Development of male and female flower in Asparagus officinalis. Search for point of transition from hermaphroditic to unisexual developmental pathway

Asparagus officinalis is a dioecious plant. The flowers start to develop as hermaphrodites and later become unisexual. In female flowers the stamens degenerate, while in male flowers the ovary stops growing without degenerating. We have examined young asparagus flowers using SEM and optical microscopy in order to determine the exact moment of transition from hermaphroditic to unisexual development. We defined 13 stages of development, starting from flower primordia up to completely mature flowers and labelled them with numbers from -6 to 7. The first five stages are fully hermaphroditic: a difference between sexes becomes visible at stage — 1 when the style begins to develop in female flowers. Degeneration of stamens in female flowers starts somewhat later. At the stage of transition, some differences between sexes also appear in the bidimensional polypeptide pattern of flowers. RNase activity shows a distinct peak at this stage (in female flowers only), probably related to stamen degeneration.     

Phylogenetic analysis of sexual systems in Inuleae (Asteraceae)

From an ancestor with bisexual flowers, plants with unisexual flowers, or even unisexual individuals have evolved in different lineages of angiosperms. The Asteraceae tribe Inuleae includes hermaphroditic, monoecious, dioecious, and gynomonoecious species. Gynomonoecy, the sexual system in which female and bisexual flowers occur on the same plant, is prevalent in the Asteraceae. We inferred one large gene phylogeny (ndhF) and two supertrees to investigate whether gynomonoecy was a stage in the evolution from hermaphroditism to monoecy. We identified transitions in sexual system evolution using the stochastic character mapping method. From gynomonoecious ancestors, both hermaphroditic and monoecious descendants have evolved. Gynomonoecy was not restricted to a stage in the evolution toward monoecy because the number of transitions and the rate of change from monoecy to gynomonoecy were much higher than the opposite. We also investigated one hypothesized association among female flowers and the development of a petaloid ray as an explanation of gynomonoecy maintenance in Asteraceae. We found that peripheral female flowers and petaloid rays were phylogenetically correlated. However, empirical evidence shows that a causal relationship between these traits is not clear.     

Bird-nest puzzle: can the study of unisexual flowers such as cucumber solve the problem of plant sex determination?

Unisexual flower development has long been used as a model system to understand the mechanism of plant sex determination. However, based on our investigation of the mechanisms regulating the development of unisexual cucumber flowers, we have realized that understanding how organ development is inhibited may not necessarily reveal how an organ is formed. We refer to this problem as a "bird-nest puzzle," meaning one cannot understand how a bird lays and hatches its eggs by understanding how its nest is ruined. To understand the biological significance of unisexual flowers, we reexamine the original meaning of sex and its application in plants. Additionally, we propose that the fundamental biological advantage for the selection and maintenance of unisexual flowers during evolution is to promote cross pollination.     

Bird–nest puzzle: can the study of unisexual flowers such as cucumber solve the problem of plant sex determination?

Unisexual flower development has long been used as a model system to understand the mechanism of plant sex determination. However, based on our investigation of the mechanisms regulating the development of unisexual cucumber flowers, we have realized that understanding how organ development is inhibited may not necessarily reveal how an organ is formed. We refer to this problem as a “bird–nest puzzle,” meaning one cannot understand how a bird lays and hatches its eggs by understanding how its nest is ruined. To understand the biological significance of unisexual flowers, we reexamine the original meaning of sex and its application in plants. Additionally, we propose that the fundamental biological advantage for the selection and maintenance of unisexual flowers during evolution is to promote cross pollination.     

Unisexual flower, spikelet, and inflorescence development in monoecious/dioecious Bouteloua dimorpha (Poaceae, Chloridoideae)

Unisexual flowers have evolved repeatedly in the angiosperms. In Poaceae, multiple transitions from bisexual to unisexual flowers are hypothesized. There appear to be at least three distinct developmental mechanisms for unisexual flower formation as found in members of three subfamilies (Ehrhartoideae, Panicoideae, Pharoideae). In this study, unisexual flower development is described for the first time in subfamily Chloridoideae, as exemplified by Bouteloua dimorpha. Scanning electron microscopy (SEM) and anatomy were used to characterize the development of male (staminate) and female (pistillate) flowers, spikelets, and inflorescences. We found the developmental pathway for staminate flowers in B. dimorpha to be distinct from that described in the other three subfamilies, showing gynoecial arrest occurs at a different stage with possible loss of some cellular contents. However, pistillate flowers of B. dimorpha had some similarity to those described in other unisexual-flowered grasses, with filament and anther differentiation in abortive stamens. Comparing our findings with previous reports, unisexual flowers seem to have evolved independently in the four examined grass subfamilies. This analysis suggests the action of different genetic mechanisms, which are consistent with previous observations that floral unisexuality is a homoplasious condition in angiosperms.     

Changes in polyamine pattern mediates sex differentiation and unisexual flower development in monoecious cucumber (Cucumis sativus L.)

Changes in the levels of polyamines are associated with fundamental physiological processes such as embryogenesis, induction of flowering, fruit development and ripening, senescence, and responses to environmental stresses, but the role of polyamines in sex differentiation and unisexual flower development has not been deeply studied. To extend the knowledge on the regulatory mechanisms of flowering in monoecious plant (producing unisexual flowers), we investigated the morphogenesis and free polyamine levels in Cucumis sativus during sex differentiation and unisexual flower development in vitro using histocytological and biochemical methods. As shown in our study, floral development in vitro was undisturbed and flowers of both sexes were produced. Sex differentiation relied on preventing the development of generative organs of the opposite sex, as we observed carpel repression in male flowers and stamen repression in female flowers. Pollen viability was negatively correlated with female flower development on the same node. Biochemical analysis revealed increased accumulation of aliphatic amines (tri, tetra‐amines) in generative (flower buds and flowers) compare to vegetative (axillary buds and leaves) organs. Undifferentiated floral buds contained elevated levels of agmatine, cadaverine, spermidine and spermine. Sex differentiation was associated with significantly decreased levels of agmatine and cadaverine. Our results showed that female flowers contained higher levels of total polyamine than male flowers. The increased level of cadaverine was associated with macrogametogenesis and female flower maturation. Putrescine was important for male flower development. Such results support the hypothesis that aliphatic amines are involved in unisexual flower development.     

Development of Sexual Organs in Taihangia rupestris--Different Temperature Requirments for Both Sexual Organ Development in a Bisexual Flower

Optimal environmental temperature for differentiation and development of stamen and pistil have been tested cultured in vitro flowers of Taihangia rupestris. The optimal temperatures for stamen development was quite different from that for pistil development. The most suitable temperature scope for stamen development was on the low verge of 1 ～ 6 'C. Temperature over 18 ℃ led to abortion of all stamens. The temperature scope appropriate for pistil development was near to 6～26 ℃. That below 3 ℃ brought about cessation of most pistil development at early stage. The course of stamen abortion induced by high temperature started from the primordium stage in the stamen development and once this occurred it became irreversible despite of lowering the temperature. It is concluded that the temperature for unisexual male flower, unisexual female flowers and bisexual flowers development should be contralled at 1～3 ℃, 6～26 ℃ and 6～12 ℃ respectively. The results of the present experiment may open up a new train of thoughts for studies on the control of sexual organ development and male sterility.     

The arrest of development of abortive reproductive organs in the unisexual flower of<Emphasis Type="Italic"> Vitis vinifera</Emphasis> ssp.<Emphasis Type="Italic"> silvestris</Emphasis>

During the first stages of development, flowers of most dioecious species are hermaphroditic, with their transition to unisexual flowers being the result of the developmental arrest of one set of reproductive organs. In this work, we describe the development of male and female flowers of the dioecious wild grape species Vitis vinifera ssp. silvestris through scanning electron microscopy analysis and cytological observations, focusing our attention on the transition from bisexual to unisexual development. We divide floral development of the wild grape into eight stages. Differences between male and female flowers appear first at stage 6, when the style and stigma start to differentiate in female but not in male flowers. Cytological analysis of the slowly growing abortive pistil of male flowers shows that megagametophyte formation is, surprisingly, not inhibited. Instead of pistil abortion in the male flower, sexual determination is accomplished through programmed death of external nucellus cells and some layers of integumentary cells. Sterility of male structures in female flowers follows a different pattern, with microspore abnormalities evident from the time of their release from the tetrad. Sterile microspores and pollen grains in female flowers display an abnormal round shape, lacking colpi and possessing uniformly thickened cell walls that impede germination.     

Functional analysis of B and C class floral organ genes in spinach demonstrates their role in sexual dimorphism

Background  

Evolution of unisexual flowers entails one of the most extreme changes in plant development. Cultivated spinach, Spinacia oleracea L., is uniquely suited for the study of unisexual flower development as it is dioecious and it achieves unisexually by the absence of organ development, rather than by organ abortion or suppression. Male staminate flowers lack fourth whorl primordia and female pistillate flowers lack third whorl primordia. Based on theoretical considerations, early inflorescence or floral organ identity genes would likely be directly involved in sex-determination in those species in which organ initiation rather than organ maturation is regulated. In this study, we tested the hypothesis that sexual dimorphism occurs through the regulation of B class floral organ gene expression by experimentally knocking down gene expression by viral induced gene silencing.     

基部被子植物雪香兰（金粟兰科）单性花的形态发生和发育

基部被子植物金粟兰科（Chloranthaceae）的单性花或两性花结构十分简单,雪香兰（Hedyosmum orientale）花单性、雌雄异株,花的形态及结构与其它属物种具有显著的差异,对于研究被子植物花特别是花被的起源和系统进化具有重要意义。该研究采用电子显微镜和光学显微镜观察了雪香兰单性花的器官发生及发育过程。结果表明,雌、雄花均为顶生和腋生,多个小花呈聚伞圆锥状排列。雄花外侧是苞片,每朵雄花上着生150–200个雄蕊,花轴基部着生少数退化的叶原体。苞片原基及其腋生的花原基最初呈圆丘状,随后伸长。在雄花发育过程中,苞片原基比雄蕊原基生长快,雄花原基纵向伸长,叶原体原基在基部发生,雄蕊原基自下而上发生。每2朵雌花底部合生形成小聚伞花序,每朵雌花被一苞叶包裹,由单心皮和三棱型子房构成,外覆三裂叶状花被。在雌花发育过程中,雌花原基比苞片原基生长快,花被原基首先于花顶端发生,随后花顶端中心凹陷,进一步发育成具有单心皮的子房原基。雪香兰的单性花发育不经过两性同体阶段,花分生组织只起始雄蕊器官或雌蕊器官的发育。研究结果支持雪香兰单性花是原始性状的观点,雄花叶原体与雌花三裂叶状花被同源,可能是花被（萼片与花瓣）的起源。     

Characterization of unisexual flower development in the endangered mahogany tree Swietenia macrophylla King. (Meliaceae)

The selection of candidate plus trees of desirable phenotypes from tropical forest trees and the rapid devastation of the natural environments in which these trees are found have created the need for a more detailed knowledge of the floral and reproductive biology of tropical tree species. In this article, the organogenic processes related to unisexual flower development in tropical mahogany, Swietenia macrophylla , are described. Mahogany inflorescences at different developmental stages were evaluated using scanning electron microscopy or optical microscopy of histological sections. The unisexual flowers of S. macrophylla are usually formed in a thyrse, in which the positions of the female and male flowers are not random. Differences between male and female flowers arise late during development. Both female and male flowers can only be structurally distinguished after stage 9, where ovule primordia development is arrested in male flowers and microspore development is aborted in female flower anthers. After this stage, male and female flowers can be distinguished by the naked eye as a result of differences in the dimensions of the gynoecium. The floral characteristics of S. macrophylla (distribution of male and female flowers within the inflorescence, and the relative number of male to female flowers) have practical implications for conservation strategies of this endangered species.  © 2008 The Linnean Society of London, Botanical Journal of the Linnean Society , 2008, 156 , 529–535.     

被子植物成花诱导和性别决定机制的研究进展

开花植物具有多样性的生殖系统,其中单性花的形成是促进异交、避免自交衰退、保持遗传多样性的重要途径。单性花物种分布于被子植物不同进化分支上的事实表明,物种的雌雄异花性可能是通过不同的机制进化形成的。本文从花发育、性染色体、植物激素和环境因素四个方面,阐述了被子植物性别分化调控机制的研究进展。     

木薯两性花及其种子的形态特征

以前期发明的一种木薯(Manihot esculenta)调控开花技术为基础, 成功研发出木薯两性花诱导技术, 并发现诱导出的木薯两性花能自花授粉结实。该文报道了木薯两性花、两性花自花授粉发育的蒴果及其种子的形态特征。木薯两性花同时具有雌蕊和雄蕊, 而单性花只具有雌蕊或雄蕊; 两性花蒴果卵圆形且长宽比大, 单性花蒴果阔椭圆形且长宽比小; 两性花种子种脊不隆起, 单性花种子种脊隆起明显。两性花和两性花种子是珍贵的木薯育种材料, 对揭示木薯的遗传特性及培育木薯新种质具有重要意义。     

Floral morphology and anatomy of Pittosporum tobira (Pittosporaceae)

Comparative studies are made on floral morphology and anatomy of female and male flowers of Pittosporum tobira. The two types of flower differ little from each other in structure at the early stage of floral development, but appear dimorphic towards anthesis. The male flower becomes cryptically bisexual, although its pistil is slender compared to that of the female flower. The stigmas of the male flower are receptive and can induce pollen germination. The structure of the style in the male flower is identical to that in the female flower. Ovules are produced on the protruded parietal placenta in the male flower, but their development is arrested at the stage of the 4–nucleate embryo sac. The female flower is clearly unisexual, with obviously aborted and sagittate anthers. Its pistil is rather plump and can produce darkish red seeds immersed in sticky pulp. The male and female flowers are similar in vascular anatomy. A conspicuous difference between the two types of flower lies in the stamens. Variation of sexual organs in the genus Pittosporum is reviewed. We assume that the flowers of Pittosporum are derived from the hermaphrodite-flowered ancestor and the female flower has become unisexual through partial reduction of sexual organs at a faster rate than the male flower.     

Morphology and development of the reproductive shoots of Lilaea scilloides (Poir.) Hauman (Alismatidae)

Shoots of adult plants of Lilaea scilloides have a sympodial form. Each unit of the sympodium bears a single sheathing prophyll (which is the only kind of foliage leaf produced in the adult) and terminates in an inflorescence. The prophyll subtends the next unit of the sympodium. A further accessory bud can form in association with each unit. This bud repeats the pattern of the main sympodium, giving the plant a tufted habit. Five different kinds of flower can be identified in the inflorescence: a unisexual male flower with a single perianth member and adnate stamen; a bisexual flower, with a single perianth member and adnate stamen, and a single carpel with an anatropous bitegmic ovule; a unisexual female flower with a single perianth member and carpel; a unisexual female flower comprising only a single carpel; and a female flower comprising only a single carpel with a very long filamentous style. The first four kinds occur in the upper part of the inflorescence which is normally elevated on a scape, while the last kind is restricted to the base of the inflorescence. In the position of the basal flowers several variations have been observed in cultivated material. These include branching associated with the basal flowers, which results in the development of additional basal flowers or inflorescences, and even total replacement of a basal flower by an inflorescence or a branching structure bearing flowers. A review of past literature includes a clarification of some persistent errors which have confused the taxonomic position of the plant and the morphological interpretation of the reproductive appendages.     

Sex-specific cell division during development of unisexual flowers in the dioecious plant Silene latifolia

We analyzed cell division patterns during the differentiation of unisexual flowers of the dioecious plant Silene latifolia using in situ hybridization with histone H4 and cyclin A1 genes. The gene expression patterns indicated that the activation of cell divisions in whorls 3 and 4 was reversed in young male and female flower buds. During maturation of flower buds, a remarkable reduction in cell division activity occurred in the male gynoecium primordium and female stamen primordia. Our analyses showed that differential activation and reduction of cell division strongly correlated with sex-specific promotion and cessation in the sex differentiation of unisexual flowers.     

文冠果可孕花与不孕花发育过程的比较研究简

利用半薄切片和透射电镜技术对文冠果可孕花和不孕花的发育过程进行观察和比较。结果显示：(1)小孢子发育初期,两种类型花花药形态无明显差别;小孢子发育双核期,可孕花花药内壁纤维层细胞壁带状加厚,无唇细胞形成。而不孕花花药同侧两个花粉囊之间唇细胞正在分化;小孢子发育成熟期,不孕花花药唇细胞完全形成;散粉期,不孕花花药开裂呈双心形,而可孕花花药则不能开裂散粉。(2)可孕花雌蕊子房内有两室,柱头细胞排列紧密,柱头逐渐发育成圆球形,周围密布乳突细胞,具中空花柱道;不孕花雌蕊柱头停止发育,无中空花柱道,子房室变小,胚囊发育退化。(3)不孕花花药绒毡层中含大量蛋白体,小泡以及乌氏体等细胞器,发育后期绒毡层解体。而可孕花花药绒毡层中细胞器和营养物质积累均较少,发育后期绒毡层解体不完全。(4)可孕花花药内花粉粒细胞壁连续无萌发孔,细胞内含物较少。不孕花花药内花粉出现3个向内凹陷的萌发孔,且花粉内含有大量造粉质体和脂类物质。     

Development of Male Flowers in Zizania aquatica (North American Wild-Rice; Gramineae)

We investigated the histology and developmental morphology of flowers of wild-rice (Zizania aquatica), a member of grass subfamily Oryzoideae, to determine whether male flowers in this species develop in a manner similar to those in the subfamily Panicoideae, a group that includes many species with unisexual flowers. Zizania has evolved unisexual flowers from hermaphrodite ancestors and is only distantly related to the Panicoideae; the origins of unisexual flowers are independent in the two taxa. Ontogenetic evidence indicates that many species within the subfamily Panicoideae develop male flowers by a process similar to that already described for maize (Zea mays), a panicoid grass. Unisexual male flowers in maize initiate both the stamen (androecium) and the pistil (gynoecium), but the gynoecium aborts early in development through the death of the subepidermal cells. Cell death in gynoecia of maize is known to be controlled by the product of the gene tasselseed2 (ts2), and an orthologue of ts2 has been shown to have the same effect in the sister genus Tripsacum. It seems likely that ts2 orthologues mediate cell death throughout the Panicoideae, but the phylogenetic range of the cell death mechanism is not known. In this study we show that male flowers of Z. aquatica show neither the distinctive pattern of cell death nor the ontogenetic timing of abortion that are characteristic of male flower formation in studied species of Panicoideae. This indicates that these unisexual flowers may be produced by an entirely different mechanism from that employed by the Panicoideae. Either ts2 does not control sex expression in Zizania, or it is deployed at a different time, and possibly in different tissues, with a different histological result. Our results indicate that the independent origins of male flowers in Gramineae apparently do not have a common system of genetic control.     

Characterization of an ethylene-inducible, calcium-dependent nuclease that is differentially expressed in cucumber flower development

? Production of unisexual flowers is an important mechanism that promotes cross-pollination in angiosperms. We previously identified primordial anther-specific DNA damage and organ-specific ethylene perception responsible for the arrest of stamen development in female flowers, but little is known about how the two processes are linked. ? To identify potential links between the two processes, we performed suppression subtractive hybridization (SSH) on cucumber (Cucumis sativus L.) stamens of male and female flowers at stage 6, with stamens at stage 5 of bisexual flowers as a control. ? Among the differentially expressed genes, we identified an expressed sequence tag (EST) encoding a cucumber homolog to an Arabidopsis calcium-dependent nuclease (CAN), designated CsCaN. Full-length CsCaN cDNA and the respective genomic DNA sequence were cloned and characterized. The CsCaN protein exhibited calcium-dependent nuclease activity. CsCaN showed ubiquitous expression; however, increased gene expression was detected in the stamens of stage 6 female flowers compared with male flowers. As expected, CsCaN expression was ethylene inducible. It was of great interest that CsCaN was post-translationally modified. ? This study demonstrated that CsCaN is a novel cucumber nuclease gene, whose DNase activity is regulated at multiple levels, and which could be involved in the primordial anther-specific DNA damage of developing female cucumber flowers.