DEVELOPMENT OF THE OVULE AND MEGAGAMETOPHYTE IN SAXIFRAGA HIERACIFOLIA

Wiggins , Ira L. (Stanford U., Stanford, Calif.) Development of the ovule and megagametophyte in Saxifraga hieracifolia. Amer. Jour. Bot. 46(10): 692–697. Illus. 1059.—Buds of Saxifraga hieracifolia collected in the vicinity of Point Barrow, Alaska, fixed, sectioned, and stained by standard methods, revealed that the archesporial cell in the ovule of this species is hypodermal and gives rise to the megaspore mother cell and a small number of parietal cells. Occasionally 2 megaspore mother cells occur within an ovule. Meiosis in the megaspore mother cell produces a linear tetrad of megaspores, the chalazal one of which normally gives rise to a monosporic, Polygonum-type megagametophyte. The polar nuclei fuse near the chalazal end of the megagametophyte and the antipodal cells disintegrate prior to fertilization. A distinct filiform apparatus and a marked lateral “spur” develop on each synergid. Vacuolation in the egg cell and in the synergids follows the usual pattern. Only a single integument surrounds the nucellus.     

MEGASPOROGENESIS AND MEGAGAMETOGENESIS IN SOYBEAN,GLYCINE MAX

Megasporogenesis and megagametogenesis were examined in Glycine max with light, fluorescence, and electron microscopy. Megasporogenesis results in a linear tetrad of four megaspores. Megagametophyte development is of the Polygonum type, with the functional chalazal megaspore undergoing three successive mitotic divisions to produce an eight-nucleate, seven-celled mature megagametophyte. The central cell becomes packed with starch. At fertilization, the antipodals are degenerate, the polar nuclei have fused, starch is diminished, and the egg occupies most of the micropylar portion of the megagametophyte. Several pollen tubes were occasionally observed at each micropyle, yet only one was involved in fertilization. Pollen tube entry occurs through a slightly reduced, viable synergid cell. Endosperm development precedes embryo growth. These results describing normal development allow important comparison with genetic mutants of soybean that affect female fertility.     

EMBRYOLOGICAL STUDIES IN THE FAMILY SAXIFRAGACEAE (S.L.). I. DEVELOPMENT OF THE OVULE AND EMBRYO SAC IN SAXIFRAGA FORTUNEI VAR. PARTITA (MAKINO) NAKAI

The development of the ovule, megaspore and megagametophyte in Saxifraga fortunei var. partita (Makino) Nakai was observed. The ovule is anatropous, bitegmic, and crassinucellate. Both integuments originate from the epidermis. The archesporium is considered to be multicellular. The primary sporogenous cell functions as the megaspore mother cell which forms a T-shaped tetrad. The chalazal member of the megaspore tetrad is functional and develops into a Polygonum-type embryo sac. In the pyriform synergids the filiform apparatus is observed, but any hook or indentations could not be recognized. The antipodal cells are detectable until the Helobial endosperm undergoes several nuclear divisions. Secondary multiplication of the nuclei or the cells of the antipodals could not be observed.     

REPRODUCTIVE FEATURES OF DENTARIA LACINIATA AND D. DIPHYLLA (CRUCIFERAE), AND THE IMPLICATIONS IN THE TAXONOMY OF THE EASTERN NORTH AMERICAN DENTARIA COMPLEX

Reproductive features including ovule development, megasporogenesis, megagametogenesis, microsporogenesis, microgametogenesis, pollen tube growth, embryogeny, and natural seed germination were studied in a single population each of Dentaria laciniata Muhl. ex. Willd. and D. diphylla Michx. to test for possible agamospermy. The population of D. laciniata studied is sexual. The archesporial cell functions directly as the megasporocyte. It undergoes two meiotic divisions, but the micropylar cell of the dyad fails to undergo meiosis II, and a linear triplet of three cells is formed. The chalazal megaspore divides to form an eight-nucleate, seven-celled megagametophyte of the Polygonum type. Simultaneous cytokinesis follows the second meiotic division of the microsporocyte yielding a tetrahedral tetrad of microspores. A three-celled pollen grain is formed prior to anther dehiscence. Following apparent fertilization, the Capsella-variation of the Onagrad type of embryogeny results in a conduplicate embryo. Endosperm is initially nuclear, but eventually becomes cellular. Seeds readily germinate in nature. Similar events are documented in one population of D. diphylla up to the organization of the embryo-sac, which disintegrates before cellularization. These reproductive events and other data indicate that the eastern North American species of Dentaria may form a sexual polyploid complex with some sexual populations and some sterile ones.     

Cytological basis for a tetraspory in Cupressus sempervirens L. megagametogenesis and its implications in genetic studies

 The processes of megasporogenesis and early megagametogenesis were cytologically investigated in Cupressus sempervirens L. in order to elucidate, at the cellular level, the origin of the megagametophyte. After pollination, sporogenous tissue developed in the chalazal region of the nucellus, but only one megaspore mother cell differentiated and divided meiotically without cell-wall formation. This led to the development of a cell with four nuclei which directly functioned as a megaspore. The C. sempervirens megagametophyte is thus tetrasporic, in contrast to the majority of conifers where the megagametophyte is monosporic. The consequenses of this observation are discussed from a genetics point of view. Received: 15 August 1997 / Accepted: 19 September 1997     

THE USE OF A NEW CLEARING TECHNIQUE FOR THE STUDY OF EARLY OVULE DEVELOPMENT,MEGASPOROGENESIS, AND MEGAGAMETOGENESIS IN FIVE SPECIES OF CORNUS L.

The application of a new clearing technique proved successful for the study of ovule development, megasporogenesis, and megagametogenesis in Cornus asperifolia Michaux., C. stricta Lam., C. amomum Miller, C. florida L., and C. alternifolia L. These were the first studies of embryological features in C. asperifolia and C. stricta. Consistent features for both species include: slightly crassinucellate nucelli, megaspore mother cells, and equal dyads with the upper member obliquely aligned to the parietal cell. Alignment of nucelli of the 4-nucleate megagametophyte is not the same for both species. More than one mature 8-nucleate megagametophyte per nucellus was recorded for C. asperifolia. Embryological features not revealed in previous investigations of the other three species include: equal dyad cells for C. alternifolia and C. amomum, although disintegration of defunct megaspores is variable for both species; vacuole formation at the micropylar end at the 2-nucleate stage of C. amomum; a 4-nucleate stage in C. amomum and C. alternifolia with different alignments of nuclei; synergids that disintegrate rapidly in both species although antipodal shapes vary for each; and fusion of polar nuclei at varying locations within different species. A prominent hypostase is formed in all five species. Because of embryological features, it is recommended that C. forida be separated from the genus Cornus.     

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.     

A COMPARATIVE STUDY OF OVULE AND MEGAGAMETOPHYTE DEVELOPMENT IN FIELD-GROWN AND GREENHOUSE-GROWN PLANTS OF GLYCINE MAX AND PHASEOLUS AUREUS (PAPILIONACEAE)

A study of ovule and megagametophyte development in field- and greenhouse-grown plants of Glycine max (L.) Merrill and Phaseolus aureus Roxb. reveals several consistent features for both species. These features include: a multiple archesporium, enlargement of a primary sporogenous cell directly into a megasporocyte, production of unequal dyad cells, a functional chalazal megaspore, Polygonum-type development, and a hypostase. A filiform apparatus was not observed in either species. Several marked differences in development also occur. Phaseolus usually produces one sporogenous cell per ovule; Glycine produces 2–3 sporogenous cells per ovule. Meiosis II is synchronous in Phaseolus but nonsynchronous in Glycine. Linear tetrads are produced in Phaseolus, whereas linear and T-shaped tetrads are found in Glycine. Starch grains accumulate in the mature megagametophyte of Glycine but are absent at that stage in Phaseolus. The usefulness of the modified clearing fluid, benzyl benzoate-4½, for the study of ovule and megagametophyte development in Glycine max and Phaseolus aureus is here demonstrated. In addition, the study indicates for both species that megagametophyte development in plants grown under field conditions is markedly similar to development in plants grown in the more uniform conditions of the greenhouse. Accordingly, these findings suggest generally that embryological data collected from plants grown under greenhouse conditions will reflect those from plants found in nature.     

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.     

A NOMARSKI INTERFERENCE STUDY OF MEGASPOROGENESIS AND MEGAGAMETOGENESIS IN SMELOWSKIA CALYCINA (CRUCIFERAE)

Ovule development, megasporogenesis and megagametogenesis in an aneuploid population of the arctic-alpine crucifer, Smelowskia calycina, were examined to test for the possibility of apomictic seed production. Whole mounts of ovules cleared in Herr's “4½” clearing fluid were examined using Nomarski differential interference microscopy. The campylotropous ovule was bitegmic, with a micropyle formed by both integuments. The single archesporial cell of a crassinucellar nucellus functioned directly as a megasporocyte, dividing to form a linear tetrad of megaspores. The chalazal megaspore divided to form an 8-nucleate, 7-celled gametophyte of the Polygonum type, having hooked synergids with a well-developed filiform apparatus and polar nuclei that fused prior to fertilization. In the absence of any anomalous development indicative of agamospermy, seed production was assumed to be sexual.     

THE FEMALE GAMETOPHYTE OF MACHAERANTHERA PATTERSONII (ASTER PATTERSONII) AND OF M. TANACETIFOLIA

The nucellus of Machaeranthera pattersonii (A. Gray) Greene (Aster pattersonii A. Gray) contains only one megaspore mother cell, and the female gametophyte develops from the chalazal megaspore of a row of four, thus conforming to the Polygonum type of development. These observations are contrary to the older work of Palm. Three nuclear divisions produce the typical eight nuclei with the egg apparatus, primary endosperm cell with two polar nuclei, and two antipodal cells, the micropylar one containing two nuclei. Usually no more antipodal cells are formed, although there is further nuclear division, apparently followed by nuclear fusion. The antipodal cells remain about the same size without forming an antipodal haustorium. Cell division accompanies the first division of the primary endosperm nucleus. The early stages of the embryo resemble those of other Compositae. Machaeranthera tanacetifolia (HBK) Nees also shows the Polygonum type of development of the female gametophyte. It is suggested that Palm may have been working on some species of Erigeron that had been wrongly identified, which would account for the difference in observations.     

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.     

OVULE ONTOGENY,MEGASPOROGENESIS, AND EARLY GAMETOGENESIS IN TRIFOLIUM REPENS (PAPILIONACEAE)

To aid in understanding of the early events in seed development, surface topography observations with the scanning electron microscope can be coupled with new methods of clearing tissues for light microscopy study. These techniques reveal that two to four ovules begin development along the placental ridge as conduplication of the carpel proceeds in Trifolium repens L. A multicellular archesporium may develop giving rise to several sporogenous cells and ultimately to more than one megasporocyte. However, meiosis is completed in only one megasporocyte to give rise to a single linear tetrad of megaspores. The chalazal megaspore functions in megagametogenesis. Megasporogenesis and megagametogenesis progress as ovule ontogeny proceeds. The outer integument develops more rapidly than the inner and contributes to the final form of the campylotropous ovule. The most dramatic change in ovule form occurs as the tetrad develops and the functional spore enlarges and divides mitotically to produce the two-nucleate megagametophyte. It can be demonstrated that this early gametophyte develops faster than it is allowed to expand in the nucellar mass. This may in part explain why there is gametophyte failure and reduced seed set in clovers.     

STUDIES ON FOSSIL AZOLLA: PRIMITIVE TYPES OF MEGASPORES AND MASSULAE FROM THE CRETACEOUS

Fossil Salviniaceae are described from the Claggett Shale and Judith River Formation, late Cretaceous (Campanian stage) of Montana. A new genus, Parazolla, from the Claggett Shale, has megaspores in which the swimming apparatus is composed of a number of elongate floats attached to the megaspore body and invested by coiled hairs. The floats separate at maturity. Massulae (bearing microspores) have simple hair-like glochidia, many of which are knobbed at their tips. Glochidia tend to resemble the perisporial hairs of the megaspore body. This resemblance provides fossil evidence of the homology of these two hair-like structures among living species of Azolla. In Azolla simplex from the Judith River Formation the megaspore has a single cap-like so-called columellate float. Massulae, which have anchor-shaped glochidia, are associated with these megaspores. A. simplex is the oldest species of Azolla and Parazolla the oldest member of the Salviniaceae so far found.     

栽培甜菜大孢子发生的超微结构

栽培甜菜(Beta vulgaris)的大孢子发生为蓼型。减数分裂时, 大孢子母细胞核中出现核液泡, 形成联会复合体, 细胞壁上有胼胝质加厚, 并存在细胞质改组现象。大孢子母细胞减数第1次分裂形成二分体, 2个细胞均被较厚的胼胝质壁包裹。合点端的二分体细胞中细胞器丰富, 线粒体和质体的形态正常, 表明完成了再分化。在大多数情况下, 珠孔端的二分体细胞在减数第2次分裂前(或分裂的过程中)退化, 合点端的细胞分裂产生大小不等的2个细胞, 形成三分体。三分体合点端的大孢子体积较大, 发育成单倍体的功能大孢子。     

Megasporogenesis and programmed cell death in <Emphasis Type="Italic">Tillandsia</Emphasis> (Bromeliaceae)

The degeneration of three of four meiotic products is a very common process in the female gender of oogamous eukaryotes. In Tillandsia (and many other angiosperms), the surviving megaspore has a callose-free wall in chalazal position while the other three megaspores are completely embedded in callose. Therefore, nutrients and signals can reach more easily the functional megaspore from the nucellus through the chalazal pole with respect to the other megaspores. The abortion of three of four megaspores was already recognized as the result of a programmed cell death (PCD) process. We investigated the process to understand the modality of this specific type of PCD and its relationship to the asymmetric callose deposition around the tetrad. The decision on which of the four megaspores will be the supernumerary megaspores in angiosperms, and hence destined to undergo programmed cell death, appears to be linked to the callose layer deposition around the tetrad. During supernumerary megaspores degeneration, events leading to the deletion of the cells do not appear to belong to a single type of cell death. The first morphological signs are typical of autophagy, including the formation of autophagosomes. The TUNEL positivity and a change in morphology of mitochondria and chloroplasts indicate the passage to an apoptotic-like PCD phase, while the cellular remnants undergo a final process resembling at least partially (ER swelling) necrotic morphological syndromes, eventually leading to a mainly lipidic cell corpse still separated from the functional megaspore by a callose layer.     

栽培甜菜大孢子发生的超微结构

栽培甜菜（Beta vulgaris）的大孢子发生为蓼型。减数分裂时,大孢子母细胞核中出现核液泡,形成联会复合体,细胞壁上有胼胝质加厚,并存在细胞质改组现象。大孢子母细胞减数第1次分裂形成二分体,2个细胞均被较厚的胼胝质壁包裹。合点端的二分体细胞中细胞器丰富,线粒体和质体的形态正常,表明完成了再分化。在大多数情况下,珠孔端的二分体细胞在减数第2次分裂前（或分裂的过程中）退化,合点端的细胞分裂产生大小不等的2个细胞,形成三分体。三分体合点端的大孢子体积较大,发育成单倍体的功能大孢子。     

竹节参雌配子体发育的研究

本文报道了竹节参(Panax japonicus C.A.Mey)雌配子体(胚囊)的发育过程。竹节参大孢子母细胞减数分裂产生线形排列的大孢子四分体。胚囊发育属蓼型,由合点端大孢子发育而成。游离核胚囊时期,胚囊珠孔端的细胞器种类和数量都较胚囊合点端多;胚囊合点端相邻的珠被细胞中有含淀粉粒的小质体,与胚囊珠孔端相邻的退化中的非功能大孢子中则有含淀粉粒的大质体和大类脂体。成熟胚囊中,反足细胞较早退化;极核融合成次生核;卵细胞高度液泡化,细胞器数量较少;助细胞则有丰富的细胞器和发达的丝状器。PAS反应表明,受精前的成熟胚囊中积累淀粉粒。次生核受精后,很快分裂产生胚乳游离核,到几十至数百个核时形成胚乳细胞。卵细胞受精后则要经过较长的休眠期。     

MECHANISMS OF APOMIXIS IN ELYMUS RECTISETUS FROM EASTERN AUSTRALIA AND NEW ZEALAND

Megasporogenesis was examined in cleared ovaries of 23 accessions of hexaploid Elymus from southeastern Queensland, northeastern New South Wales, the Australian Capital Territory, and the South Island of New Zealand. Apomixis was confined to the 17 accessions that morphologically corresponded to E. rectisetus (Nees in Lehm.) Löve et Connor. Female meiotic development followed the Polygonum type. Apomeiotic development was delayed relative to meiotic development because of a lengthy period of MMC vacuolation and nuclear stretching that occurred in place of meiosis I. Amitosis was evident in up to possibly five percent of the MMC's during nuclear stretching. A subsequent mitotic division facultatively functioned as meiosis II or the first embryo-sac mitosis to yield a 2n megaspore dyad, a hemidyad with an incomplete crosswall, or a directly binucleate embryo sac. Nuclear stretching generally resumed in the chalazal daughter nucleus from the apomeiotic division, but was not seen later in embryo sac development. When a dyad formed, its chalazal member would enlarge and develop into the embryo sac. The organized embryo sac was of the conventional eight-nucleate, seven-celled structure prior to antipodal proliferation, regardless of meiotic or apomeiotic origin. Microsporocyte meiosis was normal in both sexuals and apomicts. Deposition of a slightly birefringent substance, possibly callose, was deficient around megasporocytes, megaspores, and microsporocytes in the apomicts.