Cytological studies on meiosis and male gametophyte development in autotetraploid cucumber

With improved staining and chromosome preparation techniques, meiosis of pollen mother cells (PMCs) and male gametophyte development in autotetraploid cucumber (Cucumis sativus L.) was studied to understand the correlation between chromosomes behaviour and fertility. Various chromosome configurations, e.g. multivalent, quadrivalents, trivalents, bivalents and univalents were observed in most PMCs at metaphase I. Lagging chromosomes were frequently observed at anaphase in both meiotic divisions. In addition, chromosomes segregations were not synchronous and equal in some PMCs during anaphase II and telophase II. Dyads, triads, tetrads with micronuclei and polyads were observed at tetrad stage, and the frequencies of normal tetrad with four microcytes were only 55.4 %. The frequency of abnormal behaviour in each stage of meiosis was counted, and the average value was 37.2 %. The normal meiotic process could be accomplished to form the microspore tetrads via simultaneous cytokinesis. Most microspores could develop into fertile gametophytes with 2 cells and 3 germ pores through the following stages: single-nucleus early stage, single-nucleus late stage and 2-celled stage. The frequency of abnormalities was low during the process of male gametophyte development. The germination rate of pollen grains was 46.9 %. These results suggested that abnormal meiosis in PMCs was the reason for low pollen fertility in the autotetraploid cucumber.     

The pollinium ofLoroglossum hircinum (Orchidaceae) between pollination and pollen tube emission

The structure of the massulae composing the pollinium ofLoroglossum hircinum was studied before pollination and 12 and 24 hours afterwards. The grains are grouped in tetrads closely packed in massulae. The exine is only present on the outside of the massulae. The intine consists of two layers: a compact layer surrounding the pollen grain and a looser layer surrounding the pollen grain and a looser layer surrounding the tetrad. Twelve hours after pollination, pollen volume and the space between the tetrads increase due to vacuolization. Twenty-four hours after pollination, pollen volume and tetrad spacing are higher due to vacuolization and some grains have emitted pollen tubes. Pollen growth due to vacuole formation, and the absence of common walls between adjacent tetrads lead to crumbling of the massulae. The mature pollen grain does not have apertures: the site of pollen tube emission is determined after pollination. The first grains to germinate are those in the centre of the massula. The vegetative cell nucleus is the first to enter the pollen tube; the generative cell elongates and undergoes the second haploid mitosis shortly after entering the pollen tube.     

华北落叶松小孢子母细胞(PMC)减数分裂和花粉的发育

采用染色体压片法系统地观察了华北落叶松Larix principis-rupprechtii小孢子母细胞(PMC)减数分裂的细胞学特征和花粉发育过程。结果表明:华北落叶松PMC减数分裂始于当年秋季,以弥散双线期的特殊状态休眠越冬,次年春天又重新恢复其后续的减数分裂过程。主要特点是在同一花药中,减数分裂不同步,且各阶段分裂速度较快,3天内全部停留在四分体阶段;二价体构型主要为两臂共发生过两次以上交换的环形二价体,少数细胞中偶见一臂发生交换的棒状二价体;其平均构型为10.62II 1.38II;中期II核分裂相既有平行式样,又有互为垂直式样,形成左右对称型和四面体型四分体,兼具单、双子叶植物四分体主要类型特点。四分体持续一周后,细胞壁开始溶解,4个小孢子分离并游离在药室中,成为独立的单核花粉。其后细胞核开始有丝分裂,最后形成由2个原叶细胞、1个管细胞、1个柄细胞和1个体细胞共5个细胞组成的成熟花粉。此外,还观察到成熟花粉当中有13.6%是由4个细胞组成的。     

Microsporogenesis and pollen development in Leymus chinensis with emphasis on dynamic changes in callose deposition

Leymus chinensis is an economically and ecologically important grass that exhibits low seed production. To better understand the causes of its low sexual reproductivity, the microsporogenesis and pollen development of this species were investigated, with emphasis on dynamic changes in callose deposition. A variety of histochemical stains were employed, including Heidenhain's hematoxylin, decolorized aniline blue, DAPI, and acetocarmine, along with a temporary mount method. Microsporogenesis and pollen development generally took place from June 12 to 26. The meiosis of microspore mother cells (MMCs) was of the successive type and the tetrad was isobilateral in shape. Mature pollen grains comprised two sperms and a vegetative nucleus. Callose initially appeared in the center of the anther locule at the premeiotic phase, and then gradually and unevenly deposited around the MMC before the commencement of meiosis. At the onset of meiosis, the accumulation of callose enclosing the MMC peaked, accompanied by the disappearance of callose in the center of the locule. At the dyad and tetrad stages, the dyads and tetrads were surrounded by callose wall and the microspores in the tetrads were isolated by a crossed cell plate composed of callose. Microspores just released from tetrads were still enclosed in callose wall, and then callose gradually disappeared in the pollen wall. Ultimately, callose almost completely disappeared from the walls of mature pollen grains. In the large numbers of sections observed, most of the cases of meiosis of the MMCs, pollen development, and callose dynamics were normal, with only a few abnormities observed. The results suggest that microsporogenesis, male gametogenesis, and callose dynamics during these processes are generally normal in this species, and that the callose wall plays an important role in the production of functional pollen grains. The small numbers of abnormities of these processes that occurred likely do not adversely affect the production of viable pollen grains. Therefore, microsporogenesis and pollen development may not be factors in the low seed production of L. chinensis.     

Types of pollen dispersal units in orchids,and their consequences for germination and fertilization

The various pollen dispersal units (PDU) found in orchids are discussed together with possible evolutionary trends and the consequences for germination and fertilization. Orchids with monad and tetrad pollen form more complex dispersal units by means of pollenkitt, elastoviscin, a callosic wall, common walls or a combination of these. Evolutionary trends include (1) from pollenkitt to elastoviscin; (2) from monad to tetrads and multiples of tetrads; (3) from partially dehydrated (<30 %) to partially hydrated (>30 %) pollen; and (4) from monad pollen to PDUs with many pollen grains. The biological consequences concern both male and female reproductive systems. Some features of the male side are present in all orchids irrespective of the pollen dispersal unit, whereas other characters are found only in orchids with pollinia; the same applies for the female counterpart. Pollen grains of orchids with pollinia germinate at least 24 h after pollination because the pollen grains/tetrads must swell and make space for the growth of pollen tubes.     

Absence of microspore polarity, symmetric divisions and pollen cell fate in Brachiaria decumbens (Gramineae).

Meiotic division and male gametophyte development were analyzed in one tetraploid (2n = 4x = 36) accession of Brachiaria decumbens cv. Basilisk that showed some pollen sterility. Meiotic process was typical of polyploids in that it consisted of multiple chromosome associations. Precocious chromosome migration to the poles, laggards, and micronucleus formation were abundant in both meiosis I and II and resulted in tetrads with micronuclei. After callose dissolution, microspores were released into the anther locule and had the semblance of being normal. Although each microspore initiated its differentiation by pollen mitosis, in 43.24% of the microspores, nuclear polarization was not observed and the typical hemispherical cell plate was not detected. Division was symmetric and microspores lacked differentiation between the vegetative and the generative cell. Both nuclei were of equal size, presented equal chromatin condensation, and had a spherical shape. After the first pollen mitosis and cytokinesis, each cell underwent a new symmetric mitosis without nuclear polarization. At the end of the second pollen mitosis, four equal nuclei were observed in each pollen grain. After the second cytokinesis, the cells gave rise to four equal-sized pollen grains with a similar tetrad configuration that initially remained together. Sterile pollen grains resulted from abnormal pollen mitosis. This anomaly may be explained by a mutation, probably affecting microtubule cytoskeleton formation. The importance of this male-sterile mutation for Brachiaria breeding programs is discussed.     

An attempted analysis of the tetrads in the complex heterozygote species of the sub-genus Oenothera

Summary The statistical parameters of a pollen population of Oenothera purpurata in which the mendelian factor of pollen lethality segregates, showed that the pollen population analysed at anthesis in the Oenothera sub-genus quite faithfully conserves the segregation indicated in the tetrads. Following on this fact, the segregation active grains, inactive grains, empty grains was analysed in the pollen of the complex heterozygotes. The large standard deviation of the percentages of the three pollen classes and the value of the correlation between the percentages of grain classes taken two by two signify that the segregation of the three potential classes of microspore differs from one tetrad to another. By superimposing the correlation coefficient estimated between the percentage of the three grain classes of a bifactorial segregation of corn pollen on the correlation coefficient of the three classes of Oenothera pollen, it is possible to know with certainty the distribution of the potentially active, inactive and empty microspores in the tetrads. This distribution, which is different depending on whether the species is heterogamous or isogamous, allows us to confirm that the empty grains carry the recombined complexes which become lethal as a result of this recombination.     

二倍体和四倍体栽培苦荞的细胞学比较研究

观察和统计了二倍体和四倍体苦荞 (Fagopyrum tataricum)花粉母细胞减数分裂过程 TI、T 时期的异常程度及花粉粒败育性等多个指标 ,并比较分析了二倍体和四倍体苦荞在这些指标上的差异显著性。结果显示 :四倍体苦荞的异常 TI细胞频率、异常 T 细胞频率、TI细胞平均微核数、T 细胞平均微核数及花粉败育率均显著高于二倍体苦荞 ,这表明四倍体苦荞的遗传稳定性和花粉可育性都比二倍体苦荞差。     

Postmeiotic cytokinesis and pollen aperture number determination in eudicots: effect of the cleavage wall number

Summary.  In eudicot postmeiotic tetrads, apertures are usually joined in pairs in highly conserved areas. These appear to be located at the last points of contact persisting at the end of cytokinesis between the cytoplasm of the future microspores. In order to investigate the relationship between cytokinesis and aperture formation, aperture distribution within postmeiotic tetrads and the progression of meiosis were studied in Nicotiana tabacum cv. Ambalema. This variety (inbred line) produces about 85% tricolporate pollen and 15% tetracolporate pollen grains. In addition, about 7% of tetrads are composed of four equal-sized microspores and a supernumerary pseudomicrospore of small size and an equal proportion of tetrads exhibit unpaired apertures (these apertures are not joined in pairs within tetrads). Observation of cytokinesis indicates that both unpaired apertures and pseudomicrospores could result from the persistence of late communications between microsporocytes. Observations of tetrads indicate that an increase in the number of elements that are separated during cytokinesis is correlated with an increase in microspore aperture number. All data converge to support the hypothesis that aperture site determination is partly controlled by the number of walls formed to separate the different elements of the tetrad. Received May 22, 2002; accepted October 29, 2002; published online April 2, 2003 RID="*" ID="*" Correspondence and reprints: Laboratoire de Ecologie, Systematique et Evolution, Batiment 362, Université Paris Sud, 91405 Orsay cedex, France.     

The influence of tetrad shape and intersporal callose wall formation on pollen aperture pattern ontogeny in two eudicot species

Background and Aims

In flowering plants, microsporogenesis is accompanied by various types of cytoplasmic partitioning (cytokinesis). Patterns of male cytokinesis are suspected to play a role in the diversity of aperture patterns found in pollen grains of angiosperms. The relationships between intersporal wall formation, tetrad shape and pollen aperture pattern ontogeny are studied.

Methods

A comparative analysis of meiosis and aperture distribution was performed within tetrads in two triporate eudicot species with contrasting aperture arrangements within their tetrads [Epilobium roseum (Onagraceae) and Paranomus reflexus (Proteaceae)].

Key Results and Conclusions

Intersporal wall formation is a two-step process in both species. Cytokinesis is first achieved by the formation of naked centripetal cell plates. These naked cell plates are then covered by additional thick, localized callose deposits that differ in location between the two species. Apertures are finally formed in areas in which additional callose is deposited on the cell plates. The recorded variation in tetrad shape is correlated with variations in aperture pattern, demonstrating the role of cell partitioning in aperture pattern ontogeny.     

Embryological Studies on Narcissus tazetta var. chinensis

Chinese narcissus (Narcissus tazetta var.chinensis Roem) blooms but has no seeds.Embryological studies on the species were conducted to discover the causes of its sterility.Its anther wall is composed of four layers of cells,and its tapetum is of the secretory type.The cytokinesis of microspore mother cells is of the successive type,and the tetrad is tetrahedral.During meiosis of microspore mother cells,some chromosomes lagged,and several micronuclei were found in tetrads.Only 27.7% of the pollen grains contained full cytoplasm,and 1.3% of them germinated in culture medium.No pollen grain,however,could germinate on the stigma.The ovary is trilocular with axile placenta,and the ovules are bitegmic,tenuinucellate,and anatropous.Its embryo sac is of the polygonum type.Most embryo sacs degenerated,and only about 4.5% of the ovules contained a normal embryo sac with an egg cell,two synergids,three antipodal,and a central cell containing two polar nuclei.One reason for the sterility of Chinese narcissus is the abnormality of microsporogenesis and megasporogenesis,in which only a few functional pollen grains and embryo sacs are produced.The other reason is that the pollen grains cannot germinate on the stigma.     

Ontogenesis of monad pollen inPterostylis plumosa (Orchidaceae,Neottioideae)

Pterostylis plumosa (Neottioideae) is an orchid with monosulcate monad pollen. Tetrads may be isobilateral, decussate, tetrahedral, rhomboid or T-shaped, but all pollen grains have a similar shape. Those belonging to the same tetrads are contiguous from microspore release to opening of the anther, with the furrow oriented inwards. Sporophytic proteins are present outside the furrow. The tapetum is of the parietal type without orbicles. The increase in pollen grain size between meiosis and maturity is only three-fold. The generative cell is spherical when the pollen is mature. These features are discussed in relation to the primitive nature of the species.     

Anatomical differences on development of fertile and sterile pollen grains of Prunus salicina Lindl.

Anatomical changes occurring during the microsporogenic development of P. salicina Lindl. were studied in male fertile and male sterile genotypes. Male fertile pollen grains showed three well determined pore regions, without ektexine. Intine was thick and surrounded the vegetative cell. Vegetative cells enclosed the generative cells; their cytoplasm was rich in plastids, abundant RER and active mitochondria. Development of sterile pollen was different from the meiosis step. Microspores did not show germination pores and ektexine was continuous around the whole grain. Pollen grains showed an atypical shape. The tapetum persisted after the tetrad stage and showed hypertrophy and vacuole development, resulting in abnormal microspore development. Only a few pollen grains and rudiments of collapsed microspores close to the anther wall were formed at anthesis.     

Syncytes during male meiosis resulting in 2n pollen grain formation in Lindelofia longiflora var. falconeri

Abstract Lindelofia longiflora (Royle ex Benth.) Baill. var. falconeri (Cl.) Brand (Family: Boraginaceae) is investigated cytologically (n= 12) for the first time from the cold deserts of Pangi Valley, Chamba District (Himachal Pradesh) in India. We report the formation of syncytes and 2n pollen grains in the species. During meiosis, the majority of the pollen mother cells (PMCs) exhibited 12 bivalents, equal segregation of chromosomes during anaphases, regular tetrads, and normal‐sized pollen grain formation. Occasionally, two proximate PMCs fused during the early stages of prophase‐I and resulted in the formation of syncytes. The frequency of syncytes in the accession is rather low, at 25 out of 1866 (1.33%). Such syncyte PMCs are detectable during meiosis due to their larger size compared to typical PMCs. The syncytes or polyploid cells showed normal 24 bivalents and depicted perfectly regular meiotic course. But the products of such PMCs yield 2n or larger sized pollen grains that are almost double the size of typical normal or n pollen grains. The origin of syncytes as a consequence of the fusion of meiocytes during the early stages of meiosis‐I could be attributed to low temperature stress conditions prevailing in the Pangi Valley, where temperature during May and June dip to below freezing, the time the plants enters the reproductive/flowering bud stage. It is possible that such apparently fertile 2n pollen grains originating from syncytes might play a role in the origin of intraspecific polyploids in the species.     

Cellular and tissue mechanisms of recovery processes in Hordeum distichum L. under irradiation

Ionizing and UV-B irradiation of barley seedlings increased the number of chromosome aberrations in vegetative and generative meristems and disturbances in microsporo- and microgametogenesis. Other irradiation-induced changes included enhanced cytomixis and an increase in the number of pathologies in meiosis and tetrads and in the polymorphism and disturbances in pollen grains. Damages induced by different irradiation types differ primarily quantitatively. The dynamics of occurrence of chromosome aberrations in the root meristem reciprocally depended on the irradiation dose. Damages induced by low UV doses were detected throughout the ontogeny of plants. Increased irradiation doses activated cytolytic processes during premeiotic interphase, meiosis, and tetrad formation, as well as after the end of gametogenesis. Exposure to maximum doses of UV radiation and moderate doses of γ-radiation restored the fertility of grain pollens. Microsporocytes, microspores, and pollen grain components undergo degeneration primarily via apoptosis. This type of cell death is apparently autonomous, being induced by the cell population itself. The results of this study confirmed the hypothesis on the positive role of cell competition in the recovery processes and adaptation of plants to mutagens.     

Embryological Studies on Narcissus tazetta var. chinensis

Chinese narcissus (Narcissus tazetta var. chinensis Roem) blooms but has no seeds. Embryological studies on the species were conducted to discover the causes of its sterility. Its anther wall is composed of four layers of cells, and its tapetum is of the secretory type. The cytokinesis of microspore mother cells is of the successive type, and the tetrad is tetrahedral. During meiosis of microspore mother cells, some chromosomes lagged, and several micronuclei were found in tetrads. Only 27.7% of the pollen grains contained full cytoplasm, and 1.3% of them germinated in culture medium. No pollen grain, however, could germinate on the stigma. The ovary is trilocular with axile placenta, and the ovules are bitegmic, tenuinucellate, and anatropous. Its embryo sac is of the polygonum type. Most embryo sacs degenerated, and only about 4.5% of the ovules contained a normal embryo sac with an egg cell, two synergids, three antipodal, and a central cell containing two polar nuclei. One reason for the sterility of Chinese narcissus is the abnormality of microsporogenesis and megasporogenesis, in which only a few functional pollen grains and embryo sacs are produced. The other reason is that the pollen grains cannot germinate on the stigma. This paper was translated from Journal of Xiamen University (Natural Science), 2005, 44(1) (in Chinese)     

Pollen tetrad segregation and pollen ovule ratios in six species of Acrotriche (Styphelioideae: Ericaceae)

Pollen morphology can be useful in the determination of phylogeny and mating systems of plants. Pollen ovule ratios can give an indication of the mating system of plant species, and there are a number of theories as to reductions in response to pollination strategies. Pollen tetrad segregation occurs when a number of pollen grains within a tetrad abort; this process has evolved a number of times in the family Ericaceae. Pollen ovule ratios along with the total number of tetrads, number of viable grains and proportions of each of the tetrad types were investigated in six species of Acrotriche (Styphelioideae: Ericaceae). Pollen ovule ratios were used in comparison with related species to give indications of mating systems and to examine theories regarding reductions in pollen production. The study species can be arranged into distinct groupings based on the proportions of tetrad types that correlate with floral morphology and may be indicative of phylogentic associations.     

楸树大小孢子发生与雌雄配子体发育的研究

运用石蜡切片法和整体透明法对楸树(Catalpa bungei C.A.Meyer)大、小孢子发生及雌、雄配子体发育过程进行了研究.结果表明:楸树可育雄蕊2枚,花药4室,药壁发育属双子叶型,腺质异型绒毡层.小孢子母细胞减数分裂为同时型,四分体后小孢子不分离形成正四面体型四合花粉,偶有左右对称型和十字交叉型.成熟花粉为二细胞型,无萌发孔.子房上位,2室,中轴胎座,胚珠多数,倒生,单珠被,薄珠心,具珠被绒毡层.单孢原直接发育为大孢子母细胞,四分体线形排列,合点端大孢子发育为功能大孢子,胚囊发育为蓼型.雄蕊发育早于雌蕊,花开后雌、雄蕊趋于同熟.研究认为:虽然楸树雌、雄蕊发育过程中均存在一定比例的败育,但其花而不实"并非雄性或雌性不育所致.推测与其授粉受精和胚后发育有关.     

Unique tetrads of <Emphasis Type="Italic">Epilobium luteum</Emphasis> (Onagraceae: Onagreae) pollen from Alaska

Aperture morphology of tetrad pollen of Epilobium luteum (Onagraceae: Epilobieae) from three Alaskan collections is highly variable. The first collection appears to lack apertures altogether and is presumed to consist of immature pollen gains in a genus known to achieve mature size before the apertures become distinctly protruding. A second collection has tetrads with 3- and 4-apertured grains, the apertures in the latter are often irregularly spaced and not in apposition with the apertures of neighboring members. The third collection consists of the more typical 3-apertured members that characterize the majority of Epilobium pollen grains. In all of these collections individual pollen grains (monads) are interspersed among the tetrads. The variations in the number of apertures emphasize the importance of having a comprehensive understanding of the stage of development of the pollen (taxon) examined when describing pollen collections. In the first collection this would mean the recognition that in Onagraceae apertures occur in the later stages of microspore ontogeny. In the latter two collections a thorough background of the literature of the pollen morphology on this largest Onagraceae taxon is useful for the understanding of the significance of a range of aperture numbers on Epilobium pollen grains.     

濒危植物巴东木莲花粉母细胞减数分裂观察

对巴东木莲Manglietia patungensis及其近缘种乳源木莲M. yuyuanensis的花粉母细胞减数分裂过程的基本特征进行了比较研究。乳源木莲与巴东木莲的染色体数目和核型相同,但不经任何人为因素诱导,它们之间在减数分裂过程中的染色体行为上有明显差异。(1)巴东木莲减数分裂中期I构型为0.30IV+18.33II+0.15I,与乳源木莲构型19II不同,巴东木莲可能存在同臂内倒位杂合子,染色体结构存在一定的杂合性。(2)后期I和后期II染色体行为异常现象发生频率明显不同。以后期II为例,乳源木莲减数分裂相中有迟滞染色体的细胞占8.8%,迟滞染色体不超过2个;巴东木莲有迟滞染色体等异常现象的细胞占29.2%,迟滞染色体最高达11个,还出现染色体碎裂成断片现象。巴东木莲减数分裂过程中染色体组表现出染色体结构杂合变异和迟滞染色体与染色体的断裂频率很高的异常现象在一定程度上可能影响了雄配子体的发育。