Background and Aims
In spite of recent phylogenetic analyses for the Chenopodiaceae–Amaranthaceae complex, some morphological characters are not unambiguously interpreted, which raises homology questions. Therefore, ontogenetic investigations, emphasizing on ‘bracteoles’ in Atripliceae and flowers in Chenopodioideae, were conducted. This first paper presents original ontogenetic observations in Beta vulgaris, which was chosen as a reference species for further comparative investigation because of its unclarified phylogenetic position and its flowers with a (semi-)inferior ovary, whereas all other Chenopodiaceae–Amaranthaceae have hypogynous flowers.
Inflorescences and flowers were examined using scanning electron microscopy and light microscopy.
Floral development starts from an inflorescence unit primordium subtended by a lateral bract. This primordium develops into a determinate axis on which two opposite lateral flowers originate, each subtended by a bracteole. On a flower primordium, first five tepal primordia appear, followed by five opposite stamen primordia. Simultaneously, a convex floral apex appears, which differentiates into an annular ovary primordium with three stigma primordia, surrounding a central, single ovule. A floral tube, which raises the outer floral whorls, envelops the ovary, resulting in a semi-inferior ovary at mature stage. Similarly, a stamen tube is formed, raising the insertion points of the stamens, and forming a staminal ring, which does not contain stomata. During floral development, the calyces of the terminal flower and of one of the lateral flowers often fuse, forming a compound fruit structure.
In Beta vulgaris, the inflorescence is compound, consisting of an indeterminate main axis with many elementary dichasia as inflorescence units, of which the terminal flower and one lateral flower fuse at a later stage. Floral parts develop starting from the outer whorl towards the gynoecium. Because of the formation of an epigynous hypanthium, the ovary becomes semi-inferior in the course of floral development.
Beta vulgaris; Chenopodiaceae; floral ontogeny; gynoecial development; epigynous hypanthium; semi-inferior ovary; inflorescence ontogeny; LM; SEM
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.
Suppression of B class genes in spinach resulted in the expected homeotic transformation of stamens into carpels but also affected the number of perianth parts and the presence of fourth whorl. Phenotypically normal female flowers developed on SpPI-silenced male plants. Suppression of the spinach C class floral organ identity gene, SpAG, resulted in loss of reproductive organ identity, and indeterminate flowers, but did not result in additional sex-specific characteristics or structures. Analysis of the genomic sequences of both SpAP3 and SpPI did not reveal any allelic differences between males and females.
Sexual dimorphism in spinach is not the result of homeotic transformation of established organs, but rather is the result of differential initiation and development of the third and fourth whorl primordia. SpAG is inferred to have organ identity and meristem termination functions similar to other angiosperm C class genes. In contrast, while SpPI and SpAP3 resemble other angiosperms in their essential functions in establishing stamen identity, they also appear to have an additional function in regulating organ number and identity outside of the third whorl. We present a model for the evolution of dioecy in spinach based on the regulation of B class expression.
Background and Aims
Most of the diversity in the pseudanthia of Asteraceae is based on the differential symmetry and sexuality of its flowers. In Anacyclus, where there are (1) homogamous capitula, with bisexual, mainly actinomorphic and pentamerous flowers; and (2) heterogamous capitula, with peripheral zygomorphic, trimerous and long-/short-rayed female flowers, the floral ontogeny was investigated to infer their origin.
Floral morphology and ontogeny were studied using scanning electron microscope and light microscope techniques
Disc flowers, subtended by paleae, initiate acropetally. Perianth and androecium initiation is unidirectional/simultaneous. Late zygomorphy occurs by enlargement of the adaxial perianth lobes. In contrast, ray flowers, subtended by involucral bracts, initiate after the proximal disc buds, breaking the inflorescence acropetal pattern. Early zygomorphy is manifested through the fusion of the lateral and abaxial perianth lobes and the arrest of the adaxials. We report atypical phenotypes with peripheral ‘trumpet’ flowers from natural populations. The peripheral ‘trumpet’ buds initiate after disc flowers, but maintain an actinomorphic perianth. All phenotypes are compared and interpreted in the context of alternative scenarios for the origin of the capitulum and the perianth identity.
Homogamous inflorescences display a uniform floral morphology and development, whereas the peripheral buds in heterogamous capitula display remarkable plasticity. Disc and ray flowers follow different floral developmental pathways. Peripheral zygomorphic flowers initiate after the proximal actinomorphic disc flowers, behaving as lateral independent units of the pseudanthial disc from inception. The perianth and the androecium are the most variable whorls across the different types of flowers, but their changes are not correlated. Lack of homology between hypanthial appendages and a calyx, and the perianth double-sided structure are discussed for Anacyclus together with potential causes of its ray flower plasticity.
Anacyclus; capitulum; Compositae; disc flowers; evo-devo; inflorescence; pseudanthium; ray flowers
• Background and Aims Based on molecular phylogenetic analysis, it has been suggested recently that the Cyperaceae comprises only two subfamilies: the Mapanioideae and the Cyperoideae. In most flowers of the Cyperoideae, the whorl of inner stamens is reduced, resulting in tetracyclic flowers. In the more primitive (scirpoid) genera within the Cyperoideae, the perianth consists of two polysymmetric whorls, whereas the perianth parts in the more derived genera have been subject to modifications and/or reduction. Comparative studies of the many silky hairs of Eriophorum and of the eight bristles of Dulichium have given rise to much discussion about their homology.
• Methods The spikelet and floral ontogeny in freshly collected inflorescences was investigated using scanning electron microscopy.
• Key Results Complete floral ontogenies are presented for Scirpus sylvaticus L., Eriophorum latifolium Hoppe and Dulichium arundinaceum (L.) Britton, with special reference to the perianth. The results in S. sylvaticus confirm the trimerous monocot-like organization of the flower. It is used as a model for floral development in Cyperoideae. In the early developmental stages, the androecium of E. latifolium is surrounded by a massive perigonial primordium, from which the many hair-like bristles originate. Consequently, the stamens develop among the hair primordia, more or less simultaneously. The hairs are arranged in whorls, which develop centripetally. The development of the perianth in D. arundinaceum starts with the formation of three initial perianth primordia opposite the stamens. Subsequently, two more abaxial bristle primordia, alternating with the stamens, originate simultaneously with the appearance of three adaxial bristle primordia in the zone where an adaxial inner perianth primordium is expected.
• Conclusions The floral development in E. latifolium and D. arundinaceum can be considered as variations upon the scirpoid floral ontogenetic theme.
Dulichium arundinaceum (L.) Britton; Eriophorum latifolium Hoppe; floral ontogeny; perianth; scirpoid flower; Scirpus sylvaticus L; scanning electron microscopy
Understanding and modelling early events of floral meristem patterning and floral development requires consideration of positional information regarding the organs surrounding the floral meristem, such as the flower-subtending bracts (FSBs) and floral prophylls (bracteoles). In common with models of regulation of floral patterning, the simplest models of phyllotaxy consider only unbranched uniaxial systems. Racemose inflorescences and thyrses offer a useful model system for investigating morphogenetic interactions between organs belonging to different axes.
This review considers (1) racemose inflorescences of early-divergent and lilioid monocots and their possible relationship with other inflorescence types, (2) hypotheses on the morphogenetic significance of phyllomes surrounding developing flowers, (3) patterns of FSB reduction and (4) vascular patterns in the primary inflorescence axis and lateral pedicels.
Racemose (partial) inflorescences represent the plesiomorphic condition in monocots. The presence or absence of a terminal flower or flower-like structure is labile among early-divergent monocots. In some Alismatales, a few-flowered racemose inflorescence can be entirely transformed into a terminal ‘flower’. The presence or absence and position of additional phyllomes on the lateral pedicels represent important taxonomic markers and key features in regulation of flower patterning. Racemose inflorescences with a single floral prophyll are closely related to thyrses. Floral patterning is either unidirectional or simultaneous in species that lack a floral prophyll or possess a single adaxial floral prophyll and usually spiral in the outer perianth whorl in species with a transversely oriented floral prophyll. Inhibitory fields of surrounding phyllomes are relevant but insufficient to explain these patterns; other important factors are meristem space economy and/or the inhibitory activity of the primary inflorescence axis. Two patterns of FSB reduction exist in basal monocots: (1) complete FSB suppression (cryptic flower-subtending bract) and (2) formation of a ‘hybrid’ organ by overlap of the developmental programmes of the FSB and the first abaxial organ formed on the floral pedicel. FSB reduction affects patterns of interaction between the conductive systems of the flower and the primary inflorescence axis.
Bracteole; flower; flower-subtending bract; inflorescence; inhibitory field; pattern formation; prophyll; regulation of development; vasculature
• Background and Aims On the basis of molecular evidence Berberidopsidaceae have been linked with Aextoxicaceae in an order Berberidopsidales at the base of the core Eudicots. The floral development of Berberidopsis is central to the understanding of the evolution of floral configurations at the transition of the basal Eudicots to the core Eudicots. It lies at the transition of trimerous or dimerous, simplified apetalous forms into pentamerous, petaliferous flowers.
• Methods The floral ontogeny of Berberidopsis was studied with a scanning electron microscope.
• Key Results Flowers are grouped in terminal racemes with variable development. The relationship between the number of tepals, stamens and carpels is more or less fixed and floral initiation follows a strict 2/5 phyllotaxis. Two bracteoles, 12 tepals, eight stamens and three carpels are initiated in a regular sequence. The number of stamens can be increased by a doubling of stamen positions.
• Conclusions The floral ontogeny of Berberidopsis provides support for the shift in floral bauplan from the basal Eudicots to the core Eudicots as a transition of a spiral flower with a 2/5 phyllotaxis to pentamerous flowers with two perianth whorls, two stamen whorls and a single carpel whorl. The differentiation of sepals and petals from bracteotepals is discussed and a comparison is made with other Eudicots with a similar configuration and development. Depending on the resolution of the relationships among the basalmost core Eudicots it is suggested that Berberidopsis either represents a critical stage in the evolution of pentamerous flowers of major clades of Eudicots, or has a floral prototype that may be at the base of evolution of flowers of other core Eudicots. The distribution of a floral Bauplan in other clades of Eudicots similar to Berberidopsidales is discussed.
Aextoxicon; Berberidopsidales; Berberidopsis; core Eudicots; Streptothamnus; bracteotepals; floral development, petals; phylogeny; phyllotaxis; scanning electron microscope
We present a detailed comparative ontogenetic analysis of pseudanthia of representatives of all three subtribes of Euphorbieae (Euphorbiinae, Neoguillauminiinae, Anthosteminae) in order to clarify their homologies and interpretation. The cyathium of Euphorbia and its allies (subtribe Euphorbiinae) closely resembles a bisexual flower but is traditionally interpreted as an inflorescence bearing clusters of highly reduced male flowers surrounding a single terminal female flower. Previously unreported characters are (1) male flowers formed one above the other in the male inflorescences of some Euphorbiinae, (2) late-developing perianthlike structures in some male flowers of Neoguillauminia cleopatra, (3) evidence for a bracteate origin of the female perianth in Anthosteminae and Neoguillauminiinae, and (4) spatiotemporally independent formation of abscission zone and perianth. Indistinct boundaries between inflorescence, flower, and floral organs demonstrate that defining the cyathium neither as an inflorescence nor as a flower is entirely satisfactory and indicate a “hybrid” flower/inflorescence nature of the cyathium. Based on our current knowledge and the existing phylogenetic context, it is most parsimonious to suggest that the cyathium evolved from a determinate thyrse with a terminal female flower surrounded by dichasial male partial inflorescences. We speculate that the cyathium was formed because of strong condensation and possible overlap between expression zones of regulatory genes.
cyathium; Euphorbia; Euphorbiaceae; Euphorbieae; flower; Malpighiales; ontogeny; pseudanthium
Jatropha curcas, whose seed content is approximately 30–40% oil, is an ideal feedstock for producing biodiesel and bio-jet fuels. However, Jatropha plants have a low number of female flowers, which results in low seed yield that cannot meet the needs of the biofuel industry. Thus, increasing the number of female flowers is critical for the improvement of Jatropha seed yield. Our previous findings showed that cytokinin treatment can increase the flower number and female to male ratio and also induce bisexual flowers in Jatropha. The mechanisms underlying the influence of cytokinin on Jatropha flower development and sex determination, however, have not been clarified.
This study examined the transcriptional levels of genes involved in the response to cytokinin in Jatropha inflorescence meristems at different time points after cytokinin treatment by 454 sequencing, which gave rise to a total of 294.6 Mb of transcript sequences. Up-regulated and down-regulated annotated and novel genes were identified, and the expression levels of the genes of interest were confirmed by qRT-PCR. The identified transcripts include those encoding genes involved in the biosynthesis, metabolism, and signaling of cytokinin and other plant hormones, flower development and cell division, which may be related to phenotypic changes of Jatropha in response to cytokinin treatment. Our analysis indicated that Jatropha orthologs of the floral organ identity genes known as ABCE model genes, JcAP1,2, JcPI, JcAG, and JcSEP1,2,3, were all significantly repressed, with an exception of one B-function gene JcAP3 that was shown to be up-regulated by BA treatment, indicating different mechanisms to be involved in the floral organ development of unisexual flowers of Jatropha and bisexual flowers of Arabidopsis. Several cell division-related genes, including JcCycA3;2, JcCycD3;1, JcCycD3;2 and JcTSO1, were up-regulated, which may contribute to the increased flower number after cytokinin treatment.
This study presents the first report of global expression patterns of cytokinin-regulated transcripts in Jatropha inflorescence meristems. This report laid the foundation for further mechanistic studies on Jatropha and other non-model plants responding to cytokinin. Moreover, the identification of functional candidate genes will be useful for generating superior varieties of high-yielding transgenic Jatropha.
Electronic supplementary material
The online version of this article (doi:10.1186/1471-2164-15-974) contains supplementary material, which is available to authorized users.
Biofuel; Physic nut; Cytokinin; 454 Sequencing; Flower; Cell division
• Background and Aims The increase of molecular data and the resulting insights into legume systematics make the search for new morphological characters and a careful re‐investigation of already stated characters necessary. Bracteoles are small, reduced leaves borne close to the base of lateral branches. Although they seem unimportant in older buds, they have an ecological function in protecting the sepal primordia. Furthermore, a morphogenetic function in mediating the onset of sepal initiation is suspected in the literature. The occurrence of bracteoles varies within Papilionoideae, and their distribution is used in legume systematics. But this is open to criticism, because there is a tendency to use ‘absent’ for ‘caducous’. Thus attention here was paid to the initiation of bracteoles as well as to the sequence of sepal initiation.
• Methods The floral development of 30 taxa out of 15 tribes of Papilionoideae was investigated using scanning electron microscopy (SEM).
• Key Results In five taxa the bracteoles initiated, but suppressed early. Furthermore, a broad variability of sepal initiation was found. Besides the widely stated unidirectional pattern, modified unidirectionality, tendencies towards whorled, fully whorled, bidirectional and successive initiation of sepals were all found.
• Conclusion Initiated but suppressed bracteoles are presented as a ‘new’ character in Papilionoideae. Considering the presence of bracteoles as a plesiomorphy, their suppression can be seen as a step towards completely reduced bracteoles. The remarkable variability of the sequence of sepal initiation questions the widely stated unidirectionality of organ initiation in Papilionoideae. The different modes of sepal initiation are deducible from the helical pattern of some caesalpinioids, which is seen as a developmental link of the flowers of Papilionoideae and Caesalpinioideae. The bidirectional sepal initiation is possibly a consequence of the morphogenetic function of bracteoles, although bidirectionality is not found in all taxa with reduced bracteoles.
Bracteoles; Caesalpiniaceae; Caesalpinioideae; floral development; Fabaceae; Faboideae; Leguminosae; Papilionoideae; plastochron; sepals
Background and Aims
According to the floral ABC model, B-function genes appear to play a key role in the origin and diversification of the perianth during the evolution of angiosperms. The basal angiosperm Hedyosmum orientale (Chloranthaceae) has unisexual inflorescences associated with a seemingly primitive reproductive morphology and a reduced perianth structure in female flowers. The aim of this study was to investigate the nature of the perianth and the evolutionary state of the B-function programme in this species.
A series of experiments were conducted to characterize B-gene homologues isolated from H. orientale, including scanning electron microscopy to observe the development of floral organs, phylogenetic analysis to reconstruct gene evolutionary history, reverse transcription–PCR, quantitative real-time PCR and in situ hybridization to identify gene expression patterns, the yeast two-hybrid assay to explore protein dimerization affinities, and transgenic analyses in Arabidopsis thaliana to determine activities of the encoded proteins.
The expression of HoAP3 genes was restricted to stamens, whereas HoPI genes were broadly expressed in all floral organs. HoAP3 was able to partially restore the stamen but not petal identity in Arabidopsis ap3-3 mutants. In contrast, HoPI could rescue aspects of both stamen and petal development in Arabidopsis pi-1 mutants. When the complete C-terminal sequence of HoPI was deleted, however, no or weak transgenic phenotypes were observed and homodimerization capability was completely abolished.
The results suggest that Hedyosmum AP3-like genes have an ancestral function in specifying male reproductive organs, and that the activity of the encoded PI-like proteins is highly conserved between Hedyosmum and Arabidopsis. Moreover, there is evidence that the C-terminal region is important for the function of HoPI. Our findings indicate that the development of the proposed perianth in Hedyosmum does not rely on the B homeotic function.
Floral homeotic B function; Hedyosmum orientale; Chloranthaceae; C-terminal region; HoAP3; HoPI; homodimerization; APETALA3; PISTILLATA
Background and Aims
Variation in the relative female and male reproductive success of flowering plants is widespread, despite the fundamental hermaphroditic condition of the majority of species. In many hermaphroditic populations, environmental conditions and their influence on development and size can influence the gender expression of individuals through the formation of hermaphroditic and unisexual flowers. This study investigates the hypothesis that the bulbous, animal-pollinated, perennial Lilium apertum (Liliaceae) exhibits a form of size-dependent gender modification known as gender diphasy, in which the sexual expression of individuals depends on their size, with plants often changing sex between seasons.
Variation in floral traits was examined in relation to their size using marked individuals in natural populations, and also under glasshouse conditions. Measurements were taken of the height, flower number, floral sex expression, flower size, flower biomass and pollen production of individuals over consecutive years between 2009 and 2012 in seven populations in south-west China.
Flowers of L. apertum are either perfect (hermaphroditic) or staminate (male) and, in any given season, plants exhibit one of three sex phenotypes: only hermaphrodite flowers, a mixture of hermaphroditic and male flowers, or only male flowers. Transitions between each of these sex phenotypes were observed over consecutive years and were commonly size-dependent, particularly transitions from small plants bearing only male flowers to those that were taller with hermaphroditic flowers. Hermaphroditic flowers were significantly larger, heavier and produced more pollen than male flowers.
The results for L. apertum are consistent with the ‘size advantage hypothesis’ developed for animal species with sex change. The theory predicts that when individuals are small they should exhibit the sex for which the costs of reproduction are less, and this usually involves the male phase. L. apertum provides an example of gender diphasy, a rare sexual system in flowering plants.
Lilium apertum; gender diphasy; plant sexual systems; size-dependent gender modification; sex allocation; sex change; plant mating systems
Background and Aims
Annonaceae are one of the largest families of Magnoliales. This study investigates the comparative floral development of 15 species to understand the basis for evolutionary changes in the perianth, androecium and carpels and to provide additional characters for phylogenetic investigation.
Floral ontogeny of 15 species from 12 genera is examined and described using scanning electron microscopy.
Initiation of the three perianth whorls is either helical or unidirectional. Merism is mostly trimerous, occasionally tetramerous and the members of the inner perianth whorl may be missing or are in double position. The androecium and the gynoecium were found to be variable in organ numbers (from highly polymerous to a fixed number, six in the androecium and one or two in the gynoecium). Initiation of the androecium starts invariably with three pairs of stamen primordia along the sides of the hexagonal floral apex. Although inner staminodes were not observed, they were reported in other genera and other families of Magnoliales, except Magnoliaceae and Myristicaceae. Initiation of further organs is centripetal. Androecia with relatively low stamen numbers have a whorled phyllotaxis throughout, while phyllotaxis becomes irregular with higher stamen numbers. The limits between stamens and carpels are unstable and carpels continue the sequence of stamens with a similar variability.
It was found that merism of flowers is often variable in some species with fluctuations between trimery and tetramery. Doubling of inner perianth parts is caused by (unequal) splitting of primordia, contrary to the androecium, and is independent of changes of merism. Derived features, such as a variable merism, absence of the inner perianth and inner staminodes, fixed numbers of stamen and carpels, and capitate or elongate styles are distributed in different clades and evolved independently. The evolution of the androecium is discussed in the context of basal angiosperms: paired outer stamens are the consequence of the transition between the larger perianth parts and much smaller stamens, and not the result of splitting. An increase in stamen number is correlated with their smaller size at initiation, while limits between stamens and carpels are unclear with easy transitions of one organ type into another in some genera, or the complete replacement of carpels by stamens in unisexual flowers.
Annonaceae; basal angiosperms; Magnoliales; androecium; carpel; doubling; floral ontogeny; merism; perianth; reduction; secondary increase
Many co-sexual plants segregate female and male function among flowers on an inflorescence through dichogamy or the production of unisexual flowers. Sexual segregation may reduce self-pollination among flowers within inflorescences (geitonogamy), thereby increasing the pollen available for export to other plants. To assess these complementary roles we manipulated the simultaneously hermaphroditic (adichogamous) flowers of Eichhornia paniculata to produce ten-flowered inflorescences with either female above male flowers (female/male inflorescences) or male/female inflorescences, which competed for mating opportunities with five-flowered adichogamous inflorescences. Because of the upward movement of bumble-bees, selfing increased upward in adichogamous inflorescences (overall female selfing rate s+/-s.e.=0.320+/-0.026). Female flowers of male/female inflorescences selfed less than flowers in corresponding positions in adichogamous inflorescences so s fell to 0.135+/-0.027. In contrast, all-female flowers of female/male inflorescences selfed similarly to upper flowers on adichogamous inflorescences, elevating s (0.437+/-0.043). During 1997, male/female inflorescences sired more outcrossed seeds than female/male or adichogamous inflorescences, whereas during 1994 flowers on male/female inflorescences received fewer visits than those of adichogamous inflorescences, reducing their outcross siring success. Hence, sexual segregation limits geitonogamy and enhances outcross siring success when it does not affect pollinator behaviour, illustrating the importance of both female and male function in inflorescence design.
Sexual systems are highly variable in flowering plants and an important contributor to floral diversity. The ranunculid genus Thalictrum is especially well-suited to study evolutionary transitions in sexual systems. Homeotic transformation of sexual organs (stamens and carpels) is a plausible mechanism for the transition from hermaphroditic to unisexual flowers in this lineage because flowers of dioecious species develop unisexually from inception. The single-copy gene PISTILLATA (PI) constitutes a likely candidate for rapid switches between stamen and carpel identity. Here, we first characterized the expression pattern of all B class genes in the dioecious species T. dioicum. As expected, all B class orthologs are expressed in stamens from the earliest stages. Certain AP3 lineages were also expressed late in sepal development. We then tested whether orthologs of PI could potentially control sexual system transitions in Thalictrum, by knocking-down their expression in T. dioicum and the hermaphroditic species T. thalictroides. In T. dioicum, we found that ThdPI-1/2 silencing caused stamen primordia to develop into carpels, resulting in male to female flower conversions. In T. thalictroides, we found that ThtPI silencing caused stamen primordia to develop into supernumerary carpels, resulting in hermaphroditic to female flower conversions. These phenotypes illustrate the ability for homeotic mutations to bring about sudden and potentially adaptive changes by altering the function of a single gene. We propose a two-step evolutionary model where transitions from hermaphroditic to unisexual plants in Thalictrum result from two independent mutations at a B class gene locus. Our PI knockdown experiments in T. thalictroides recapitulate the second step in this model: the evolution of female plants as a result of a loss-of-function mutation in a B class gene.
B class genes; PISTILLATA; VIGS; RNAi; ranunculid; ABC model; sex determination; MADS box genes
Lepidium sisymbrioides, a polyploid New Zealand endemic, is the sole dioecious species in Brassicaceae and therefore the closest dioecious relative of the model plant Arabidopsis thaliana. The attractiveness of developing this system for future studies on the genetics of sex determination prompted us to investigate historical and developmental factors surrounding the evolution of its unisexual flowers. Our goal was to determine the evolutionary pattern of polyploidization of L. sisymbrioides and the timing and process of flower reproductive organ abortion. To that end, we used a combination of phylogenetics to place this species within the complex history of polyploidization events in Lepidium and histology to compare its floral ontogeny to that of its closest hermaphroditic relatives and to A. thaliana.
Using a nuclear locus (PISTILLATA), we reconstructed the gene tree among Lepidium taxa and applied a phylogenetic network analysis to identify ancestral genomes that contributed to the evolution of L. sisymbrioides. Combining this phylogenetic framework with cytological and genome size data, we estimated L. sisymbrioides as an allo-octoploid resulting from three hybridization events. Our investigations of flower development showed that unisexual flowers appear to abort reproductive organs by programmed cell death in female flowers and by developmental arrest in male flowers. This selective abortion occurs at the same floral developmental stage in both males and females, corresponding to Arabidopsis stage nine.
Dioecy in Brassicaceae evolved once in L. sisymbrioides following several allopolyploidization events, by a process of selective abortion of reproductive organs at intermediate stages of flower development. Different developmental processes, but similar timing of abortions, affect male versus female flower development. An increased understanding of how and when reproductive organs abort in this species, combined with our estimates of ancestral genome contributions, ploidy and genome size, lay the foundation for future efforts to examine the genetic mechanisms involved in the evolution of unisexual flowers in the closest dioecious relative of the best studied model plant.
Electronic supplementary material
The online version of this article (doi:10.1186/2041-9139-5-42) contains supplementary material, which is available to authorized users.
allopolyploidy; programmed cell death; dioecy; floral ontogeny; genome size; organ arrest; phylogenetic network; PISTILLATA; sex differentiation; unisexual flowers
Background and Aims
Balsaminaceae consist of two genera, the monospecific Hydrocera and its species-rich sister Impatiens. Although both genera are seemingly rather similar in overall appearance, they differ in ecology, distribution range, habitat preference and morphology. Because morphological support for the current molecular phylogenetic hypothesis of Impatiens is low, a developmental study is necessary in order to obtain better insights into the evolutionary history of the family. Therefore, the floral development of H. triflora and I. omeiana was investigated, representing the most early-diverged lineage of Impatiens, and the observations were compared with the literature.
Flowers at all developmental stages were examined using scanning electron microscopy and light microscopy.
In Hydrocera, two whorls of five free perianth primordia develop into a less zygomorphic perianth compared with its sister genus. The androecial cap originates from five individual stamen primordia. Post-genital fusion of the upper parts of the filaments result in a filament ring below the anthers. The anthers fuse forming connivent anther-like units. The gynoecium of Hydrocera is pentamerous; it is largely synascidiate in early development. Only then is a symplicate zone formed resulting in style and stigmas. In I. omeiana, the perianth is formed as in Hydrocera. Five individual stamen primordia develop into five stamens, of which the upper part of the filaments converge with each other. The gynoecium of I. omeiana is tetramerous; it appears annular in early development.
Comparison of the present results with developmental data from the literature confirms the perianth morphocline hypothesis in which a congenital fusion of the parts of the perianth results in a shift from pentasepalous to trisepalous flowers. In addition, the development of the androecial cap and the gynoecium follows several distinct ontogenetic sequences within the family.
Balsaminaceae; androecium; floral development; gynoecium; Hydrocera triflora; Impatiens omeiana
Background and Aims
Dimorphism among floral traits can evolve through variation in selection intensity between female and male performance, especially when sex functions are separated between flowers on a plant (monoecy), or between individuals (dioecy). In animal-pollinated species, male floral traits are predicted to be larger because competition for pollinators should favour larger displays. Floral dimorphism may be greater in dioecious than monoecious populations because of trade-offs between female and male function and opportunities for selfing in hermaphrodites.
These predictions were tested by surveying flower size, total flowers per inflorescence and daily display size in the insect-pollinated Sagittaria latifolia (Alismataceae). This species is useful for comparative analysis because populations are mostly either monoecious or dioecious. We examined floral dimorphism in 13 monoecious and 16 dioecious populations in eastern North America.
Male flowers were significantly larger than female flowers in monoecious and dioecious populations, but there was no evidence for greater flower size dimorphism in dioecious populations despite their larger flower sizes overall. Although inflorescences in both dioecious and monoecious populations produced more male flowers, daily floral displays were significantly larger for female than male function due to more synchronous female flower opening. Daily floral display dimorphism was significantly greater in dioecious populations, due to greater female daily floral displays. There was a positive relationship between mean flower size and total flowers per inflorescence for both sexes in dioecious populations, but no relationship for either sex function in monoecious populations. Flower size dimorphism was positively correlated with the frequencies of females in dioecious populations.
The increased size and number of male flowers and protracted male floral displays in S. latifolia are probably shaped by sexual selection for more effective pollen dispersal.
Sexual dimorphism; flower size; daily floral display; sexual selection; sex ratios; monoecy; dioecy; Sagittaria latifolia
Background and Aims
Eriocaulaceae (Poales) is currently divided in two subfamilies: Eriocauloideae, which comprises two genera and Paepalanthoideae, with nine genera. The floral anatomy of Actinocephalus polyanthus, Leiothrix fluitans, Paepalanthus chlorocephalus, P. flaccidus and Rondonanthus roraimae was studied here. The flowers of these species of Paepalanthoideae are unisexual, and form capitulum-type inflorescences. Staminate and pistillate flowers are randomly distributed in the capitulum and develop centripetally. This work aims to establish a floral nomenclature for the Eriocaulaceae to provide more information about the taxonomy and phylogeny of the family.
Light microscopy, scanning electron microscopy and chemical tests were used to investigate the floral structures.
Staminate and pistillate flowers are trimerous (except in P. flaccidus, which presents dimerous flowers), and the perianth of all species is differentiated into sepals and petals. Staminate flowers present an androecium with scale-like staminodes (not in R. roraimae) and fertile stamens, and nectariferous pistillodes. Pistillate flowers present scale-like staminodes (except for R. roraimae, which presents elongated and vascularized staminodes), and a gynoecium with a hollow style, ramified in stigmatic and nectariferous portions.
The scale-like staminodes present in the species of Paepalanthoideae indicate a probable reduction of the outer whorl of stamens present in species of Eriocauloideae. Among the Paepalanthoideae genera, Rondonanthus, which is probably basal, shows vascularized staminodes in their pistillate flowers. The occurrence of nectariferous pistillodes in staminate flowers and that of nectariferous portions of the style in pistillate flowers of Paepalanthoideae are emphasized as nectariferous structures in Eriocaulaceae.
Eriocaulaceae; Paepalanthoideae; nectariferous structures; staminodes; staminate flowers; pistillate flowers; floral anatomy; monocotyledons; Poales
• Background and Aims In 1976 the monotypic genus Hellmuthia was placed in the Hypolytreae s.l., but was subsequently ascribed to the Mapanioideae, tribe Chrysitricheae, mainly because of the presence in Hellmuthia of two lateral, mapanioid-like floral scales with ciliated keels, the anatomy of the nutlet, the embryo and the inflorescence. Recently, based on cladistic analyses and supported by pollen ontogenetic evidence, Hellmuthia was transferred to a Cyperaceae, tribe Cypereae, clade mainly consisting of Ficinia and Isolepis. In this study, the floral ontogeny in Hellmuthia was investigated and compared with the floral ontogeny in Paramapania, with special attention for the floral scales.
• Methods Freshly collected inflorescences of Hellmuthia membranacea and Paramapania parvibractea were investigated using scanning electron and light microscopy.
• Key Results In the conical ‘spikelet’ in Hellmuthia, proximal bracts occur, each axillating an axis with empty glume-like structures, or a reduced spikelet. Hence, it is a reduced partial inflorescence. In Hellmuthia, the stamen primordia originate before the primordia of the perianth–gynoecium appear. Moreover, a third adaxially positioned ‘floral scale’ was observed for the first time. The position and relative time of appearance of the floral scales in Hellmuthia are typical for perianth parts in Cyperoideae. The basal position of Hellmuthia within a clade of species with usually perianthless flowers, allows the presence of rudiments of a perianth in Hellmuthia to be interpreted as a primitive state. Development of the lateral ‘scales’ in Paramapania follows a different pattern. Therefore, it was decided that the lateral ‘scales’ in Paramapania are different from the lateral perianth parts in Hellmuthia. The pollen grains in Hellmuthia are cyperoid, with one polar and five lateral apertures, of which the membrane is covered with sexinous bodies. The pollen surface is granulate and perforate with microspines.
• Conclusions The floral ontogeny in Hellmuthia occurs according to the general cyperoid pattern. The lateral scales in Hellmuthia are perianth parts, and they are not homologous to the lateral ‘scales’ in Paramapania.
Floral scales; Paramapania; floral ontogeny; Cyperaceae; Hellmuthia; SEM; homology
The gynoecium is the female reproductive structure of angiosperm flowers. In Arabidopsis thaliana the gynoecium is composed of two carpels that are fused into a tube-like structure. As the gynoecial primordium arises from the floral meristem, a specialized meristematic structure, the carpel margin meristem (CMM), develops from portions of the medial gynoecial domain. The CMM is critical for reproductive competence because it gives rise to the ovules, the precursors of the seeds. Here we report a functional role for the transcription factor PERIANTHIA (PAN) in the development of the gynoecial medial domain and the formation of ovule primordia. This function of PAN is revealed in pan aintegumenta (ant) as well as seuss (seu) pan double mutants that form reduced numbers of ovules. Previously, PAN was identified as a regulator of perianth organ number and as a direct activator of AGAMOUS (AG) expression in floral whorl four. However, the seu pan double mutants display enhanced ectopic AG expression in developing sepals and the partial transformation of sepals to petals indicating a novel role for PAN in the repression of AG in floral whorl one. These results indicate that PAN functions as an activator or repressor of AG expression in a whorl-specific fashion. The seu pan double mutants also display enhanced floral indeterminacy, resulting in the formation of “fifth whorl” structures and disruption of WUSCHEL (WUS) expression patterns revealing a novel role for SEU in floral meristem termination.
ovule; gynoecium; flowers; agamous; wuschel; organ identity; indeterminate growth
Background and Aims
In the sedge subfamily Mapanioideae there are considerable discrepancies between the standard trimerous monocot floral architecture expected and the complex floral and inflorescence morphologies seen. Decades of debate about whether the basic reproductive units are single flowers or pseudanthia have not resolved the question. This paper evaluates current knowledge about Mapaniid reproductive structures and presents an ontogenetic study of the Mapaniid genus Lepironia with the first floral protein expression maps for the family, localizing the products of the APETALA1/FRUITFULL-like (AP1/FUL) MADS-box genes with the aim of shedding light on this conundrum.
A range of reproductive developmental stages, from spikelet primordia through to infructescence material, were processed for anatomical and immunohistochemical analyses.
The basic reproductive unit is subtended by a bract and possesses two prophyll-like structures, the first organs to be initiated on the primordium, which grow rapidly, enclosing two whorls of initiating leaf-like structures with intervening stamens and a central gynoecium, formed from an annular primordium. The subtending bract and prophyll-like structures possess very different morphologies from that of the internal leaf-like structures and do not show AP1/FUL-like protein localization, which is otherwise strongly localized in the internal leaf-like structures, stamens and gynoecia.
Results support the synanthial hypothesis as the evolutionary origin of the reproductive unit. Thus, the basic reproductive unit in Lepironia is an extremely condensed pseudanthium, of staminate flowers surrounding a central terminal pistillate female flower. Early in development the reproductive unit becomes enclosed by a split-prophyll, with the whole structure subtended by a bract.
Floral development; Lepironia articulata; Mapanioideae; Chrysitricheae; Cyperaceae; Hypolytreae; SEM; immunolocalization; AP1-FUL-like MADS-box genes
This study deals specifically with floral organogenesis and the development of the inflorescence of Philodendron squamiferum and P. pedatum. Pistillate flowers are initiated on the lower portion of the inflorescence and staminate flowers are initiated on the distal portion. An intermediate zone consisting of sterile male flowers and atypical bisexual flowers with fused or free carpels and staminodes is also present. This zone is located between the sterile male and female floral zones. In general, the portion of bisexual flowers facing the male zone forms staminodes, and the portion facing the female zone develops an incomplete gynoecium with few carpels. The incomplete separation of some staminodes from the gynoecial portion of the whorl shows that they belong to the same whorl as the carpels. There are two levels of aberrant floral structures in Philodendron. The first one is represented by the presence of atypical bisexual flowers, which are intermediates between typical female flowers and typical sterile male flowers. The second one is the presence of intermediate structures between typical carpels and typical staminodes on a single atypical bisexual flower. The atypical bisexual flowers of P. squamiferum and P. pedatum are believed to be a case of homeosis where carpels have been replaced by sterile stamens on the same whorl. A quantitative analysis indicates that in both species, on average, one staminode replaces one carpel.
Inflorescence; development; positional information; sex determination
Background and Aims
Bisexual flowers of Carica papaya range from highly regular flowers to morphs with various fusions of stamens to the ovary. Arabidopsis thaliana sup1 mutants have carpels replaced by chimeric carpel–stamen structures. Comparative analysis of stamen to carpel conversions in the two different plant systems was used to understand the stage and origin of carpeloidy when derived from stamen tissues, and consequently to understand how carpeloidy contributes to innovations in flower evolution.
Floral development of bisexual flowers of Carica was studied by scanning electron microscopy and was compared with teratological sup mutants of A. thaliana.
In Carica development of bisexual flowers was similar to wild (unisexual) forms up to locule initiation. Feminization ranges from fusion of stamen tissue to the gynoecium to complete carpeloidy of antepetalous stamens. In A. thaliana, partial stamen feminization occurs exclusively at the flower apex, with normal stamens forming at the periphery. Such transformations take place relatively late in development, indicating strong developmental plasticity of most stamen tissues. These results are compared with evo-devo theories on flower bisexuality, as derived from unisexual ancestors. The Arabidopsis data highlight possible early evolutionary events in the acquisition of bisexuality by a patchy transformation of stamen parts into female parts linked to a flower axis-position effect. The Carica results highlight tissue-fusion mechanisms in angiosperms leading to carpeloidy once bisexual flowers have evolved.
We show two different developmental routes leading to stamen to carpel conversions by late re-specification. The process may be a fundamental aspect of flower development that is hidden in most instances by developmental homeostasis.
Arabidopsis; Carica papaya; bisexual flowers; carpeloidy; ectopic ovules; evo-devo; feminization; floral development; sex conversion; sup1 mutants
The aim of this paper is to discuss the controversial origins of petals from tepals or stamens and the links between the morphological expression of petals and floral organ identity genes in the core eudicots.
I challenge the widely held classical view that petals are morphologically derived from stamens in the core eudicots, and sepals from tepals or bracts. Morphological data suggest that tepal-derived petals have evolved independently in the major lineages of the core eudicots (i.e. asterids, Santalales and rosids) from Berberidopsis-like prototypes, and that staminodial petals have arisen only in few isolated cases where petals had been previously lost (Caryophyllales, Rosales). The clear correlation between continuous changes in petal morphology, and a scenario that indicates numerous duplications to have taken place in genes controlling floral organ development, can only be fully understood within a phylogenetic context. B-gene expression plays a fundamental role in the evolution of the petals by controlling petaloidy, but it does not clarify petal homology.
An increased synorganization of the flower in the core eudicots linked with the establishment of floral whorls restricts the petaloid gene expression to the second whorl, reducing the similarities of petals with tepals from which they were originally derived. An increased flower size linked with secondary polyandry or polycarpelly may lead to a breakdown of the restricted gene expression and a reversal to ancestral characteristics of perianth development. An altered ‘sliding boundary’ hypothesis is proposed for the core eudicots to explain shifts in petaloidy of the perianth and the event of staminodial petals. The repetitive changes of function in the perianth of the core eudicots are linked with shifts in petaloidy to the outer perianth whorl, or losses of petal or sepal whorls that can be secondarily compensated for by the inclusion of bracts in the flower. The origin and evolution of petals appears to be as complex on a molecular basis as it is from a morphological point of view.
Apetala 3; Berberidopsis; bract-derived petals; core eudicots; gene expression; perianth evolution; petaloidy; phylogeny; staminodial petals
Almost all species of the orchid genus Ophrys are pollinated by sexual deception. The orchids mimic the sex pheromone of receptive female insects, mainly hymenopterans, in order to attract males seeking to copulate. Most Ophrys species have achromatic flowers, but some exhibit a coloured perianth and a bright, conspicuous labellum pattern. We recently showed that the pink perianth of Ophrys heldreichii flowers increases detectability by its pollinator, males of the long-horned bee Eucera berlandi. Here we tested the hypothesis that the bright, complex labellum pattern mimics the female of the pollinator to increase attractiveness toward males. In a dual-choice test we offered E. berlandi males an O. heldreichii flower and a flower from O. dictynnae, which also exhibits a pinkish perianth but no conspicuous labellum pattern. Both flowers were housed in UV-transmitting acrylic glass boxes to exclude olfactory signals. Males significantly preferred O. heldreichii to O. dictynnae flowers. In a second experiment, we replaced the perianth of both flowers with identical artificial perianths made from pink card, so that only the labellum differed between the two flower stimuli. Males then chose between both stimuli at random, suggesting that the presence of a labellum pattern does not affect their choice. Spectral measurements revealed higher colour contrast with the background of the perianth of O. heldreichii compared to O. dictynnae, but no difference in green receptor-specific contrast or brightness. Our results show that male choice is guided by the chromatic contrast of the perianth during the initial flower approach but is not affected by the presence of a labellum pattern. Instead, we hypothesise that the labellum pattern is involved in aversive learning during post-copulatory behaviour and used by the orchid as a strategy to increase outcrossing.
Ophrys heldreichii; Colour vision; Sexual deception; Signal evolution; Object detection; Pollination