Floral development and the formation of unisexual spikelets in the Andropogoneae (Poaceae)

We investigated spikelet development in four distantly related species of the grass tribe Andropogoneae to determine whether spikelet development and the formation of unisexual florets are uniform throughout the tribe. We studied development in Bothriochloa bladhii, Coelorachis aurita, Heteropogon contortus, and Hyparrhenia hirta, and compared these with Panicum, a member of the sister tribe Paniceae. Many aspects of spikelet development in the species we have studied correlate with what is already known for Tripsacum and maize (both Andropogoneae), despite variation in how unisexual florets are distributed on the plant. The formation of unisexual spikelets is also uniform. All florets initiate both pistil and stamen primordia. In florets destined to be male, cell death occurs in the subepidermal layers of the gynoecium after the formation of a gynoecial ridge. In florets destined to be female, there is no apparent cell death in the stamens, but growth ceases after anther formation. The similarity in spikelet development and the formation of unisexual florets point to a common genetic mechanism for sex determination throughout the Andropogoneae and possibly the entire Panicoideae. Use of a cell death pathway to cause gynoecial abortion may be the basis of one morphological character that defines the subfamily.     

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.     

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.     

Molecular evolution of the maize sex-determining gene TASSELSEED2 in Bouteloua (Poaceae)

The majority of angiosperms produce hermaphrodite flowers, while a lesser number (20-30%) produce unisexual flowers. Little is known about the molecular biology of sex-determination in angiosperms, however, a few sex-determining genes have been cloned from the model system Zea mays. One of these genes is Tasselseed2 (Ts2) which has been shown to be involved in the arrest of developing pistils in male flowers. In this study, we sequenced a putative homologue of Ts2 in species of Bouteloua, a genus in the grass subfamily Chloridoideae. We found significant genetic variation at Ts2 in Bouteloua relative to other developmental genes characterized in maize and other grass species. We also found that in Bouteluoua, Ts2 is evolving non-neutrally in the hermaphrodite-flowered Bouteloua hirsuta while no difference from neutral expectation was detected at Ts2 in the monoecious/dioecious Bouteloua dimorpha. The putatively neutral gene Alcohol Dehydrogenase1 (Adh1) was also examined for the same species of Bouteloua, and no departure from neutral expectation was detected. Our results suggest that purifying selection may be acting on Ts2 in the hermaphrodite-flowered B. hirsuta while no evidence of selection was detected at Ts2 in the monoecious/dioecious B. dimorpha.     

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.     

Multiple developmental processes underlie sex differentiation in angiosperms

The production of unisexual flowers has evolved numerous times in dioecious and monoecious plant taxa. Based on repeated evolutionary origins, a great variety of developmental and genetic mechanisms underlying unisexual flower development is predicted. Here, we comprehensively review the modes of development of unisexual flowers, test potential correlations with sexual system, and end with a synthesis of the genetics and hormonal regulation of plant sex determination. We find that the stage of organ abortion in male and female flowers is temporally correlated within species and also confirm that the arrest of development does not tend to occur preferentially at a particular stage, or via a common process.     

The developmental floral morphology of Montrichardia arborescens (Araceae) revisited

The spadix of Montrichardia arborescens contains unisexual flowers without a perianth. The pistillate flowers are located in the basal portion of the inflorescence, and the staminate flowers are located in the apical portion. There is a narrow :zone between male flowers and female flowers consisting of atypical flowers. The portion of the atypical flowers facing the staminate zone exhibits staminate characters (stamens), and the portion facing the pistillate zone has an aborted gynoecium. The floral development of Montrichurdia is compared with that of Philodendron and a new interpretation of the morphology of atypical flowers of Montrichardia is proposed. Ontogenetic evidence supports relationships with Philodendron rather than Cercestis. 2001 The Linnean Society of London     

Molecular evolution of the maize sex-determining gene TASSELSEED2 in Bouteloua (Poaceae)

The majority of angiosperms produce hermaphrodite flowers, while a lesser number (20–30%) produce unisexual flowers. Little is known about the molecular biology of sex-determination in angiosperms, however, a few sex-determining genes have been cloned from the model system Zea mays. One of these genes is Tasselseed2 (Ts2) which has been shown to be involved in the arrest of developing pistils in male flowers. In this study, we sequenced a putative homologue of Ts2 in species of Bouteloua, a genus in the grass subfamily Chloridoideae. We found significant genetic variation at Ts2 in Bouteloua relative to other developmental genes characterized in maize and other grass species. We also found that in Bouteluoua, Ts2 is evolving non-neutrally in the hermaphrodite-flowered Bouteloua hirsuta while no difference from neutral expectation was detected at Ts2 in the monoecious/dioecious Bouteloua dimorpha. The putatively neutral gene Alcohol Dehydrogenase1 (Adh1) was also examined for the same species of Bouteloua, and no departure from neutral expectation was detected. Our results suggest that purifying selection may be acting on Ts2 in the hermaphrodite-flowered B. hirsuta while no evidence of selection was detected at Ts2 in the monoecious/dioecious B. dimorpha.     

Developmental Events Associated with the Critical Stage for Sex Determination in Wild-Rice Florets

The developmental events of florets and a critical stage for sex determination in two wild-rice populations (Zizania palustris cv. Franklin and Zizania palustris cv. K-2Pi) have been identified. Formation of bisexual florets precedes the development of both male and female florets. Developmental indicators, established by measuring the length of florets and panicles, indicate that the critical stage for sex determination occurs when floret and panicle lengths are 1-2 mm and 3 cm, respectively. The stage of floret development at which sex determination occurs is the same in the two investigated wild-rice populations. Organ suppression in bisexual florets is an essential step for sex determination during the formation of unisexual florets. Histological examination of suppressed stamens or pistils in unisexual florets of wild-rice indicates that cell death does not occur during sex determination. In addition, the length of anthers and pistils in bisexual florets indicates that floral development in the transition zone is normal when compared with the male florets in the male spikelets and female florets in the female spikelets.     

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.     

Phylogeny and a new tribal classification of the Panicoideae s.l. (Poaceae) based on plastid and nuclear sequence data and structural data

? Premise of the study: The subfamily Panicoideae (Poaceae) encompasses nearly one-third of the diversity of grass species, including important crops such as maize and sugarcane. Previous analyses recovered strong support for a Panicoideae+Centothecoideae lineage within the diverse Panicoideae+Arundinoideae+Chloridoideae+Micrairoideae+Aristidoideae+Danthonioideae (PACMAD) clade, although support for internal relationships was inconsistent. The objectives of this research were to (1) further test the monophyly of each subfamily and previously recovered clades within the Panicoideae+Centothecoideae lineage, (2) establish phylogenetic relationships among these groups, and (3) propose a new tribal classification for this lineage based explicitly on the phylogeny. ? Methods: Maximum parsimony and Bayesian inference analyses of 37 taxa were based on previously published sequences (ndhF and rpl16 intron) and on new plastid and nuclear (rbcL and granule-bound starch synthase I) sequence data as well as structural data. ? Key results. The Panicoideae+Centothecoideae lineage and a majority of the clades identified in previous analyses continue to be robustly supported, but resolution along the backbone of the topology remains elusive. Support for the monophyly of both subfamilies was lacking although support values for some clades increased. The tribes Centotheceae and Arundinelleae were confirmed as polyphyletic. ? Conclusions: Subfamily Centothecoideae is formally submerged into the Panicoideae, and a new tribal classification for the expanded Panicoideae is proposed based explicitly on the phylogeny. This classification includes 12 tribes of which Chasmanthieae and Zeugiteae are segretated from the Centotheceae; Tristachyideae is segregated from Arundinelleae, and a new tribe, Cyperochloeae, is validated to accommodate two isolated genera. A key to the tribes is provided.     

A Systematic and Evolutionary Study of Zizania L. (Gramineae) Pollen Morphology

This paper deals with pollen morphology of Zizania L. and its relatives. A total of 7 genera, 13 species, 3 varieties and 1 form were examined under light microscope and scanning electron microscope. The results are as follows: 1. The genus Zizania belongs to tribe Oryzeae as shown by pollen characters, i, e. subspheroidal to ovoid in shape, monoporate, exine two-layered, with minute granules under LM. 2. The evolutionary trend of these taxa seems to be from minute granules free (Zizania latifolia, Z. texana, Zizaniopsis milicea and Oryza sativa) to minute granules aggregated in a group of 2-4 (many) (Zizania aquatica, Z. palustris, Leersia hexandra etc.). The genus Zizania may be derived from the ancient stock which has also given rise to the genus Oryza, and therefore parallel evolution may have taken place in Oryzeae, i. e. from perennial species to annual species in Zizania in one line, and from the genus Oryza to Leersia, Chikusichloa etc. in the other. 3. The characters of pollen morphology under LM and SEM support the division of the genus, Zizania into 4 species, 2 subspecies in the world, i. e. Z. latifolia (Griseb.) Turcz. ex Stapf, Z. texana Hitchc., Z. aquatica subsp. aquatica, Z. aquatica subsp. brevis (Fassett) S. L.Chen, Z. palustris subsp. palustris, and Z. palustris subsp. interior (Fassett) S. L. Chen.     

BREEDING SYSTEM IN FICUS CARICA,THE COMMON FIG. I. FLORAL DIVERSITY

Coevolution in Ficus carica (Moraceae) and the fig wasp (Blastophaga psenes, Agaonidae, Chalcidoidea) has resulted in a complex breeding system involving two tree morphs (Caprifig and Edible fig), three floral forms (long-styled female, short-styled female, and male flowers) and the insect pollinator. The two female floral forms have been reported to differ only in style length and stigma shape. In the present study, we demonstrate that the two female flowers differ from inception—short-styled flower primordia are smaller and exhibit significantly greater individual variation than do those of the long-styled flower, and the relative growth rate of each flower type differs. Mature forms exhibit disparity in style length, in stigma characteristics, and in degree of fusion of stylar lobes. Female flowers of both tree morphs are unisexual from inception. Male flowers of the Caprifig tree morph are initiated as hermaphrodites and gynoecium abortion occurs before megaspore mother cell stage. A single inflorescence therefore expresses two pathways to unisexuality. Hermaphrodite flower primordia were repeatedly found in the supposedly unisexual female syconium of the Edible fig tree morph. Based on its developmental morphology, Ficus carica appears to be of gynomonoecious ancestry.     

Whole chloroplast genome comparison of rice,maize, and wheat: implications for chloroplast gene diversification and phylogeny of cereals

The fully sequenced chloroplast genomes of maize (subfamily Panicoideae), rice (subfamily Bambusoideae), and wheat (subfamily Pooideae) provide the unique opportunity to investigate the evolution of chloroplast genes and genomes in the grass family (Poaceae) by whole-genome comparison. Analyses of nucleotide sequence variations in 106 cereal chloroplast genes with tobacco sequences as the outgroup suggested that (1) most of the genic regions of the chloroplast genomes of maize, rice, and wheat have evolved at similar rates; (2) RNA genes have highly conservative evolutionary rates relative to the other genes; (3) photosynthetic genes have been under strong purifying selection; (4) between the three cereals, 14 genes which account for about 28% of the genic region have evolved with heterogeneous nucleotide substitution rates; and (5) rice genes tend to have evolved more slowly than the others at loci where rate heterogeneity exists. Although the mechanism that underlies chloroplast gene diversification is complex, our analyses identified variation in nonsynonymous substitution rates as a genetic force that generates heterogeneity, which is evidence of selection in chloroplast gene diversification at the intrafamilial level. Phylogenetic trees constructed with the variable nucleotide sites of the chloroplast genes place maize basal to the rice-wheat clade, revealing a close relationship between the Bambusoideae and Pooideae.     

Floral development and vasculature in Eriocaulon (Eriocaulaceae) provide insights into the evolution of Poales

Background and AimsFloral developmental studies are crucial for understanding the evolution of floral structures and sexual systems in angiosperms. Within the monocot order Poales, both subfamilies of Eriocaulaceae have unisexual flowers bearing unusual nectaries. Few previous studies have investigated floral development in subfamily Eriocauloideae, which includes the large, diverse and widespread genus Eriocaulon. To understand floral variation and the evolution of the androecium, gynoecium and floral nectaries of Eriocaulaceae, we analysed floral development and vasculature in Eriocaulon and compared it with that of subfamily Paepalanthoideae and the related family Xyridaceae in a phylogenetic context.MethodsThirteen species of Eriocaulon were studied. Developmental analysis was carried out using scanning electron microscopy, and vasculature analysis was carried out using light microscopy. Fresh material was also analysed using scanning electron microscopy with a cryo function. Character evolution was reconstructed over well-resolved phylogenies.Key ResultsPerianth reductions can occur due to delayed development that can also result in loss of the vascular bundles of the median sepals. Nectariferous petal glands cease development and remain vestigial in some species. In staminate flowers, the inner stamens can emerge before the outer ones, and carpels are transformed into nectariferous carpellodes. In pistillate flowers, stamens are reduced to staminodes and the gynoecium has dorsal stigmas.ConclusionsFloral morphology is highly diverse in Eriocaulon, as a result of fusion, reduction or loss of perianth parts. The nectariferous carpellodes of staminate flowers originated first in the ancestor of Eriocaulaceae; petal glands and nectariferous branches of pistillate flowers originated independently in Eriocaulaceae through transfer of function. We present a hypothesis of floral evolution for the family, illustrating a shift from bisexuality to unisexuality and the evolution of nectaries in a complex monocot family, which can contribute to future studies on reproductive biology and floral evolution in other groups.     

臭椿花器官分化的初步研究

利用扫描电镜(SEM)和光镜(LM)对臭椿花序及花器官的分化和发育进行了初步研究,表明:1)臭椿花器官分化于当年的4月初,为圆锥花序;2)分化顺序为花萼原基、花冠原基、雄蕊原基和雌蕊原基。5个萼片原基的发生不同步,并且呈螺旋状发生;5个花瓣原基几乎同步发生且其生长要比雄蕊原基缓慢;雄蕊10枚,两轮排列,每轮5个原基的分化基本是同步的;雌蕊5,其分化速度较快;3)在两性花植株中,5个心皮顶端粘合形成柱头和花柱,而在雄株中,5个心皮退化,只有雄蕊原基分化出花药和花丝。本研究着重观察了臭椿中雄花及两性花发育的过程中两性花向单性花的转变。结果表明,臭椿两性花及单性花的形成在花器官的各原基上是一致的(尽管时间上有差异),雌雄蕊原基同时出现在每一个花器官分化过程中,但是,可育性结构部分的形成取决于其原基是否分化成所应有的结构:雄蕊原基分化形成花药与花丝,雌蕊原基分化形成花柱、柱头和子房。臭椿单性花的形成是由于两性花中雌蕊原基的退化所造成,其机理有待于进一步研究。     

The correlation between development of atypical bisexual flowers and phylogeny in the Aroideae (Araceae)

 In the intermediate zone of the inflorescence of genera of Aroideae one can find flowers with male and female characteristics. Until now, two types of developmental sequences of atypical bisexual flowers (ABFs) have been recognized: the Philodendron type and the Cercestis type. In the Philodendron type, bisexual flowers generally consist of functional carpels and staminodes inserted on the same whorl. In the Cercestis type, the gynoecium and stamens are inserted on two different whorls. These different ontogenetic patterns represent two different pathways in the evolution of unisexual flowers in this subfamily. A molecular phylogenetic analysis of 33 genera of Araceae, based on the chloroplast trnL intron and trnL–F intergenic spacer sequences was carried out. We use this phylogenetic analysis and those published by French et al. (1995) and Mayo et al. (1997) to examine the distribution of the two types of ABFs in selected genera. Our results suggest that the two developmental patterns of ABFs in Aroideae sensu Mayo et al. (1997) do not correspond to two separate evolutionary lineages but rather are more or less consistent within clades. Although this new molecular phylogeny does not include all aroid genera, it corroborates in general, at the subfamily level, the molecular analysis of French et al. (1995) based on chloroplast DNA restriction site data and the analysis of Mayo et al. (1997) based on morphological and anatomical data. Received March 15, 2001 Accepted October 11, 2001