ascospore fromation in KMnO₄-fixed Saccobolus kerverni apothecia with the electron microscope reveal the following sequence. Ascus formation is preceded by the development of croziers whose fine structure differs little from that of vegetative hyphae. Following fusion of the two nuclei in the ascus mother cell, the resultant ascus elongates, and two large vacuoles appear, first below and later above the fusion nucleus. These vacuoles soon occupy dominant positions at the tip and bottom of the ascus and assume a flocculent appearance. Nuclear blebbing occurs during meiosis, mitosis, and the subsequent spore delimitation process in the central cytoplasmic portion of the ascus. Each spore initial is surrounded by two membranes, the plasma and investing membranes, between which the spore wall is deposited in two layers, an inner primary wall and an outer secondary wall. Following primary wall deposition the spores clump; secondary wall deposition begins outside the primary wall at the places where the spores are contiguous. Interdigitation of these walls and disappearance of the investing membranes in the sutures lead to the envelopment of all eight ascospores in a common secondary wall. A flocculent material in the epiplasmic vacuoles aggregates around the mature spore balls.Based on a portion of a dissertation presented to the Faculty of the Graduate School of the University of Texas in partial fulfillment of the requirements for the degree of Doctor of Philosophy.     

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.     

F-actin redistributions at the division site in livingTradescantia stomatal complexes as revealed by microinjection of rhodamine-phalloidin

Summary Microinjection of rhodamine-phalloidin into living cells of isolatedTradescantia leaf epidermis and visualisation by confocal microscopy has extended previous results on the distribution of actin in mitotic cells of higher plants and revealed new aspects of actin arrays in stomatal cells and their initials. Divisions in the stomatal guard mother cells and unspecialised epidermal cells are symmetrical. Asymmetrical divisions occur in guard mother precursor cells and subsidiary mother cells. Each asymmetrical division is preceded by migration of the nucleus and the subsequent accumulation of thick bundles of anticlinally oriented actin filaments localised to the area of the anticlinal wall closest to the polarised nucleus. During prophase, in all cell types, a subset of cortical actin filaments coaligns to form a band, which, like the preprophase band of microtubules, accurately delineates the site of insertion of the future cell wall. Following the breakdown of the nuclear envelope, F-actin in these bands disassembles but persists elsewhere in the cell cortex. Thus, cortical F-actin marks the division site throughout mitosis, firstly as an appropriately positioned band and then by its localised depletion from the same region of the cell cortex. This sequence has been detected in all classes of division inTradescantia leaf epidermis, irrespective of whether the division is asymmetrical or symmetrical, or whether the cell is vacuolate or densely cytoplasmic. Taken together with earlier observations on stamen hair cells and root tip cells it may therefore be a general cytoskeletal feature of division in cells of higher plants.Abbreviations GMC guard mother cell - MT microtubule - PPB preprophase band - Rh rhodamine - SMC subsidiary mother cell     

Evidence for the production of a fusion factor during in vitro infection of sheep choroid plexus cells by visna virus]

One of the most important early events during the infection of sheep choro?d plexus cells in culture by Visna virus is the synthesis by these cells of a fusion factor distinct from the virus. The cell fusion activity of this factor does not seem transmissible. It promotes cell shape changes and a migration of the nucleus towards the cell wall.     

STRUCTURE AND DEVELOPMENT OF THE CARPOSPOROPHYTE OF NEMALION HELMINTHOIDES (NEMALIONALES,RHODOPHYTA)1

Following fertilization, the carposporophyte of Nemalion helminthoides (Velley in With.) Batters differentiates into four distinct regions: the fusion cell, the sterile gonimoblast cells, the carposporangial mother cells and the carposporangia. The gonimoblast is formed by apically dividing, monopodial filaments of limited growth which may later become pseudodichotomous. Upon differentiation of a terminal carposporangium, a gonimoblast filament may continue to grow sympodially. A single carposporangial mother cell may produce carposporangia in several different directions as well as proliferate successive carposporangia within the sporangial walls that remain after carpospore liberation. As the carposporophyte matures, the gonimoblast initial, the stalk cell, the hypogynous and subhypogynous cells fuse. Except for the fusion cell, all cells of the carposporophyte show organelle polarity and contain a distally located, lobed chloroplast and proximal nucleus.