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.     

COMPARATIVE MEGASPOROGENESIS IN PAPILIONACEAE

Megasporogenesis in 16 species of Papilionaceae has been investigated. Archesporial development in Papilionaceae is usually hypodermal, but subhypodermal origin has been observed. The archesporial cell develops directly into the megasporocyte, or divides to produce a parietal cell and a primary sporogenous cell. The parietal cell is always oriented toward the micropylar end of the developing ovule. It may or may not divide to contribute to the nucellar mass as the primary sporogenous cell enlarges to form the megasporocyte. A single megasporocyte is produced in most species investigated. Of those species that form more than one megasporocyte, few develop more than one tetrad of megaspores, and in no case is more than one megagametophyte produced. Meiosis occurs in the megasporocyte to form a tetrad of megaspores. Several variations in tetrad patterns are explored here. Monosporic development is the prevalent condition in Papilionaceae; however, bisporic development does occur.     

Cytological and histological studies on female gametophyte of Leucojum aestivum (Amaryllidaceae)

In this study, gynoeceum, development of megasporangium, megasporogenesis, megagametogenesis and female gametophyte of Leucojum aestivum were examined cytologically and histologically. Ovules of L. aestivum are of anatropous, bitegmic and crassinucellate type. Inner integument forms the micropyle. Archesporial cell develops directly into a megasporocyte. Embryo sac development is of bisporic Allium type. Filiform apparatus is observed in synergids. Polar nuclei fuse before fertilization to form secondary nucleus near the antipodals.     

小盐芥大孢子发生和雌配子体发育

本论文研究了小盐芥(Thellungiella halophila)大孢子发生和雌配子体发育过程及该阶段与花蕾、花、果实外部形态的相关性。结果如下: 小盐芥雌蕊由2心皮组成, 侧膜胎座, 每室胚珠多数, 弯生, 双珠被, 薄珠心。孢原细胞位于珠心表皮之下, 直接起大孢子母细胞的功能。大孢子四分体线形排列, 合点端大孢子为功能大孢子, 胚囊发育为蓼型。     

小蓬草的胚胎学研究

对小蓬草(Conyza canadensis)大小孢子发生、雌雄配子体形成、受精、胚及胚乳发育过程进行了研究,主要结果如下:花药四室,药壁由表皮、药室内壁、中层和绒毡层组成.表皮退化;药室内壁宿存,细胞柱状伸长,纤维状加厚;中层细胞退化较早,在小孢子母细胞减数分裂开始时仅存残迹;绒毡层于小孢子母细胞减数第一次分裂前期开始原位变形退化,属于腺质型绒毡层;小孢子母细胞减数分裂为同时型,四分体的排列方式主要为四面体形和左右对称形;成熟花粉粒多为3细-胞花粉粒,偶见2细-胞花粉粒.子房下位,2心皮,1室,单胚珠,基生胎座;单珠被,薄珠心,倒生胚珠,具发达的珠被绒毡层.珠心表皮下分化出大孢子孢原细胞,孢原细胞直接发育为大孢子母细胞,大孢子母细胞减数分裂形成4个大孢子直线形排列,仅合点端的大孢子发育成功能大孢子母细胞,胚囊发育为蓼型.两个极核在受精前融合为次生核,珠孔受精.胚乳发育属于核型,胚胎发育为紫菀型;具胚乳吸器.     

小盐芥大孢子发生和雌配子体发育

本论文研究了小盐芥（Thellungiella halophila）大孢子发生和雌配子体发育过程及该阶段与花蕾、花、果实外部形态的相关性。结果如下：小盐芥雌蕊由2心皮组成,侧膜胎座,每室胚珠多数,弯生,双珠被,薄珠心。孢原细胞位于珠心表皮之下,直接起大孢子母细胞的功能。大孢子四分体线形排列,合点端大孢子为功能大孢子,胚囊发育为蓼型。     

小蓬草的胚胎学研究

对小蓬草（Conyzacanadensis）大小孢子发生、雌雄配子体形成、受精、胚及胚乳发育过程进行了研究,主要结果如下：花药四室,药壁由表皮、药室内壁、中层和绒毡层组成。表皮退化;药室内壁宿存,细胞柱状伸长,纤维状加厚;中层细胞退化较早,在小孢子母细胞减数分裂开始时仅存残迹;绒毡层于小孢子母细胞减数第一次分裂前期开始原位变形退化,属于腺质型绒毡层;小孢子母细胞减数分裂为同时型,四分体的排列方式主要为四面体形和左右对称形;成熟花粉粒多为3-细胞花粉粒,偶见2-细胞花粉粒。子房下位,2心皮,1室,单胚珠,基生胎座;单珠被,薄珠心,倒生胚珠,具发达的珠被绒毡层。珠心表皮下分化出大孢子孢原细胞,孢原细胞直接发育为大孢子母细胞,大孢子母细胞减数分裂形成4个大孢子直线形排列,仅合点端的大孢子发育成功能大孢子母细胞,胚囊发育为蓼型。两个极核在受精前融合为次生核,珠孔受精。胚乳发育属于核型,胚胎发育为紫菀型;具胚乳吸器。     

The Mac1 Gene: Controlling the Commitment to the Meiotic Pathway in Maize

The switch from the vegetative to the reproductive pathway of development in flowering plants requires the commitment of the subepidermal cells of the ovules and anthers to enter the meiotic pathway. These cells, the hypodermal cells, either directly or indirectly form the archesporial cells that, in turn, differentiate into the megasporocytes and microsporocytes. We have isolated a recessive pleiotropic mutation that we have termed multiple archesporial cells1 (mac1) and located it to the short arm of chromosome 10. Its cytological phenotype suggests that this locus plays an important role in the switch of the hypodermal cells from the vegetative to the meiotic (sporogenous) pathway in maize ovules. During normal ovule development in maize, only a single hypodermal cell develops into an archesporial cell and this differentiates into the single megasporocyte. In mac1 mutant ovules several hypodermal cells develop into archesporial cells, and the resulting megasporocytes undergo a normal meiosis. More than one megaspore survives in the tetrad and more than one embryo sac is formed in each ovule. Ears on mutant plants show partial sterility resulting from abnormalities in megaspore differentiation and embryo sac formation. The sporophytic expression of this gene is therefore also important for normal female gametophyte development.     

RELATIONSHIP BETWEEN MALE AND FEMALE GAMETOPHYTE DEVELOPMENT IN RYE

The development of male gametophyte and female gametophyte within a floret of rye (Secale cereale L.) was examined. Generally, meiosis in microsporocytes and in megasporocytes occurs simultaneously in most florets, but the period from zygotene to tetrad meiosis in the megasporocyte progresses more slowly than that in the microsporocyte. When the female gametophyte has one nucleus and no vacuoles, the male gametophyte has a single, eccentric nucleus. By the time the female gametophyte develops to the vacuolated one-, two-, four-, and eight-nucleate stages and to the growth and differentiation of the egg apparatus stage, the male gametophyte reaches the two-celled pollen stage. As the female gametophyte matures, the male gametophyte also reaches maturity. The duration of male gametophyte development from microspore mother cell and the duration of female gametophyte development from megaspore mother cell are the same in most florets. The relationship between sexual development of cross-pollinated rye is similar to that of self-pollinated wheat (Triticum aestivum L.). It seems that the relationship is not related to the breeding system.     

Cross talk between the sporophyte and the megagametophyte during ovule development

In seed plant ovules, the diploid maternal sporophytic generation embeds and sustains the haploid generation (the female gametophyte); thus, two independent generations coexist in a single organ. Many independent studies on Arabidopsis ovule mutants suggest that embryo sac development requires highly synchronized morphogenesis of the maternal sporophyte surrounding the gametophyte, since megagametogenesis is severely perturbed in most of the known sporophytic ovule development mutants. Which are the messenger molecules involved in the haploid–diploid dialogue? And furthermore, is this one way communication or is a feedback cross talk? In this review, we discuss genetic and molecular evidences supporting the presence of a cross talk between the two generations, starting from the first studies regarding ovule development and ending to the recently sporophytic identified genes whose expression is strictly controlled by the haploid gametophytic generation. We will mainly focus on Arabidopsis studies since it is the species more widely studied for this aspect. Furthermore, possible candidate molecules involved in the diploid–haploid generations dialogue will be presented and discussed.     

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.     

星星草大、小孢子发生与雌、雄配子体发育的观察

利用常规石蜡制片法研究了星星草[Puccinellia tenutiflora(Griseb．)Scribn．et Merr．]大、小孢子发生及其雌、雄配子体的发育过程。主要结论是：(1)小孢子母细胞减数分裂过程中的胞质分裂为连续型,四分孢子为左右对称型;(2)成熟的花粉为三细胞型,具单萌发孔;(3)花药壁由4层结构组成,最外层为表皮,其内分别为药室内壁、中层、绒毡层,绒毡层为分泌型,花药壁的发育属于单子叶型;(4)星星草为单子房,单胚珠,双珠被,薄珠心,倒生型胚珠。大孢子母细胞经减数分裂形成线形排列的4个大孢子,合点端大孢子具功能;(5)胚囊发育属于蓼型,成熟胚囊形成时,反足细胞经无丝分裂形成4～6个反足细胞,反足细胞内可能存在多次DNA复制过程。     

短柄五加大,小孢子发生和雌,雄配子体发育的研究

短柄五加花药５枚,每个花药四个花粉囊。小孢子母细胞减数分裂时,胞质分裂为同时型,产生正四面体形的四分体。花药壁由表皮、药室内壁、中层和绒毡层四层细胞组成,其发育类型为双子叶型。腺质绒毡层,其细胞为二核。三细胞型花粉。子房５室,每室两个胚珠,上胚珠败育,下胚珠可育。下胚珠倒生,具单珠被,厚珠心。大孢子母细胞减数分裂形成线性排列的四个大孢子,雌配子体发育属蓼型。开花当天,花粉散开,雌配子体尚未成熟,处     

罗汉果大孢子发生及雌配子体发育与花部形态、胚珠变化的关系

为弄清罗汉果(Siraitia grosvenorii)大孢子发生、雌配子体发育过程与花部形态特征、胚珠的关系,运用石蜡切片法对罗汉果子房进行了显微观察。结果表明,罗汉果的胚珠倒生,双珠被,厚珠心,大孢子四分体呈线型排列,合点端一个大孢子分化为功能大孢子,成熟胚囊为蓼型。花蕾形态、胚珠变化与大孢子发生、雌配子体的发育时期具有一定相关性,当子房长度为7.0 mm≤L<9.0 mm,珠心呈椭圆形时,约有45.83%的大孢子母细胞处于减数分裂时期。因此,依据罗汉果花部形态可有效确定大孢子发生与雌配子体发育的时期。     

APOMIXIS IN AMELANCHIER LAEVIS,SHADBUSH (ROSACEAE,MALOIDEAE)

We studied ovule and megagametophyte development in tetraploid (n = 34) individuals of Amelanchier laevis in Maine. Nomarski differential interference contrast microscopy of cleared, whole ovules and conventional microscopy of sectioned, stained material show no clear evidence for the successful completion of meiosis. Instead, the megasporocyte or its derivatives degenerate and one to six nearby cells develop into aposporous initials. Usually more than one of these divide to form eight-nucleate, Polygonum-type megagametophytes. The egg apparently forms a proembryo parthenogenetically, but seed maturation requires pollination. This evidence for apospory and pseudogamy, the first to be reported in Amelanchier, conforms to the general pattern found in other apomictic genera of the Maloideae.     

马哈利樱桃大孢子发生和雌配子体的发育

采用石蜡切片法对马哈利樱桃大孢子发生和雌配子体发育过程进行观察研究。结果表明:(1)马哈利樱桃雌配子体发育早期,在单室子房内可以看到2个倒生胚珠,但在后期其中一个退化,另一个发育为种子;其胚珠具双珠被,为厚珠心。(2)大孢子母细胞减数分裂形成直线型四分体,功能大孢子位于合点端;胚囊发育为蓼型,成熟胚囊为七细胞八核。(3)根据不同时间花的外部形态特征与内部解剖学对比的观察结果,在陕西关中地区,三月下旬是马哈利樱桃雌性生殖细胞分化和发育的重要时期,果园在此期间应加强肥水管理。     

Somatic small RNA pathways promote the mitotic events of megagametogenesis during female reproductive development in Arabidopsis

Female gamete development in Arabidopsis ovules comprises two phases. During megasporogenesis, a somatic ovule cell differentiates into a megaspore mother cell and undergoes meiosis to produce four haploid megaspores, three of which degrade. The surviving functional megaspore participates in megagametogenesis, undergoing syncytial mitosis and cellular differentiation to produce a multicellular female gametophyte containing the egg and central cell, progenitors of the embryo and endosperm of the seed. The transition between megasporogenesis and megagametogenesis is poorly characterised, partly owing to the inaccessibility of reproductive cells within the ovule. Here, laser capture microdissection was used to identify genes expressed in and/or around developing megaspores during the transition to megagametogenesis. ARGONAUTE5 (AGO5), a putative effector of small RNA (sRNA) silencing pathways, was found to be expressed around reproductive cells during megasporogenesis, and a novel semi-dominant ago5-4 insertion allele showed defects in the initiation of megagametogenesis. Expression of a viral RNAi suppressor, P1/Hc-Pro, driven by the WUSCHEL and AGO5 promoters in somatic cells flanking the megaspores resulted in a similar phenotype. This indicates that sRNA-dependent pathways acting in somatic ovule tissues promote the initiation of megagametogenesis in the functional megaspore. Notably, these pathways are independent of AGO9, which functions in somatic epidermal ovule cells to inhibit the formation of multiple megaspore-like cells. Therefore, one somatic sRNA pathway involving AGO9 restricts reproductive development to the functional megaspore and a second pathway, inhibited by ago5-4 and P1/Hc-Pro, promotes megagametogenesis.     

Ovule and seed development of Lacandonia schismatica (Lacandoniaceae)

Ovule and seed development is described for Lacandonia schismatica, a species whose androecium is surrounded by the gynoecium. The ovule in each carpel is basal, anatropous, tenuinucellate, and bitegmic. The female gametophyte is formed by the micropylar megaspore cell, after a coenocytic stage of the four megaspore nuclei. The mature female gametophyte has the normal complement of seven cells and eight nuclei. We propose a new type of female gametophyte development on the basis of the coenocytic stage of the tetrad, the cellularization of the tetrad, and the survival of the micropylar spore. At seed dispersal time, the embryo has ~10-20 cells. Endosperm development is of the nuclear type. At maturity, endosperm cells show starch and protein inclusions as well as polysaccharides in their thick walls. The seed coat is formed from the outer integument; the inner one disappears. The exotesta contains tannin. The fruit (achene) wall is two-layered. The maturation of the fruits in a flower is synchronous, and they separate from the receptacle for dispersal.     

Cytological analysis of ginseng carpel development

Panax ginseng Meyer, commonly known as ginseng, is considered one of the most important herbs with pharmaceutical values due to the presence of ginsenosides and is cultivated for its highly valued root for medicinal purposes. Recently, it has been recognized that ginseng fruit contains high contents of triterpene such as ginsenoside Re as pharmaceutical compounds. However, it is unclear how carpel, the female reproductive tissue of flowers, is formed during the three-year-old growth before fruit is formed in ginseng plants. Here, we report P. ginseng carpel development at the cytological level, starting from the initial stage of ovule development to seed development. The carpel of P. ginseng is composed of two free stigmas, two free styles, and one epigynous bilocular ovary containing one ovule in each locule. Based on our cytological study, we propose that the female reproductive development in P. ginseng can be classified into seven stages: early phase of ovule development, megasporogenesis, megagametogenesis, pre-fertilization, fertilization, post-fertilization, and seed development. We also describe the correlation of the female and male gametophyte development and compare morphological differences in carpel development between ginseng and other higher plants. One unique feature for ginseng seed development is that it takes 40 days for the embryo to develop to the early torpedo stage and that the embryo is small relative to the seed size, which could be a feature of taxonomic importance. This study will provide an integral tool for the study of the reproductive development and breeding of P. ginseng.