Chalazal Nuclei Research Articles

Megasporogenesis, megagametogenesis, and embryogenesis in Maxillaria crassifolia (Lindl.) Rchb.f. (Cymbidieae, Orchidaceae).

Maxillaria crassifolia (Lindl.) Rchb.f. belongs to the polyphyletic genus Maxillaria Ruiz & Pav., which currently is the subject of several taxonomic research. There are conflicting descriptions of megasporogenesis, megagametogenesis, and embryogenesis in orchids from the tribe Cymbidieae, in general, and in the genus Maxillaria, in particular. In the present report, all stages of embryonic development of M. crassifolia were examined using confocal fluorescence microscopy. Some features of the development of the ovule and embryo, which distinguish M. crassifolia from other species of the tribe Cymbidieae were identified. The T-shaped arrangement of megaspores is formed by dividing the micropylar megaspore of the dyad. The megagametophyte develops according to the modified Polygonum-type with an unstable number of nuclei in the embryo sacs. The nucleus of the central cell varies in composition and may include unfused micropylar and chalazal nuclei and daughter nuclei formed during their division. The sequence of embryonal divisions is strictly structured. A special variant of embryogenesis, the Cymbidium-type Maxillaria-variant, has been described. Its characteristic features are the strictly apical nature of embryonic divisions, the absence of basal cell (cb) division, the formation of one to three pairs of tubular suspensor cells, and the localization of all suspensor cells within the inner integument.

Megasporogenesis, megagametogenesis, and embryogenesis in Dendrobium nobile (Orchidaceae).

The orchid reproductive strategy, including the formation of numerous tiny seeds, is achieved by the elimination of some stages in the early plant embryogenesis. In this study, we documented in detail the formation of the maternal tissues (the nucellus and integuments), the structures of female gametophyte (megaspores, chalazal nuclei, synergids, polar nuclei), and embryonic structures in Dendrobium nobile. The ovary is unilocular, and the ovule primordia are formed in the placenta before the pollination. The ovule is medionucellate: the two-cell postament and two rows of nucellar cells persist until the death of the inner integument. A monosporic eight-nucleated embryo sac is developed. After the fertilization, the most common central cell nucleus consisted of two joined but not fused polar nuclei. The embryogenesis of D. nobile is similar to the Caryophyllad-type, and it is characterized by the formation of all embryo cells from the apical cell (ca) of a two-celled proembryo. The only exception is that there is no formation of the radicle and/or cotyledons. The basal cell (cb) does not divide during the embryogenesis, gradually transforming into the uninuclear suspensor. Then the suspensor goes through three main stages: it starts with an unbranched cell within the embryo sac, followed by a branched stage growing into the integuments, and it ends with the cell death. The stage-specific development of the female gametophyte and embryo of D. nobile is discussed.

The embryo sac ofVanilla imperialis(Orchidaceae) is six-nucleate, and double fertilization and formation of endosperm are not observed

Double fertilization and subsequent endosperm formation are two of the most important synapomorphies of the angiosperms. Endosperm is generally lacking in Orchidaceae, but Swamy reported the formation of up to ten endosperm nuclei in Vanilla planifolia in 1947. The observation was documented only by line drawings and has not been confirmed; however, assumptions about endosperm formation occurring in Orchidaceae are primarily founded on Swamy's study. The current study provides the first detailed description of embryo sac formation and early embryogeny in Vanilla since Swamy, and is the first using modern imaging techniques. Flowers of Vanilla imperialis were artificially pollinated at 1-week intervals and the resulting fruits were fixed, embedded and sectioned for light microscopy of embryo sac formation, fertilization and early development of the embryo. Three-dimensional reconstructions of embryo sacs were obtained using confocal laser scanning microscopy on fixed ovules. The mature embryo sac contains only six nuclei as a result of the arrested development of the chalazal nuclei prior to the formation of three antipodals and a polar nucleus. Fertilization was observed 7 weeks after pollination, but double fertilization and the formation of endosperm did not occur. © 2014 The Linnean Society of London, Botanical Journal of the Linnean Society, 2015, 177, 202–213.

Female Gametophyte Development in Maize: Microtubular Organization and Embryo Sac Polarity.

The developmental stages of the maize embryo sac were correlated with the corresponding silk lengths of ear florets in the female inflorescence. The development of embryo sacs in the ovules of spikes occurs in a gradient pattern with the initiation of the embryo sac beginning at the base of the ear and progressing to the top. At the beginning of meiosis, the presence of conspicuous cortical microtubules coincides with the extensive elongation of the megasporocyte. The spindles at metaphase I and II align along the long axis of the megasporocyte leading to the linear alignment of the dyad and tetrad of megaspores. During megagametogenesis, micropylar and chalazal nuclei of the embryo sac undergo synchronized divisions and migration at the second and third mitosis. Radiate perinuclear microtubules are present during the interphase of the second and third mitosis, and inter-sister nuclear microtubules occur at the late four-nucleate embryo sac. The configuration and orientation of the spindles, phragmoplasts, and pairs of nuclei result in precise positioning of the nuclei. The fusion of the polar nuclei and the formation of a microtubule organizing center-like structure in the filiform apparatus occur right after the first division of the antipodal cells. The different patterns of organization of microtubules in the cells of the mature embryo sac reflect their structural adaptations for their future function.

Polarity of nuclear and organellar DNA during megasporogenesis and megagametogenesis in Plumbago zeylanica

Megasporogenesis and megagametogenesis of Plumbago zeylanica were studied using isolated megasporocytes, megaspores, and embryo sacs labeled with Hoechst 33258 for nuclear and organellar (presumably plastid) DNA. Megasporogenesis conforms to the tetrasporic Plumbago type, producing a coenomegaspore with four megaspore nuclei. Organeller DNA is polarized in the micropylar end of the coenomegaspore and embryo sac, reflecting the site of egg cell formation. The three remaining nuclei are somewhat displaced to the chalazal pole, producing a variable number of accessory cells and a 4N secondary central cell nucleus. Ultimately, the mature embryo sac consists of two to five cells including an egg cell, a central cell, zero to two lateral cells, and zero to one antipodal cell depending on the degeneration of the lateral or chalazal nuclei during megagametogenesis.

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.

Embryology of Epidendrum ibaguense. I. Ovule development

The orchids are unique among angiosperms in that ovule development is initiated after successful pollination. The monandrous orchid Epidendrum ibaguense has three placental ridges at anthesis. After pollination, mitotic activities result in the formation of a dichotomously branching system of outgrowths. The tip of each branch consists of five to six nucellar cells covered by the epidermis. A subterminal nucellar cell differentiates into the archesporial cell approximately 12 days after pollination. By day 18, it differentiates directly into a megasporocyte. The first meiotic cell division produces a dyad in which the micropylar cell begins prompt degeneration. The second meiotic cell division results in the formation of two megaspores of unequal size. The larger cell at the chalazal end will become the functional megaspore. Callose is present in the walls of the megasporocyte, the micropylar dyad cell, and the megaspore destined to degenerate. The development of the megagametophyte conforms to the Polygonum type. One of the chalazal nuclei delays its final mitotic division until fertilization, making it appear that only two antipodals are present. The mature ovules are bitegmic and have an anatropous orientation.

Megagametophyte development in Hordeum vulgare. 1. Early megagametogenesis and the nature of cell wall formation

Megagametophyte development in barley (Hordeum vulgare 'Atsel') was studied using Nomarski-interference optics and transmission electron microscopy. Stages described include the functional megaspore to cell wall formation. Aspects of the transition from the free nuclear stage of the embryo sac to the cellular embryo sac indicate involvement of elongate cell plates associated with clusters of microtubules. Initial cell walls among micropylar and chalazal nuclei are composed of beads derived from dictyosome vesicles. Fusion of growing cell plates occurs, especially within the antipodal apparatus.

A contribution to the embryology ofCirrhopetalum fimbriatum Lindl

The embryo sac ofCirrhopetalum fimbriatum is 6-nucleate, generally bisporic, and occasionally monosporic. The reduction of nuclei is due to the ‘strike’ phenomenon. The two chalazal nuclei receive the micropylar polar. The secondary nucleus is triploid, and comes to lie below the egg. The embryo (in ripe seed) is undifferentiated. The cellsca, m, n, andn′ contribute to the embryo. The suspensor is single-celled, hypertrophied and cap-like.

The development of the ovule and megagametophyte in Rhexia mariana

The ovules of Rhexia mariana are bitegmic and anatropous or rarely atropous. Periclinal divisions in the nucellar epidermis at the micropylar end produces a nucellar cap. A terminal pore in the nucellus is formed by the suppression of anticlinal divisions in the epidermis and subsequent separation of the cells during nucellar enlargement. A single hypodermal archesporium divides, producing a primary parietal cell and primary sporogenous cell. The primary parietal cell establishes a prominent parietal tissue as the primary sporogenous cell differentiates into the megasporocyte. Two prominent nucleoli that consistently appear in the archesporium persist in the primary sporogenous cell and the megasporocyte. Meiosis produces a tetrad of megaspores in either linear or approximately T-shaped arrangement. Cells of the nucellus adjacent to the sporogenous cell, megasporocyte, and tetrad rarely give rise to megagametophytes aposporically. The chalazal megaspore functions in megagametogenesis. Shortly after the first nuclear division, vacuoles migrate and coalesce between the nuclei. Two more nuclear divisions establish the four- and eight-nucleate megagametophytes. The chalazal nuclei are situated laterally and are noticeably smaller than the micropylar nuclei in both four- and eight-nucleate stages. The two polar nuclei remain in close contact, usually near the egg apparatus, but sometimes near the center of the megagametophyte. The three antipodal nuclei are ephemeral with degeneration completed by the time the egg apparatus is established. Orientation of the egg apparatus places the synergids on the side of the megagametophyte adjacent to the funiculus and the egg cell on the opposite side. The similarities and differences in ovule and megagametophyte development between Rhexia mariana and the tropical representatives of Melastomataceae previously investigated are discussed.

Embryological studies on Spiranthes australis Lindl.1

SUMMARY The flower comprises two trimerous whorls of perianth lobes, of which the labellum is supplied by four to six vascular strands and the rest by three bundles each. Some of the mesophyll cells become enlarged and contain raphides. The fertile stamen is quadrilocular. At the tetrad stage only the epidermis and the fibrous endothecium persist, while the two middle layers and tapetum disintegrate. The tapetal cells are uninucleate and the microspores remain together in tetrads. The pollen grains are bicelled at the time of shedding. The tricarpellary inferior ovary bears an indefinite number of bitegmic anatropus ovules arising from three parietal placentae. The hypodermal archesporial cell differentiates directly into the megaspore mother cell which gives rise to a linear tetrad of megaspores. The upper dyad cell rarely degenerates before the division is completed. In one instance a row of five cells was noticed. This might have arisen from two superposed megaspore mother cells or from an extra division of one of the megaspores. The chalazal megaspore functions. The mature embryo sac is usually six-nucleate, probably due to a suppression of the last division of the two chalazal nuclei. The single antipodal cell degenerates early and may not be seen in mature embryo sacs. The two polar nuclei may fuse to form the secondary nucleus or degenerate without fusion. Syngamy occurs, but triple fusion is delayed or is omitted altogether so that there is no endosperm formation. The first division of the egg is transverse. The mature embryo is ovoid in shape and undifferentiated. It has no suspensor haustoria and is enclosed within the seed coat formed by the outer integument. Various abnormalities have been recorded. Evidence is presented to indicate that parthenogenetic development of the egg is possible and does occur in some ovules. In such embryo sacs the undischarged pollen tube is usually seen beside the egg, but parthenogenetic embryos have also been observed in embryo sacs that had apparently not received a pollen tube at all. These observations have been confirmed by chromosome counts during division stages of the egg as well as in cells of the pro-embryo. Occasionally an embryo sac receives more than one pollen tube. Both the male nuclei of a pollen tube or the male nuclei of different pollen tubes may fertilize the egg, indicating the possibility of polyploid embryos.

MEGASPOROGENESIS AND DEVELOPMENT OF MEGAGAMETOPHYTE IN ULMUS

MEGASPOROGENESIS AND DEVELOPMENT OF MEGAGAMETOPHYTE IN ULMUS

Embryological Studies of Two Members of the Podostemaceae

1. This paper deals with some aspects of the morphology and gametogenesis of Lawia zeylanica and Griffithella hookeriana, members of the Podostemaceae. Some of the structural details of the two plants are described. 2. Microsporogenesis was investigated in Griffithella. The anthers are quadrilocular, inclosing paired pollen grains. At the shedding stage there is a lenticular generative cell and a large tube cell in each pollen grain. 3. Development of the female gametophyte was studied in both Lawia and Griffithella. The ovules are anatropous and bitegmic. The archesporial cell functions directly as the megaspore mother cell. The embryo-sac development corresponds to a reduced Allium type, in which there is reduction at the chalazal end, and consequently a complete elimination of the lower polar nucleus. 4. The mature embryo sac of L. zeylanica is usually four- or rarely five-nucleate, but the chalazal nucleus may soon degenerate and disappear, so that only four nuclei are seen in the older stages. Sometimes the chalazal nucleus may divide to give a six-nucleate embryo sac. 5. In G. hookeriana the mature embryo sac is four-nucleate because of early degeneration of the chalazal nucleus. A polar cell is formed during the development of the embryo sac. This may be an unusual feature among angiosperms. 6. A prominent multinucleate pseudo embryo sac is seen in both the species, resulting from the disintegration of the axial row of cells.

On the Life-History of Vanilla planifolia

1. In the orchid Vanilla planifolia the microspores do not remain united to form compound grains, massulae, or pollinia, but lie free inside the anther. The pollen grains are shed at the two-celled stage. 2. The nucellus shows a greater development than in most other orchids, and the nucellar cells at the chalazal end persist even in the mature seed. 3. The outer integument consists of four to six layers of cells which persist in the seed. In other orchids only the outermost layer is recognizable in the seed. 4. The inner integument is also two-to three-layered and, although flattened and crushed, remains distinguishable for a long time. 5. The embryo sac arises from the chalazal cell of a row of three cells, consisting of a micropylar undivided dyad cell and two megaspores. After the four-nucleate stage, the two chalazal nuclei do not divide further, resulting in a six-nucleate embryo sac. 6. Double fertilization takes place normally, but, since there is no lower polar nucleus, the primary endosperm nucl...

Osservazioni cito-ebriologiche sulCornus masL

Summary A comparative cyto-embryological study was carried out both in the fertile diploid (2n = 18) and in a sterile natural autotriploid (2n=3x=27) biotype of Cornus mas L. In the diploid individuals a quite normal meiosis occurs, functional ripe pollen grains are formed and an entbryo-sac development following the Euphorbia dulcis-type (improperly named Fritillaria-type)) is present. In the autotriploid biotype—on the contrary—a disturbed meiosis with a varying degree of chromosome pairing was observed. Generally 1–3 trivalents and 1–5 bivalents are formed at diacinesis; but some cases of asyndesis were also detected. As a consequence polycary is very frequent and the pollen grains are completely sterile. Embryo-sac development generally follows the Euphorbia dulcis-type (8,7 and 6-nucleate embryo-sac); more rarely 5-nucleate embryo-sacs of Clintonia-type and 7-nucleate embryo-sacs with three chalazal nuclei of sporic nature (Pyrethrum parthenifolium-type) are formed. Pre and post-meiotic abortion phen...

Embryo Sac and Fertilization in Cypripedium spectabile

1. The embryo sac of Cypripedium spectabile develops according to the Allium-type; and up to the 2-nucleate stage, my observations are in agreement with those of Pace. 2. The primary micropylar nucleus now divides twice to form the four micropylar nuclei, which typically organize into two synergids-the egg cell and the upper polar nucleus. The primary chalazal nucleus usually divides only once, but sometimes one or both of the resultant nuclei divide once again, resulting in 6-, 7-, or 8-nucleate embryo sacs of the kind seen in many other members of the Orchidaceae and certain of the Alismaceae (3). In all cases one nucleus from the chalazal group functions as the lower polar nucleus. The respective interpretations of Pace, Rutgers, and the writer are shown in figure 8. 3. No indication could be found in favor of Pace's supposition that during fertilization one synergid nucleus moves downward and enters into triple fusion with the polar nucleus and the second male gamete.

Megasporogenesis and Development of the Embryo Sac of Cypripedium parviflorum

1. The haploid number of chromosomes in Cypripedium parviflorum is ten rather than eleven, as previously reported. 2. Two potential megaspore nuclei enter into the formation of the embryo sac, making it bisporic, and placing this species of Cypripedium in the Scilla-type of Schnarf, or the Allium-type of Maheshwari. 3. After the 4-nucleate stage of the embryo sac, the two micropylar nuclei divide and two of the resulting four nuclei become synergids, one becoming the egg and the other the polar nucleus. The two chalazal nuclei do not divide and usually fuse. They may, however, simply lie in contact. At this stage the embryo sac contains either six or five nuclei, depending on whether the chalazal nuclei have fused. 4. The polar nucleus from the micropylar group usually migrates into contact with the fused or contiguous chalazal nuclei and may never actually fuse with them. 5. Double fertilization occurs. One sperm fuses with the egg and the second may unite with or lie in close contact with the chalazal g...

Macrosporogenesis and Development of the Embryo Sac of Lilium Henryi

1. The egg is removed from the macrospore mother cell of Lilium henryi by four divisions instead of three as has usually been described for members of this genus. 2. The nucleus of the macrospore mother cell and its daughter nuclei pass through the heterotypic and homoeotypic divisions respectively without cell division, so that the embryo sac is 4-nucleate. 3. Three of these nuclei pass to the chalazal end of the embryo sac; all four nuclei divide simultaneously. 4. The spindles of the three chalazal nuclei unite and a single bipolar spindle bearing 36 chromosomes is formed. 5. As a result of the third division the embryo sac is again 4-nucleate, two haploid nuclei being at the micropylar end and two triploid nuclei at the chalazal end. 6. Three of these four nuclei, the two with n chromosomes and one with 3n, pass through typical mitoses; the other chalazal nucleus divides in an abortive manner. 7. The cells of the embryo sac are formed as a result of cell-plate formation across the spindle of this last...

Studies in the familyAlismaceæ

The embryo sac ofSagittaria sagittifolia L. follows the same lines of development as described by me forLimnophyton andSagittaria guayanensis, and by Dahlgren forAlisma plantago, Damasonium alisma, Elisma natans, Echinodorus ranunculoides, E. macrophyllus andSagittaria sagittifolia. The only important fact that has been overlooked by Prof. Dahlgren is the division of the two chalazal nuclei of the four-nucleate embryo sac. Sometimes one, and rarely both of these divide, thus producing three or four nuclei in the chalazal end of the embryo sac respectively. One of these always functions as the lower polar nucleus while the remaining two or three are the antipodal nuclei which may sometimes organise into definite cells.

Early Development of Floral Organs and Embryonic Structures of Scrophularia marylandica

1. The order of the development of the floral parts of Scrophularia marylandica is calyx, stamens, corolla, and pistil. The stamens and the corolla arise from a common outgrowth. 2. The archesporium of the megaspore consists of a single hypodermal cell, which functions as a megaspore mother cell. 3. The megaspore mother cell by two successive divisions gives rise to an axial row of four potential megaspores; the embryo sac arises from the chalazal one, while the other three degenerate. 4. The mature embryo sac contains one egg, two large synergids, an endosperm nucleus, and three antipodal nuclei which soon degenerate. 5. A secondary endosperm nucleus, which grows larger as it migrates toward the egg, was observed. A polar nucleus from the chalazal end was seen to fuse with a polar nucleus from the micropylar end. 6. The first division of the fertilized egg is transverse, and is followed by a longitudinal division of the chalazal nucleus, while the other nucleus fails to divide until later. 7. The nucellu...