Chalazal Megaspore Research Articles

Development and structure of the female gametophyte in Austrobaileya scandens (Austrobaileyaceae)

Austrobaileyales, comprising the four families Austrobaileyaceae, Trimeniaceae, Schisandraceae, and Illiciaceae, are included in the basal angiosperms along with Amborellaceae and Nymphaeaceae. Here, we present the first developmental study of the female gametophyte in Austrobaileya scandens, the only species of Austrobaileyaceae, which are sister to the rest of the Austrobaileyales. Austrobaileya scandens has a four-celled/four-nucleate embryo sac as in the derived families of the order, e.g., Illiciaceae and Schisandraceae. It is monosporic, with the chalazal megaspore of a tetrad developing into the embryo sac composed of an egg cell, two synergids, and one polar nucleus. This mode of embryo sac formation was first reported in Schisandra over 40 years ago and should now be established as the Schisandra type. Its occurrence in A. scandens shows that the Schisandra-type embryo sac is likely common to the whole Austrobaileyales as well as to Nymphaeaceae. Amborellaceae were recently reported to have an eight-celled/nine-nucleate embryo sac, clarifying that none of the basal angiosperms has the seven-celled/eight-nucleate Polygonum-type embryo sac found in the majority of angiosperms, and that the Polygonum-type embryo sac represents a derived character state in angiosperms.

Microsporogenesis, Megasporogenesis, and the Development of Male and Female Gametophytes in Eustoma grandiflorum

Embryological characters during microsporogenesis, megasporogenesis, and the development of male and female gametophytes in Eustoma grandiflorum were observed by microscopy. The results are as follows. 1. The formation of anther walls was of the dicotyledonous type. The tapetum was of the heteromorphic and glandular type. Tapetal cells on the connective side elongated radially. 2. Cytokinesis in microsporocyte meiosis was of the simultaneous type and microspore tetrads were tetrahedral. 3. Mature pollen grains were 2-celled and had 3 germ furrows. 4. The ovary was bicarpellary syncarpous and unilocular, having parietal placentas. Ovules were numerous and anatropous. 5. The archespore under the nucellar epidermis directly developed a megaspore mother cell, which in turn underwent meiotic division to form 4 megaspores arranged in a line or T-shape. The chalazal megaspore was observed to be functional. 6. The formation of the embryo sac was of the polygonum type. Before fertilization, the 2 polar nuclei fused into a secondary nucleus. The mature embryo sac was made up of 7 cells. 7. It was found at a very low rate that there were 2 megasporocytes or 2 embryo sacs in an ovule.

Cytological and histological studies on reproductive system of hexaploid Bellevalia edirnensis Ozhatay & Mathew (hyacinthaceae).

In this study anther wall, meiosis, sporogenesis and the development of female gametophytes of Bellevalia edirnensis were examined cytologically and histologically. The young anther wall consists of an epidermis, a single layered fibrous endothecium, 1-2 middle layers which disappear early, and a single-layered tapetum. In the glandular tapetum; normal mitosis, the "sticky" type of division and secondary nucleus divisions were observed. In most microspore mother cells the course of meiosis is regular. In some cells irregularities were observed including chromosome bridges and univalents. Cytokinesis is of the successive type and the tetrads are isobilateral. The ovules are anatrapous, bitegmic and crassinucellate. The micropyle is formed by the inner integument. The female archesporium is unicelled and directly functions as the megaspore mother cell. The female gametophyte formed by the functioning chalazal megaspore is monosporic, of the Polygonum type.

Embryology of Macroptilium arenarium (Leguminosae)

Macroptilium arenarium (Bacigalupo) S.I.Drewes & R.A.Palacios produces two floral morphs, aerial chasmogamous flowers and cleistogamous flowers in geophyte racemes. A comparative study of the sporogenesis, gametogenesis and the development of the related sporophytic structures in both floral morphs is reported. The anther is tetrasporangiate, its wall consists of epidermis, endothecium, one or two middle layers and an uninucleate secretory tapetum. The mature endothecium presents fibrilar thickenings that are more developed in cleistogamous flowers. Pollen grains are tricolporate, angulaperturate, and are shed at bicellular stage. The ovule is crassinucelate, bitegmic and anacampylotropous. Megaspore tetrads with linear arrangement have been observed in chasmogamous flowers, whereas only megaspore dyads have been found in cleistogamous flowers. In both floral morphs the chalazal megaspore develops into an embryo sac of Polygonum type. Apomixis is considered as a possible replacement for sexual reproduction in cleistogamous flowers.

Structure and development of the ovule and seed in Aristolochiaceae, with particular reference to Saruma

All members of Aristolochiaceae have anatropous, bitegmic, crassinucellate ovules, which are endostomic except in Saruma and Asarum arifolium where ovules are amphistomic. The outer integument is two cell-layered and the inner integument is three cell-layered. The chalazal megaspore is the functional one. All these conditions appear to be plesiomorphic for the order Piperales, which consists of five families, Aristolochiaceae, Hydnoraceae, Lactoridaceae, Piperaceae and Saururaceae. The embryo sac in Aristolochiaceae is eight-nucleate and corresponds to the Polygonum type; a hypostase is frequently present in this family. The seed coat of Aristolochia s.l., Asarum, Saruma and some Thottea species consists primarily of a two cell-layered testa, and a three cell-layered tegmen. In some species the cells of the outer epidermis become radially elongated, forming reticulate wall thickenings. Cells of the inner layer of the testa have crystals and thickened inner walls. The three layers of the tegmen are tangentially elongated, and become cross fibres at maturity, as fibres of the outer and inner layers are parallel to the seed axis, whereas those of the middle layer are perpendicular to it. This type of seed coat anatomy is synapomorphic for Aristolochiaceae. In addition, the gross morphology of the seed and elaiosome histology are remarkably similar in Asarum and Saruma, thus supporting a sister-group relationship between them. Embryological and seed characters do not supply any synapomorphy that support a close relationship between Aristolochiaceae, Hydnoraceae and Lactoridaceae. Instead, some seed features such as the absence of seed appendages and the collapsed cells of endotesta may indicate a close relationship of Lactoris with Piperaceae plus Saururaceae, although this is the subject of further analysis.

Development and ultrastructure of the megagametophyte in Passiflora caerulea L. (Passifloraceae)

Megasporogenesis and megagametogenesis of Passiflora caerulea L. were studied using light and transmission electron microscopy. The archesporial tissue is generally formed by one cell. The megaspore mother cell gives rise to a linear tetrad of megaspores. The chalazal megaspore is the functional one, and originates a Polygonum-type female gametophyte. The antipodals are ephemeral. Abundant starch is found in the nucellar cells, specially the ones adjacent to the megagametophyte. The two synergids show ultrastructural differences, involving the filiform apparatus, the nucleolus and the endoplasmic reticulum; these differences suggest a functional differentiation, probably related to the reception of the pollen tube. This is the first report in angiosperms of substantial morphological differences between the two synergids. © 2003 The Linnean Society of London, Botanical Journal of the Linnean Society, 2003, 142, 73–81.

Ovule ontogeny of Tabebuia pulcherrima Sandwith (Bignoniaceae): embryo sac development

The chalazal megaspore develops in a Polygonum-type embryo sac. The amyloplast-rich endothelium is partially degraded during the expansion of the micropylar portion of the megagametophyte. Organization of the mature embryo sac is determined by the patterns of vacuolation, nuclear migration, spindle orientation and cellularization. The egg cell is slightly chalazal in relation to the synergids, and its micropylar end does not touch the micropylar channel. At the chalazal pole of the egg apparatus, the common walls between the synergids, the egg and central cells, despite their tenuity, are present in the mature megagametophyte. The polar nuclei do not fuse before fertilization and the antipodals are persistent until the first stages of endosperm formation. The taxonomic significance of some embryological characters for the Bignoniaceae is discussed.

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.

Embryology of Clitoria ternatea (Fabaceae)

ABSTRACT The embryology of Clitoria ternatea was studied. Anthers contain four sporangia. The anther wall comprises an epidermis, an endothecium, a middle layer and a glandular tapetum. Microspore tetrads are tetrahedral and pollen grains are shed at the 2-cell stage. The ovule is campylotropous, bitegmic and crassinucellate. The micropyle is formed by both the integuments. The megaspore tetrad is linear or T-shaped. The chalazal megaspore is functional and embryo sac development follows the monosporic Polygonum type. Endosperm development is of the nuclear type. The chalazal part of the endosperm forms a haustorium. Embryo development follows the Onagrad type.

Desarrollo del gametofito femenino de Tagetes patula (Asteraceae)

La forma común de reproducción en el género Tagetes es sexual, a través de semilla. Sin embargo, recientes estudios morfológicos y de hibridación realizados en la especie Tagetes patula L. sugieren la presencia de apomixis (formación de semillas sin fecundación). Con la finalidad de esclarecer el origen sexual o apomíctico del embrión, el presente estudio describe la megasporogénesis, megagametogénesis y desarrollo temprano del embrión. Tagetes patula L. tiene el óvulo típico de la familia Asteraceae: anátropo, unitégmico, tenuinucelado y de placentación basal. Una célula arquesporial hipodermal se desarrolla directamente como la célula madre de la megaspora. La megasporogénesis es normal (división meiótica, con formación de tétrade) y el saco embrional se desarrolla de la megaspora calazal. El saco embrional es de tipo Polygonum. Las flores femeninas (radiales) muestran irregularidades del megagametofito como: formación de más de ocho núcleos, polaridad invertida, y diferenciación incompleta de células del megagametofito en flores maduras. Tales irregularidades no representan una evidencia contundente de la reproducción apomíctica.

Megagametophyte Development, Fertilization, and Cytoplasmic Inheritance inTaxus brevifolia

Taxus brevifolia is a tree of evolutionary and medicinal importance. This study describes the megagametophyte development, archegonial development, and fertilization in this species. In Victoria, British Columbia, megasporogenesis and free nuclear mitosis within the megagametophyte occur anytime between early February until the end of June. Within the differentiated sporogenesis tissue, a megaspore mother cell develops and undergoes meiosis, usually forming a linear tetrad of megaspores. Though up to three of the megaspores may be functional, the chalazal megaspore develops faster than the others and is usually the functional megaspore. Cellularization of the megagametophyte begins in mid‐April and continues until early June. The presence of an ephemeral ventral canal nucleus was confirmed. Fertilization was observed in June 1996. The mature egg cell cytoplasm and sperm structure are used to infer paternal inheritance of plastids and biparental inheritance of mitochondria. This anatomic and developmental study supports the inclusion of Taxus in the order Coniferales.

Floral morphology and embryo sac development in Burretiodendron kydiifolium Y. C. Hsu et R. Zhuge (Tiliaceae) and their systematic significance

Floral morphology and development of megaspore and megagametophyte in Burretiodendron kydiifolium are described. The ovule is anatropous, crassinucellate and bitegmic. The archesporium is multicellular and usually two cells develop to megasporocytes. The tetrad is linear or very rarely T-shaped. The chalazal megaspore functions and develops into a Polygonum-type of embryo sac. The antipodals degenerate very early and the fertilizable embryo sac consists of only one egg cell, two synergids and two polar nuclei. In a fertilizable embryo sac the micropyle is formed by both outer and inner integuments, which are three to four and five cells thick, respectively. The epidermal cells of the nucellus divide to form a nucellar cap. A group of special cells with large nuclei and less staining are observed below the sporogeneous cells. Comparison of the embryological features of Bumtiodendmn S.S. with those of Tiliaceae indicates that this genus differs greatly from other members of Tiliaceae and no close relationship is established among Burretiodendmn and other tiliaceous taxa. Most embryological features of Bunetiodendmn are shared by the genus Ptmspennum of Sterculiaceae although they differ considerably in gross morphology.

Floral morphology and embryo sac development inBurretiodendron kydiifoliumY. C. Hsu et R. Zhuge (Tiliaceae) and their systematic significance

Floral morphology and development of megaspore and megagametophyte inBurretiodendron kydiifoliumare described. The ovule is anatropous, crassinucellate and bitegmic. The archesporium is multicellular and usually two cells develop to megasporocytes. The tetrad is linear or very rarely T-shaped. The chalazal megaspore functions and develops into a Polygonum-type of embryo sac. The antipodals degenerate very early and the fertilizable embryo sac consists of only one egg cell, two synergids and two polar nuclei. In a fertilizable embryo sac the micropyle is formed by both outer and inner integuments, which are three to four and five cells thick, respectively. The epidermal cells of the nucellus divide to form a nucellar cap. A group of special cells with large nuclei and less staining are observed below the sporogeneous cells. Comparison of the embryological features ofBurretiodendron s.s.with those of Tiliaceae indicates that this genus differs greatly from other members of Tiliaceae and no close relationship is established amongBurretiodendronand other tiliaceous taxa. Most embryological features ofBurretiodendronare shared by the genusPterospermumof Sterculiaceae although they differ considerably in gross morphology.

Cytological and Cytoembryological Studies on Paeonia tenuifolia L.

The cytology and cytoembryology of Paeonia tenuifolia L. (Paeoniaceae) at Ortakçi Village near Edirne during the years 1989-1991 are considered in this study. Chromosome counts were done on adventive roots, and the results were found to be 2n=10. In the karyotype, 3 median (M), 1 submedian (SM) and 1 subterminal (ST) chromosome pairs were found, in two of which SAT-chromosomes were detected. The anther wall of P. tenuifolia consists of an epidermis , a 1-3 layered endothecium, 1-2 middle layers and a 1-2 layered tapetum. In the secretory tapetum, normal mitosis and the ÒstickyÓ type of division were observed. In most microspore mother cells, the course of meiosis is regular. In some cells irregularities were observed, including chromosome bridges, univalent and fragments. Microspore tetrads are isobilateral and tetrahedral, polen grains are tricalporate and are shed at the 2 celled stage. In rare cases, polyspory and multinucleate pollen grains were observed. The course of meiosis in the megaspore mother cell is regular. The ovules are anatrapous, bitegmic and crassinucleate. The female archesporium is multicelled. The female gametophyte formed by the functioning chalazal megaspore is monosporic, of the Polygonum type, in the mature embryo sac, the egg apparatus, the normal appearence of the secondary nucleus and of the antipodal cells. The occurence of double and triple embryo sacs due to the further development of chalazal megaspores of double and triple tetrads was observed.

Ultrastructure of Megasporogenesis and Early Megagametogenesis in Arundo formosana Hack. (Poaceae)

The ovule of Arundo formosana Hack. is tenuinucellate, the megasporocyte originating from a hypodermal cell of the ovule primordium. Embryo sac development is of the Polygonum type. At its inception, the megasporocyte appears to be identical to the surrounding nucellar cells except for its larger size and dominant nucleus. The onset of megasporogenesis is indicated by the expansion of the megasporocyte, which is accompanied by a decrease in cytoplasmic density. Callose deposition in the expanded megasporocyte originates from the micropylar end and extends laterally. During megasporocyte expansion, the nucleus retains the micropylar location. Meiosis results in the formation of a linear, T-shaped, or intermediate tetrad: the chalazal megaspore is functional; all others will degenerate. The functional megaspore appears least dense among the cells of the tetrad and seldom contains vacuoles. As the megaspore enlarges, the organelles and the cytoplasmic density increase, the callose wall is gradually digested, and a electron-dense wall appears. The vacuoles are randomly distributed and then gradually migrate to both poles of the megaspore. A well-developed megaspore shows organelle polarity such that most plastids and dictyosomes are distributed in the micropylar region. The polarized dictyosome and plastid distributions persist through the two- and four-nucleate stage of the embryo sac, respectively. The two- and four-nucleate embryo sac is characterized by the presence of large central and chalazal vacuoles. Plasmodesmata persist in the chalazal wall during megasporogenesis and megagametogenesis to the four-nucleate stage.

The embyology and relationship of the Byblidaceae

The relationships of the Byblidaceae have been the subject of a number of recent molecular phylogenetic studies where their traditional relationships with the Roridulaceae and other members of the Rosidae have been overturned in favour of affinities with the Asteridae, in particular to the Lentibulariaceae in the Lamiales. Although the embryological relationships between these families were the subject of an earlier study, the data for the Byblidaceae were incomplete. The family has tetrasporangiate, bilocular anthers with a glandular bi-nucleate tapetum. Formation of the anther wall appears to be of the Dicotyledonous type, and the anthers have ephemeral middle layers and apical fibrous thickenings. Simultaneous microsporogenesis results in tetrahedral tetrads of bi-nucleate pollen grains. The ovules are anatropous, unitegmic and tenuinucellate. Megasporogenesis is direct from the archesporal cell, which in Byblis liniflora Salisb. divides to produce a linear tetrad from which a chalazal megaspore is derived (contrary to a report of a micropylar megaspore in B. gigantea Lindl.). Megagametogenesis conforms to the Polygonum type, and the antipodal cells are persistent. The endosperm is ab initio Cellular and there are well developed micropylar and chalazal haustoria. Embryogenesis appears to conform to the Onagrad type noted for several of the other members of the Lamiales s.l., including the Lentibulariaceae, to which the Byblidaceae have been related in recent molecular studies.

Ultrastructural features of megasporogenesis inTorreya nucifera (Taxaceae)

In megasporogenesis ofTorreya nucifera (Taxaceae) more than one product of meiosis can start to germinate, a process previously observed inTaxus. Ultrastructural analysis ofT. nucifera revealed that this behaviour is cytologically determined by the presence of a transfer cell type wall-membrane apparatus and by the presence of uncommonly complex plasmalemmasomes in the megasporocyte. These features may determine a uniform supply of nutrients to the entire megasporocyte cytoplasm, so that after meiosis the germination potential is not restricted to the chalazal megaspore. Even if more than one product of meiosis can originate the female gametophyte in bothTaxus andTorreya, this process involves different ultrastructural aspects in the two species. In conclusion, the extension of the germination potential is a common characteristic to both species but it is most probably the result of some evolutionary convergence.

A contribution to the embryology of Desmodium motorium (Houtt.) Merr. (= D. gyrans (L.f.) DC.) (Fabaceae)

Abstract The anthers are tetrasporangiate. The anther wall comprises epidermis, fibrous endothecium, middle layer and tapetal layer. The tapetum is of the Glandular type and its cells remain uninucleate. Meiosis in pollen mother cells is normal and simultaneous cytokinesis leads to the formation of tetrahedral and decussate microspore tetrads. The pollen grains are shed at 2-celled stage. The ovule is campylotropous, bitegmic and crassinucellate. Meiosis in megaspore mother cell results in the formation of linear or occasionally T-shaped megaspore tetrad. The chalazal megaspore develops into Monosporic Polygonum type of embryo sac. Endosperm development is of the Nuclear type.

Ovule Development and Determination of Seed Number Per Pod in Oilseed Rape (Brassica napusL.)

Seed number per pod at maturity over the terminal raceme of single plants of oilseed rape is closely correlated to the percentage of ovules with complete embryo sacs (ovule fertility) at flower opening. Approximately one-third of the ovules did not contain an embryo sac and sterility, due to the absence of embryo sac, accounted for most of the difference between the numbers of ovules and seeds. Within the terminal raceme, both a decreased proportion of fertile ovules and a lower number of ovules per ovary in apical flowers contributed to the lower number of seeds per pod in the mature apical pods compared to the basal ones. A study of ovule development before flower opening showed that differences in the differentiation of the embryo sacs arose before the buds were 40 mm long and probably involved the stages of meiosis II and/or differentiation of the chalazal megaspore.

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

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.