Embryo sac development in Arabidopsis thaliana

Summary Aspects of megasporogenesis in Arabidopsis thaliana have been investigated using a variety of histochemical techniques to visualize general cell organization, DNA and callose in whole ovules and sections by bright field, fluorescence, differential interference contrast and scanning electron microscopy. The microtubular cytoskeleton has been studied using immunofluorescence localization of tubulin in sections and whole cells. The observations deviate from reports of preceding studies in that the megasporocyte was found to undergo both meiotic divisions followed by simultaneous cytokinesis (i.e. without an intermediate dyad stage) to give a multiplanar tetrad of megaspores. This represents a variation of monosporic development not previously described. Polarized distribution of organelles prior to meiosis ensures that the functional megaspore receives the largest share. Aberrant wall formation is common between degenerating megaspores. Localized callose deposition in the tetrad separates these cells from the active megaspore. Their pattern of degeneration and displacement is extremely flexible within the embryo sac space. The microtubular cytoskeleton is extensive and largely cytoplasmic, as distinct from cortical, throughout megasporogenesis. In the megasporocyte, megaspores and one-nucleate embryo sac, randomly oriented microtubules throughout the cells may serve to maintain cytoplasmic integrity and position organelles. Numerous microtubules (MTs) associate closely with the nucleus and some radiate from it, perhaps functioning in nuclear positioning. During meiosis MTs are restricted to the spindle configurations and later to the phragmoplasts which form between daughter nuclei. The lack of interphase cortical arrays suggests that the role of internal influences on cell shape is small.     

The microtubular cytoskeleton during megasporogenesis in the Nun orchid, Phaius tankervilliae

This study examines the microtubular cytoskeleton during megasporogenesis in the Nun orchid, Phaius tankervilliae . The subepidermal cell located at the terminal end of the nucellar filament differentiates first into an archesporial cell and then enlarges to become the megasporocyte. The megasporocyte undergoes the first meiotic division, giving rise to two dyad cells of unequal size. Immunostaining reveals that microtubules become more abundant as the megasporocyte increases in size. Microtubules congregate around the nucleus forming a distinct perinuclear array and many microtubules radiate directly from the nuclear envelope. In the megasporocyte, prominent microtubules are readily detected at the chalazal end of the cell cytoplasm. After meiosis I, the chalazal dyad cell expands in size at the expense of the micropylar dyad cell. At this stage, new microtubule organizing centres can be found at the corners of the cells. The appearance of these structures is stage-specific and they are not found at any other stages of megasporogenesis. The functional dyad cell undergoes the second meiotic division, resulting in the formation of two megaspores of unequal size. The chalazal megaspore enlarges and eventually gives rise to the embryo sac. As the functional megaspore expands, the microtubules again form a distinct perinuclear array with many microtubules radiating from the nuclear envelope. A defined cortical array of microtubules has not been found in P. tankervilliae during the course of megasporogenesis.     

A new type of microtubular cytoskeleton in microsporogenesis of Lavatera thuringiaca L.

Summary. In Lavatera thuringiaca, kariokinesis and simultaneous cytokinesis during the meiotic division of microsporogenesis follow a procedure similar to that which takes place in the majority of members of the class Angiospermae. However, chondriokinesis occurs in a unique way found only in species from the family Malvaceae. Chondriokinesis in such species is well documented, but the relationship between the tubulin cytoskeleton and rearrangement of cell organelles during meiosis in L. thuringiaca has not been precisely defined so far. In this study, the microtubular cytoskeleton was investigated in dividing microsporocytes of L. thuringiaca by immunofluorescence. The meiotic stages and positions of cell organelles were identified by staining with 4′,6-diamidino-2-phenylindole. We observed that, during prophase I and II, changes in microtubular cytoskeleton configurations have unique features, which have not been described for other plant species. At the end of prophase I, organelles (mostly plastids and mitochondria) form a compact envelope around the nucleus, and the subsequent phases of kariokinesis take place within this arrangement. At this point of cell division, microtubules surround the organelle envelope and separate it from the peripheral cytoplasm, which is devoid of plastids and mitochondria. In telophase I, two newly formed nuclei are tightly surrounded by the cell organelle envelopes, and these are separated by the phragmoplast. Later, when the phragmoplast disappears, cell organelles still surround the nuclei but also move a little, starting to occupy the place of the disappearing phragmoplast. After the breakup of tetrads, the radial microtubule system is well developed, and cell organelles can still be observed as a dense envelope around the nuclei. At a very late stage of sporoderm development, the radial microtubule system disappears, and cell organelles become gradually scattered in the cytoplasm of the microspores. Using colchicines, specific inhibitors of microtubule formation, we investigated the relationship between the tubulin cytoskeleton and the distribution of cell organelles. Our analysis demonstrates that impairment of microtubule organization, which constitutes only a single component of the cytoskeleton, is enough to disturb typical chondriokinesis in L. thuringiaca. This indicates that microtubules (independent of microfilaments) are responsible for the reorganization of cell organelles during meiotic division. Correspondence: D. Tchórzewska, Department of Plant Anatomy and Cytology, Maria Curie-Skłodowska University, Akademicka 19, 20-033 Lublin, Poland.     

Confocal Microscopic Observations on Microtubular Cytoskeleton Changes During Megasporogenesis and Megagametogenesis in Phaius tankervilliae (Aiton) Bl.

In nun orchid (Phaius tankervilliae (Alton) B1. ) embryo sac development follows the monosporic pattern. Changes in the pattern of organization of the microtubular cytoskeleton during megasporogenesis and megagametogenesis in this orchid were studied using the immunofluorescence technique and eonfocal microscopy. At the initial stage of development the microtubules in the arehesporium were randomly oriented into a network. Later the archesporial cell elongated to form the megasporocyte. The cytoskeleton in the elongated megasporoeyte was radially organized in which microtubules extending from the nuclear envelope to the peripheral region of the cell. The megasporoeyte then underwent meiosis 1 to form a dyad. The dyad cell at the chalazal end was larger than the cell at the micropylar end. Microtubules in the dyad cell were radially oriented. The dyad underwent meiosis to give rise to a linear array of four megaspores (i. e. tetrad formation). The chalazal-far most megaspore survived and became the functional megaspore, which contained a set of randomly oriented microtubules. The microtubules in the other 3 megaspore disappeared as the cells degenerated. The functional megaspore then underwent mitotic division giveing rise to a 2 nucleate embryo sac. The nuclei of the 2-nucleate embryo sac were separated by a set of longitudinally oriented microtubules which ran parallel to the long axis of the embryo sac. Each nucleus in the embryo sac was surrounded by a set of perinuelear microtubules. The gnucleate embryo sac again underwent mitotic division to form a 4-nucleate embryo sac. The division of the two nuclei was synchronous. But the orientation of the division plan of the two spindles was different (i. e. the spindle microtubules at the chalazal end ran parallel with the long axis of the embryo sac and those at the mieropylar end ran at right angle to the axis of the embryo sac). The 4 nuclei of the 4-nucleate embryo sac were all tightly surrounded by randomly oriented microtubules. Later the paired nuclei at the micropylr end and at the chalazal end as well underwent mitotic division in seguence. At this time when the embryo sac had reached the 8-nucleate embryo sac stage. The pattern of organization of the microtubules was very complex. Initially the nuclei were surrounded by a set of randomly oriented microtubules, but after the two polar nuclei had moved to the central region of the embryo sac, three different organizational zones of microtubules appeared, viz: a randomly oriented set of microtubules surrounding each nucleus in the chalazal zone: a set (in the form of a basket) of cortical microtubules which surrounded the vacuoles and the two polar nuclei in the central zone and a loosely knitted network of microtubules surrounding the nucleus that later became the egg cell nucleus in the micropylar zone. The two nuclei that would become the nuclei of the synergids were surrounded by a set of more densely packed mierotubules. Towards far the most micropylar end some microtubules formed thick bundles. The site of appearance of these thick bundles coincided with the site of development of the filiform apparatus. The pattern of microtubule organization after cellularization (i. e. at the beginning of embryo sac maturation) did not change much. The author's results indicated that various patterns of microtubule organization observed in the developing embryo sac of nun orchid reflected the complexity and dynamism of the embryo sac.     

鹤顶兰胚囊发育过程中微管变化的共焦显微镜观察

光镜的观察确定了鹤顶兰（Ｐｈａｉｕｓ ｔａｎｋｅｒｖｉｌｌｉａｅ （Ａｉｔｏｎ） Ｂｌ．）胚囊发育属单孢子蓼型。应用免疫荧光标记技术及共焦镜观察了胚囊发育过程中微管分布的变化。当孢原细胞初形成时,细胞内的微管呈网状分布。之后,孢原细胞体积增大发育为大孢子母细胞。大孢子母细胞延长,进入减数分裂Ⅰ。微管由分裂前的网状分布变为辐射状排列。二分体的两个细胞内的微管分布一样,呈辐射状。四分体的近珠孔端的３ 个大孢子解体,细胞内的微管消失。靠合点端的功能大孢子内有许多微管呈网状分布。当功能大孢子进入第一次有丝分裂时,细胞内的微管由网状变为辐射状,从核膜伸展至周质。再经两次有丝分裂形成八核胚囊。在核分裂之前微管一般是呈网状分布并紧包围着核。在分裂期间二核和四核胚囊都呈极性现象,微管系统也呈极性分布。微管在八核胚囊内的分布变化情形特别复杂。首先,八核分别作不同程度的移动,其中两个核移向胚囊中央,珠孔端和合点端的３ 个核分别互相靠拢,形成３ 个区,即中央区、反足区和卵器区。胚囊未形成区时,８ 个核都被网状分布的微管包围着。当胚囊明显分成区时,反足区内的微管仍作网状分布。中央区的微管分布则趋疏松,形成篮形结构,包围着液泡和两个极核。在     

Calcium changes during megasporogenesis and megaspore degeneration in lettuce (<Emphasis Type="Italic">Lactuca sativa</Emphasis> L.)

Potassium pyroantimonate was used to localize loosely-bound calcium in young ovules of lettuce (Lactuca sativa L.) during megasporogenesis to investigate the relationship between ionically available calcium and megaspore degeneration. At the megasporocyte (megaspore mother cell) stage, few calcium precipitates were located in the ovule. Following meiosis in the megasporocyte, a linear tetrad of four megaspores is formed, with three of the four megaspores degenerating from the micropylar end inward. Only the chalazal-most megaspore continues to develop, becoming the functional megaspore. A decrease in amount of calcium precipitates in the megaspore, particularly in the nucleus, precedes the breakdown of the micropylar megaspores, which subsequently undergo structural disintegration and loss of recognizable cellular features. A partial recovery of calcium precipitates occurs during later degeneration. The functional megaspore retains a consistently higher concentration of calcium precipitates during development, which is retained in the developing embryo sac. This, to our knowledge, is the first report related to calcium dynamics during megaspore degeneration, and may facilitate future research aimed at elucidating the mechanisms of megasporogenesis.     

The microtubular cytoskeleton of three dinoflagellates: an immunofluorescence study

Summary The sub-thecal microtubular cytoskeleton of the dinoflagellatesAmphidinium rhynchocephalum, Gymnodinium sanguineum, andGymnodinium. sp has been investigated by indirect immunofluorescence microscopy. In these cells, the majority of cytoskeletal microtubules lie in the anterior-posterior plane. These longitudinal microtubules clearly originate from one of two radially arranged microtubular bands that correspond in location with the anterior and posterior edge of the cingolar depression. Despite the morphological variability of these gymnodinioid dinoflagellates, our data indicate that the microtubular cytoskeleton perfectly reflects the spatial patterning of the epicone and hypocone in each cell.Abbreviations ALB Anterior longitudinal microtubular bundles - ATB Anterior transverse microtubular bands - C cingulum - CLB Cingular longitudinal microtubular bundles - E Epicone - H Hypocone - PLB Posterior longitudinal microtubular bundles - PTB Posterior transverse microtubular bands - S Sulcus     

EMBRYOLOGY OF CALYPSO BULBOSA. I. OVULE DEVELOPMENT

Calypso bulbosa is a terrestrial orchid that grows in north temperate regions. Like many orchids, the Calypso has ovules that are not fully developed at anthesis. After pollination, the ovule primordia divide several times to produce a nucellar filament which consists of five to six cells. The subterminal cell of the nucellar filament enlarges to become the archesporial cell. Through further enlargement and elongation, the archesporial cell becomes the megasporocyte. An unequal dyad results from the first meiotic division. A triad of one active chalazal megaspore and two inactive micropylar megaspores are the end products of meiotic division. Callose is present in the cell wall of the megaspore destined to degenerate. In the mature embryo sac the number of nuclei is reduced to six when the chalazal nuclei fail to divide after the first mitotic division. The chalazal nuclei join the polar nucleus and the male nucleus near the center of the embryo sac subsequent to fertilization.     

Structure and function of the microtubular cytoskeleton during pollen development in Gasteria verrucosa (Mill.) H. Duval

In a study of pollen development in Gasteria verrucosa, the changes in the spatial organization of microtubules were related to the processes of cell division, nuclear movement and cytomorphogenesis. Sections of polyethylene-glycol-embedded anthers of G. verrucosa were processed immunocytochemically to record the structure and succession of fluorescently labeled microtubular configurations. Using microspectrophotometric measurements the relative quantity of tubulin in microtubules per unit of cytoplasm was determined. Cell dimensions and nuclear positions were measured to relate changes in cell shape and nuclear movements to microtubular configurations. Microtubules were detected in the different cells during microsporogenesis and microgametogenesis. In microspore mother cells which are approximately isodiametric at interphase, microtubules were predominantly arranged in a criss-cross pattern. The microtubules probably function as a flexible cytoskeleton which sustains the integrity of the cytoplasm. Bundles of microtubules were observed in the microspores, in the generative cells and during nuclear division, where they functioned in establishing and maintaining cell and spindle shapes. Microtubules radiating from nuclear membranes appeared to fix the nucleus in position. In prophase of meiosis and after microspore mitosis, periods a high fluorescence intensity were distinguished indicating a variation in the quantity of microtubules.Abbreviation MT microtubule     

Pollen wall formation in Lilium: The effect of chaotropic agents,and the organisation of the microtubular cytoskeleton during pattern development

Using a combination of electron-microscopic and immunocytochemical techniques the behaviour of the microtubular cytoskeleton has been followed throughout microsporogenesis in Lilium henryi Thunb. Cells treated with colchicine at specific stages and then permitted to develop to near maturity were used to investigate any participation by microtubules in the regulation of pollen wall patterning. The microtubular cytoskeleton assumes four principal forms during the meiotic process; in pre-meiosis it resembles that characteristic of meristematic somatic cells, during meiotic prophase it becomes associated with a nuclear envelope and, perhaps, with the chromosomes and, as the nuclear and cell divisions commence, it takes the form of a normal spindle apparatus. In the young microspores, microtubules assume a radial organisation extending from sites at the nuclear envelope to the inner face of the plasma membrane. No firm evidence was found linking any one of these forms of cytoskeleton with the generation of patterning on the cell surface. Experiments with colchicine revealed that the drug would readily dislocate the colpus, but did not affect the general reticulate patterning. The radial cytoskeleton was present during the deposition of the early primexine, but evidence from these and other studies (J.M. Sheldon and H.G. Dickinson 1983, J. Cell. Sci. 63, 191–208; H.G. Dickinson and J.M. Sheldon, 1984, Planta 161, 86–90) indicates patterning to be imprinted upon the plasma membrane prior to the appearance of this type of cytoskeleton. These results are discussed in terms of a recent model proposed to explain pattern generation on the surface of Lilium pollen grains, based on the self-assembly of patterning determinants within the plasma membrane.Abbreviation MTOC microtubule-organising centre     

Organization of the actin cytoskeleton during pollen development inGasteria verrucosa (Mill.) H. Duval visualized with rhodamine-phalloidin

The three-dimensional organization of the microfilamental cytoskeleton of developingGasteria pollen was investigated by light microscopy using whole cells and fluorescently labelled phalloidin. Cells were not fixed chemically but their walls were permeabilized with dimethylsulphoxide and Nonidet P-40 at premicrospore stages or with dimethylsulphoxide, Nonidet P-40 and 4-methylmorpholinoxide-monohydrate at free-microspore and pollen stages to dissolve the intine.Four strikingly different microfilamentous configurations were distinguished. (i) Actin filaments were observed in the central cytoplasm throughout the successive stages of pollen development. The network was commonly composed of thin bundles ramifying throughout the cytoplasm at interphase stages but as thick bundles encaging the nucleus prior to the first and second meiotic division. (ii) In released microspores and pollen, F-actin filaments formed remarkably parallel arrays in the peripheral cytoplasm. (iii) In the first and second meiotic spindles there was an apparent localization of massive arrays of phalloidin-reactive material. Fluorescently labelled F-actin was present in kinetochore fibers and pole-to-pole fibers during metaphase and anaphase. (iv) At telophase, microfilaments radiated from the nuclear envelopes and after karyokinesis in the second meiotic division, F-actin was observed in phragmoplasts.We did not observe rhodamine-phalloidin-labelled filaments in the cytoplasm after cytochalasin-B treatment whereas F-actin persisted in the spindle. Incubation at 4° C did not influence the existence of cytoplasmic microfilaments whereas spindle filaments disappeared. This points to a close interdependence of spindle microfilaments and spindle tubules.Based on present data and earlier observations on the configuration of microtubules during pollen development in the same species (Van Lammeren et al., 1985, Planta165, 1-11) there appear to be apparent codistributions of F-actin and microtubules during various stages of male meiosis inGasteria verrucosa.Abbreviation DMSO dimethylsulfoxide     

Inorganic salts modify embryo sac development in sexual and aposporous Cenchrus ciliaris

Summary Sexual and aposporously apomictic plants of buffelgrass (Cenchrus ciliaris L.) form megaspore tetrads. In sexual plants the chalazal megaspore develops into a single Polygonum type embryo sac. In aposporous plants the megaspores degenerate, and one or more un-reduced nucellar cells form Panicum type embryo sacs. Apospory is conditioned by gene A; the dominant allele of gene B is epistatic to A and preserves sexual reproduction. We recently observed that heavy application of (NH₄)₂SO₄ to the soil induced multiple embryo sacs in a sexual line. Therefore we tested the effect of salt stress on embryo sac formation in sexual and aposporous genotypes. One molar solutions of CaCl₂, NaCl, (NH₄)₂SO₄, NH₄Cl, NaNO₃, or Na₂SO₄ were applied to the soil of greenhouse plants every day or two starting at the archespore stage. Some of the pistils in salt-treated plants of sexual genotypes AaBb, aaBb, and aabb showed features not seen in untreated controls: (1) multiple Polygonum type embryo sacs in 1%–7% of pistils depending upon the salt; (2) embryo sacs without antipodals (0%–7%); (3) embryo sacs protruding through the micropyle (1%–16%). Some pistils of salt-treated obligately aposporous lines, but not controls, developed Polygonum type embryo sacs (4%–13%) and protruding embryo sacs (0%–6%). There was no ion specificity for induction of abnormal features. We postulate that salt stress suppresses the developmental priority of nucellar embryo sacs over megaspores in aposporous lines and of the chalazal megaspore over other megaspores in all lines. This may permit megaspores of aposporous plants to form reduced Polygonum type gametophytes, and permit more than one megaspore to form reduced embryo sacs in all lines. Protrusion of sacs and failure of antipodal formation in reduced embryo sacs may be the consequence of uncoordinated expansion of the embryo sacs and surrounding tissue.Joint contribution of the Department of Biology, The Pennsylvania State University, and USDA-ARS, U.S. Regional Pasture Research Laboratory. Names of products are included for the benefit of the reader and do not imply endorsement or preferential treatment by USDA     

Callose in cell walls during megasporogenesis in angiosperms

Summary Callose was detected by fluorescence microscopy in megasporogenesis in all investigated species with mono- and bisporic embryo-sac development. Callose occurs first in the meiotic prophase in the chalazal part of the megasporocyte wall and by the first meiotic metaphase the whole cell is enveloped in a callose-containing wall. Later, there is a marked decrease of callose fluorescence, usually at the chalazal end of the megasporocyte. In Oenothera, where the micropylar megaspore is active, decrease of fluorescence takes place at the micropylar pole of the megasporocyte. Callose appears centrifugally in the cell plates forming eventually the walls dividing the megaspores. It disappears from the walls of the megaspores during degeneration and differentiation.     

鹅毛竹大小孢子及雌雄配子体发育

利用扫描电镜、透射电镜、石蜡切片,对鹅毛竹的花芽分化、大、小孢子及雌、雄配子体的发育进行了详细观察.结果发现:鹅毛竹花药具4个药室,花药壁由表皮、药室内壁、中层、绒毡层4层结构组成,花药壁发育为单子叶型,绒毡层为腺质型,小孢子母细胞减数分裂中的胞质分裂为连续型,产生左右对称型小孢子.鹅毛竹成熟花粉大多2细胞型,都具1个萌发孔.鹅毛竹子房为单子房,子房1室,侧膜胎座,一个倒生胚珠,双珠被,薄珠心.大孢子母细胞由一个雌性孢原细胞直接发育而成,大孢子四分体呈线型,合点端一个大孢子分化为功能大孢子,由功能大孢子经过3次有丝分裂形成8核胚囊,发育类型为蓼型,位于核点端的3个细胞核进行多次分裂形成多个反足细胞.至此,成熟胚囊形成.并就鹅毛竹不结实的原因进行了探讨.     

Changes in the density of microtubular networks in mesophyll cells and mesophyll derived protoplasts of Nicotiana and Triticum during leaf development

Changes in the density of microtubular mesh-works were analysed in mesophyll cells and mesophyll derived protoplasts of Nicotiana tabacum L. and Triticum aestivum L. during leaf development. The main purpose of this study was to test whether the low density, if not lack, of microtubular networks recently described in protoplasts that had been isolated from fully differentiated mesophyll cells happened during protoplast isolation or whether the loss of microtubules actually occurred during differentiation of the leaf tissue. Immunofluorescence microscopy showed that the density of the microtubular cytoskeleton in the leaf tissue decreased steadily after cessation of cell growth in both species. Nevertheless, in Triticum microtubule disappearance was swifter and occurred along a gradient from the base to tip of the leaf, a phenomenon reflecting the differences in the ontogeny between the dicotyledonous Nicotiana and the mono-cotyledonous Triticum leaves. Protein extraction from leaf tissues and Western blot analysis indicated that in both species the disappearance of microtubules was the result of a degradation of tubulin and not only due to a depolymerisation into tubulin subunits. When the cell walls were removed from live cells and the protoplasts released, the original patterns of the microtubules became obscured and, particularly in differentiated cells, the integrity and density of the microtubule strands deteriorated. The potential application of the density of the microtubular cytoskeleton as a marker in studies on differentiation and dedifferentiation in mesophyll cells and protoplasts is discussed.We wish to thank Silke Heichel for excellent technical assistance. We also express our thanks to the group of A.M. Lambert at CNRS, Strasbourg, France, for advice during establishment of our Western blot system. The work was supported by a grant of the German Ministry of Science and Technology (BMFT).     

中国水韭属两个四倍体新种

水韭属(Iso&#235;tes)是起源最为古老的水生维管植物,全属物种均被列为国家一级重点保护植物。通过对全国水韭属植物的调查和研究发现,不同产地的四倍体植株在形态上存在显著差异。基于形态学、孢粉学和细胞学证据,将分布于中国湖南省长沙地区和怀化地区的四倍体居群分别命名为隆平水韭(Iso&#235;tes longpingii)和湘妃水韭(I. xiangfei),并详细描述了其形态特征。隆平水韭形态上与中华水韭(I. sinensis)相似,不同之处在于其大孢子具小的瘤状或冠状纹饰,叶细长而柔弱,长达60 cm; 该种也与六倍体东方水韭(I. orientalis)相似,不同之处在于其染色体44条,大孢子具瘤状或冠状纹饰。湘妃水韭的大孢子纹饰虽与二倍体云贵水韭(I. yunguiensis)相似,但在小孢子纹饰、孢子囊形状和染色体数目方面却不同。隆平水韭仅少数植株生长于湖南省宁乡市一处池塘,完全沉水生长,而湘妃水韭则分布于怀化市通道县和会同县的湿地。由于这两个新种的分布区狭窄,野生居群数量和个体数较少,栖息地环境受到人为干扰,因此根据IUCN红色名录评估标准,将隆平水韭评为极危(CR)等级,湘妃水韭评为易危(VU)等级。所编制的中国已知水韭属物种的分种检索表,为本属物种的鉴定和保护工作提供了重要参考。     

开口箭大小孢子发生及雌雄配子体发育

利用石蜡切片技术,对百合科植物开口箭(Tupistra chinensis Baker)大小孢子发生及雌雄配子体发育进程进行胚胎学观察分析,以明确开口箭胚胎发育的特征,为百合科植物的研究提供生殖生物学依据。结果表明:(1)开口箭花药具有4个药室,花药壁的发育方式为基本型,由表皮、药室内壁、中层及绒毡层组成;绒毡层发育类型为分泌型,到四分体花药阶段绒毡层细胞开始解体退化,花药成熟时完全消失。(2)花粉母细胞减数分裂为连续型,依次形成二分体、四分体,四分体为左右对称形;成熟花粉为2-细胞花粉,具单萌发沟。(3)子房3室,倒生型胚珠6枚,双珠被,薄珠心;在花部的分化早期,由珠心顶端表皮下方分化出雌性孢原细胞,孢原细胞经过一次平周分裂形成周缘细胞和造孢细胞,造孢细胞发育为大孢子母细胞;大孢子母细胞第一次减数分裂后形成二分体,珠孔端的二分体孢子退化,合点端的二分体孢子继续第二次分裂,形成两个子细胞依次发育为二核胚囊、四核胚囊和八核胚囊;开口箭的胚囊发育类型为葱型。     

Spatio-temporal reorganization of intracytoplasmic microtubules is associated with nuclear selection and differentiation during the developmental process in the ciliateTetrahymena thermophila

Summary Indirect immunofluorescence has revealed various intracytoplasmic microtubular structures, which are transiently polymerized in specific subcellular locations during the developmental process of conjugation in the ciliateTetrahymena thermophila. These structures include: (1) micronuclear spindles, (2) perimicronuclear microtubules, (3) microtubular baskets surrounding migrating pronuclei, and (4) microtubules interconnecting the pronuclei with the conjugants' junctional zone. Furthermore, a peripheral network of intracytoplasmic microtubules related to the cell cortex is present in both vegetative cells and in conjugants. Comparative observations made on cells undergoing normal conjugation and defective conjugation (occurring either spontaneously or induced by taxol) has revealed some rules governing the pattern of deployment of conjugation-specific microtubules. The presence of perinuclear microtubular arrays during early postmeiotic stages of development is strictly limited to more anteriorly located nuclei which includes the selected haploid nucleus that further divides to form the stationary and migratory pronuclei. These perinuclear microtubules may be involved in the positional control of nuclear fates leading to effective nuclear selection. Microtubular bundles associated with pronuclei and connecting the junctional zone are only formed in the presence of functional pronuclei, and may be involved in the guidance of pronuclei leading to their fusion. The mechanism of cytoplasmic control of nuclear differentiation of derivatives of the zygotic nucleus appear to be associated with a coordinate action of two microtubular arrays: spindle microtubules of the second postzygotic division and the peripheral intracytoplasmic network of microtubules, leading to a proper subcortical positioning of the postzygotic nuclei at opposite poles of the cell.Abbreviations MTs Microtubules     

Nuclear DNA amount during sporogenesis and gametogenesis in sexual and aposporous buffelgrass

Nuclear DNA amounts (C values) were measured in Feulgen-stained sections of anthers and ovules of sexual plant B-2s (genotype aaaa) and aposporous cultivar Higgins (genotype AAaa) of buffelgrass (Pennisetum ciliare). The mass of the unreplicated nuclear genome of a gamete equals 1C DNA. In both lines, pollen mother cell nuclei were 4C before leptotene; anther wall, dyad, 1-nucleate pollen, and generative cell nuclei were 2C; microspore tetrad, enlarging microspore, and sperm nuclei were 1C. The tapetum persisted as uninucleate cells with 4C DNA. Archespores (2-4C) of both lines initiated meiosis to form megaspore tetrad nuclei with 1-2C DNA. In B-2s, chalazal megaspores (2-4C) formed reduced 8-nucleate Polygonum type embryo sacs, and sacs at 2- and 4-nucleate stages showed distributions with peaks near C1 and C2, corresponding to G1 and G2 cell cycle phases; this is characteristic of active mitosis. Nuclei of 8-nucleate sacs and of eggs and polars were 1C, indicating chromosomes were not duplicated before fertilization. Antipodal nuclei had levels from 1 to 36C, possibly due to polyteny or endopolyploidy. In Higgins, aposporous initials and 2-nucleate embryo sacs showed bimodal distributions of 2n nuclei with peaks at 2C and 4C DNA. Nuclei of newly formed 4-nucleate Panicum type aposporous sacs and of polars were 2C; aposporous eggs stained too faintly for reliable measurement.Names of products are included for the benefit of the reader and do not imply endorsement or preferential treatment by USDA     

Structure and function of the microtubular cytoskeleton during megasporogenesis and embryo sac development in Gasteria verrucosa (Mill.) H. Duval

Summary Using immunocytochemical techniques, tubulin distribution in various stages of meiosis and embryo sac development was studied. In the archespore cell some microtubules appeared to be randomly oriented. During zygotene and pachytene, when the cell volume increases, a large number of microtubules in dispersed configurations and bundles were observed. During this stage the nucellar cells divide, and their parallel cortical microtubules play an important role in preparing the direction of cell enlargement. The protoderm cells show anticlinal-directed cortical microtubules. It can be concluded that the enlargement of the meiocyte during these early meiotic stages is influenced both by its own cytoskeleton and by growth of the nucellus. Thereafter, the microtubules function directly in meiosis and disappear for the greater part until the two-nucleate coenocyte is formed. In a four-nucleate coenocyte microtubules reappear around the nucleus; in a young synergid, randomly oriented microtubules are involved in cell shaping during the formation of the filiform apparatus; in the synergids of the mature embryo sac, many parallel arrays of microtubules are present. Microtubules are less abundant in other cells. It is concluded that the cytomorphogenesis of the developing coenocyte and embryo sac are due to cell growth of the nucellar cells together with vacuolation of the coenocyte.