Background and Aims
Floral symmetry presents two main states in angiosperms, actinomorphy (polysymmetry or radial symmetry) and zygomorphy (monosymmetry or bilateral symmetry). Transitions from actinomorphy to zygomorphy have occurred repeatedly among flowering plants, possibly in coadaptation with specialized pollinators. In this paper, the rules controlling the evolution of floral symmetry were investigated to determine in which architectural context zygomorphy can evolve.
Floral traits potentially associated with perianth symmetry shifts in Asteridae, one of the major clades of the core eudicots, were selected: namely the perianth merism, the presence and number of spurs, and the androecium organ number. The evolution of these characters was optimized on a composite tree. Correlations between symmetry and the other morphological traits were then examined using a phylogenetic comparative method.
The analyses reveal that the evolution of floral symmetry in Asteridae is conditioned by both androecium organ number and perianth merism and that zygomorphy is a prerequisite to the emergence of spurs.
The statistically significant correlation between perianth zygomorphy and oligandry suggests that the evolution of floral symmetry could be canalized by developmental or spatial constraint. Interestingly, the evolution of polyandry in an actinomorphic context appears as an alternative evolutionary pathway to zygomorphy in Asteridae. These results may be interpreted either in terms of plant–pollinator adaptation or in terms of developmental or physical constraints. The results are discussed in relation to current knowledge about the molecular bases underlying floral symmetry.
Floral symmetry; architectural constraints; Asteridae; comparative analysis; composite tree; correlated evolution; evolutionary scenario
Background and Aims Fumarioideae (20 genera, 593 species) is a clade of Papaveraceae (Ranunculales) characterized by flowers that are either disymmetric (i.e. two perpendicular planes of bilateral symmetry) or zygomorphic (i.e. one plane of bilateral symmetry). In contrast, the other subfamily of Papaveraceae, Papaveroideae (23 genera, 230 species), has actinomorphic flowers (i.e. more than two planes of symmetry). Understanding of the evolution of floral symmetry in this clade has so far been limited by the lack of a reliable phylogenetic framework. Pteridophyllum (one species) shares similarities with Fumarioideae but has actinomorphic flowers, and the relationships among Pteridophyllum, Papaveroideae and Fumarioideae have remained unclear. This study reassesses the evolution of floral symmetry in Papaveraceae based on new molecular phylogenetic analyses of the family.
Methods Maximum likelihood, Bayesian and maximum parsimony phylogenetic analyses of Papaveraceae were conducted using six plastid markers and one nuclear marker, sampling Pteridophyllum, 18 (90 %) genera and 73 species of Fumarioideae, 11 (48 %) genera and 11 species of Papaveroideae, and a wide selection of outgroup taxa. Floral characters recorded from the literature were then optimized onto phylogenetic trees to reconstruct ancestral states using parsimony, maximum likelihood and reversible-jump Bayesian approaches.
Pteridophyllum is not nested in Fumarioideae. Fumarioideae are monophyletic and Hypecoum (18 species) is the sister group of the remaining genera. Relationships within the core Fumarioideae are well resolved and supported. Dactylicapnos and all zygomorphic genera form a well-supported clade nested among disymmetric taxa.
Conclusions Disymmetry of the corolla is a synapomorphy of Fumarioideae and is strongly correlated with changes in the androecium and differentiation of middle and inner tepal shape (basal spurs on middle tepals). Zygomorphy subsequently evolved from disymmetry either once (with a reversal in Dactylicapnos) or twice (Capnoides, other zygomorphic Fumarioideae) and appears to be correlated with the loss of one nectar spur.
Character evolution; disymmetry; floral evolution; floral symmetry; Fumarioideae; nectar spur; Papaveraceae; Papaveroideae; Pteridophyllum; phylogeny; Ranunculales; reversible-jump Markov chain Monte Carlo; zygomorphy
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
Background and Aims
Based on molecular phylogenetic studies, the unigeneric family Eupteleaceae has a prominent phylogenetic position at or near the base of Ranunculales, which, in turn, appear at the base of eudicots. The aim of the present paper is to reveal developmental features of the flowers and to put the genus in a morphological context with other basal eudicots.
Flowers in all developmental stages of Euptelea pleiosperma were collected in the wild at intervals of 7–10 d in the critical stages and studied with a scanning electron microscope.
Remnants of a perianth are lacking throughout flower development. Floral symmetry changes from monosymmetric to asymmetric to disymmetric during development. Asymmetry is expressed in that the sequence of stamen initiation is from the centre to both lateral sides on the adaxial side of the flower but starting from one lateral side and proceeding to the other on the abaxial side. Despite the pronounced floral disymmetry, a dimerous pattern of floral organs was not found. The carpel primordia arise between the already large stamens and alternate with them. Stamens and carpels each form a somewhat irregular whorl. The carpels are ascidiate from the beginning. The stigma differentiates as two crests along the ventral slit of the ovary. The few lateral ovules alternate with each other.
Although the flowers have some unusual autapomorphies (wind pollination, lack of a perianth, pronounced disymmetry of the floral base, long connective protrusion, long temporal gap between androecium and gynoecium initiation, small space for carpel initiation), they show some plesiomorphies at the level of basal eudicots (free carpels, basifixed anthers, whorled phyllotaxis), and thus fit well in Ranunculales.
Basal eudicots; Euptelea; Eupteleaceae; floral development; floral phyllotaxis; floral symmetry; Ranunculales; systematics
Gene duplication and loss provide raw material for evolutionary change within organismal lineages as functional diversification of gene copies provide a mechanism for phenotypic variation. Here we focus on the APETALA1/FRUITFULL MADS-box gene lineage evolution. AP1/FUL genes are angiosperm-specific and have undergone several duplications. By far the most significant one is the core-eudicot duplication resulting in the euAP1 and euFUL clades. Functional characterization of several euAP1 and euFUL genes has shown that both function in proper floral meristem identity, and axillary meristem repression. Independently, euAP1 genes function in floral meristem and sepal identity, whereas euFUL genes control phase transition, cauline leaf growth, compound leaf morphogenesis and fruit development. Significant functional variation has been detected in the function of pre-duplication basal-eudicot FUL-like genes, but the underlying mechanisms for change have not been identified. FUL-like genes in the Papaveraceae encode all functions reported for euAP1 and euFUL genes, whereas FUL-like genes in Aquilegia (Ranunculaceae) function in inflorescence development and leaf complexity, but not in flower or fruit development. Here we isolated FUL-like genes across the Ranunculales and used phylogenetic approaches to analyze their evolutionary history. We identified an early duplication resulting in the RanFL1 and RanFL2 clades. RanFL1 genes were present in all the families sampled and are mostly under strong negative selection in the MADS, I and K domains. RanFL2 genes were only identified from Eupteleaceae, Papaveraceae s.l., Menispermaceae and Ranunculaceae and show relaxed purifying selection at the I and K domains. We discuss how asymmetric sequence diversification, new motifs, differences in codon substitutions and likely protein-protein interactions resulting from this Ranunculiid-specific duplication can help explain the functional differences among basal-eudicot FUL-like genes.
gene duplication; APETALA1; FRUITFULL; basal-eudicots; FRUITFULL-like; Ranunculales
Background and Aims
Ranunculaceae presents both ancestral and derived floral traits for eudicots, and as such is of potential interest to understand key steps involved in the evolution of zygomorphy in eudicots. Zygomorphy evolved once in Ranunculaceae, in the speciose and derived tribe Delphinieae. This tribe consists of two genera (Aconitum and Delphinium s.l.) comprising more than one-quarter of the species of the family. In this paper, the establishment of zygomorphy during development was investigated to cast light on the origin and evolution of this morphological novelty.
The floral developmental sequence of six species of Ranunculaceae, three actinomorphic (Nigella damascena, Aquilegia alpina and Clematis recta) and three zygomorphic (Aconitum napellus, Delphinium staphisagria and D. grandiflorum), was compared. A developmental model was elaborated to break down the successive acquisitions of floral organ identities on the ontogenic spiral (all the species studied except Aquilegia have a spiral phyllotaxis), giving clues to understanding this complex morphogenesis from an evo-devo point of view. In addition, the evolution of symmetry in Ranunculaceae was examined in conjunction with other traits of flowers and with ecological factors.
In the species studied, zygomorphy is established after organogenesis is completed, and is late, compared with other zygomorphic eudicot species. Zygomorphy occurs in flowers characterized by a fixed merism and a partially reduced and transformed corolla.
It is suggested that shifts in expression of genes controlling the merism, as well as floral symmetry and organ identity, have played a critical role in the evolution of zygomorphy in Delphinieae, while the presence of pollinators able to exploit the peculiar morphology of the flower has been a key factor for the maintenance and diversification of this trait.
Delphinieae; development; evolution; evo-devo; nectar spurs; ontogenic spiral; Ranunculaceae; zygomorphy
Floral bilateral symmetry (zygomorphy) has evolved several times independently in angiosperms from radially symmetrical (actinomorphic) ancestral states. Homologs of the Antirrhinum majus Cycloidea gene (Cyc) have been shown to control floral symmetry in diverse groups in core eudicots. In the basal eudicot family Ranunculaceae, there is a single evolutionary transition from actinomorphy to zygomorphy in the stem lineage of the tribe Delphinieae. We characterized Cyc homologs in 18 genera of Ranunculaceae, including the four genera of Delphinieae, in a sampling that represents the floral morphological diversity of this tribe, and reconstructed the evolutionary history of this gene family in Ranunculaceae. Within each of the two RanaCyL (Ranunculaceae Cycloidea-like) lineages previously identified, an additional duplication possibly predating the emergence of the Delphinieae was found, resulting in up to four gene copies in zygomorphic species. Expression analyses indicate that the RanaCyL paralogs are expressed early in floral buds and that the duration of their expression varies between species and paralog class. At most one RanaCyL paralog was expressed during the late stages of floral development in the actinomorphic species studied whereas all paralogs from the zygomorphic species were expressed, composing a species-specific identity code for perianth organs. The contrasted asymmetric patterns of expression observed in the two zygomorphic species is discussed in relation to their distinct perianth architecture.
Bocconia and Macleaya are the only genera of the poppy family (Papaveraceae) lacking petals; however, the developmental and genetic processes underlying such evolutionary shift have not yet been studied.
We studied floral development in two species of petal-less poppies Bocconiafrutescens and Macleayacordata as well as in the closely related petal-bearing Stylophorum diphyllum. We generated a floral transcriptome of B. frutescens to identify MADS-box ABCE floral organ identity genes expressed during early floral development. We performed phylogenetic analyses of these genes across Ranunculales as well as RT-PCR and qRT-PCR to assess loci-specific expression patterns. We found that petal-to-stamen homeosis in petal-less poppies occurs through distinct developmental pathways. Transcriptomic analyses of B. frutescens floral buds showed that homologs of all MADS-box genes are expressed except for the APETALA3-3 ortholog. Species-specific duplications of other ABCE genes in B. frutescens have resulted in functional copies with expanded expression patterns than those predicted by the model.
Petal loss in B. frutescens is likely associated with the lack of expression of AP3-3 and an expanded expression of AGAMOUS. The genetic basis of petal identity is conserved in Ranunculaceae and Papaveraceae although they have different number of AP3 paralogs and exhibit dissimilar floral groundplans.
Electronic supplementary material
The online version of this article (doi:10.1186/s13227-016-0054-6) contains supplementary material, which is available to authorized users.
ABCE model; AGAMOUS; APETALA3; Apetaly; Bocconia; Homeosis; Macleaya; Papaveraceae; Stylophorum
• 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
Background and Aims
The perianths of the Lardizabalaceae are diverse. The second-whorl floral organs of Sinofranchetia chinensis (Lardizabalaceae) are nectar leaves. The aim of this study was to explore the nature of this type of floral organ, and to determine its relationship to nectar leaves in other Ranunculales species, and to other floral organs in Sinofranchetia chinensis.
Approaches of evolutionary developmental biology were used, including 3′ RACE (rapid amplification of cDNA ends) for isolating floral MADS-box genes, phylogenetic analysis for reconstructing gene evolutionary history, in situ hybridization and tissue-specific RT-PCR for identifying gene expression patterns and SEM (scanning electron microscopy) for observing the epidermal cell morphology of floral organs.
Fourteen new floral MADS-box genes were isolated from Sinofranchetia chinensis and from two other species of Lardizabalaceae, Holboellia grandiflora and Decaisnea insignis. The phylogenetic analysis of AP3-like genes in Ranunculales showed that three AP3 paralogues from Sinofranchetia chinensis belong to the AP3-I, -II and -III lineages. In situ hybridization results showed that SIchAP3-3 is significantly expressed only in nectar leaves at the late stages of floral development, and SIchAG, a C-class MADS-box gene, is expressed not only in stamens and carpels, but also in nectar leaves. SEM observation revealed that the adaxial surface of nectar leaves is covered with conical epidermal cells, a hallmark of petaloidy.
The gene expression data imply that the nectar leaves in S. chinensis might share a similar genetic regulatory code with other nectar leaves in Ranunculales species. Based on gene expression and morphological evidence, it is considered that the nectar leaves in S. chinensis could be referred to as petals. Furthermore, the study supports the hypothesis that the nectar leaves in some Ranunculales species might be derived from stamens.
Nectar leaves; perianth; petals; Ranunculales; Lardizabalaceae; Sinofranchetia chinensis; MADS-box; expression pattern; evolutionary developmental biology
A striking aspect of flowering plant (angiosperm) diversity is variation in flower symmetry. From an ancestral form of radial symmetry (polysymmetry, actinomorphy), multiple evolutionary transitions have contributed to instances of non-radial forms, including bilateral symmetry (monosymmetry, zygomorphy) and asymmetry. Advances in flowering plant molecular phylogenetic research and studies of character evolution as well as detailed flower developmental genetic studies in a few model species (e.g. Antirrhinum majus, snapdragon) have provided a foundation for deep insights into flower symmetry evolution. From phylogenetic studies, we have a better understanding of where during flowering plant diversification transitions from radial to bilateral flower symmetry (and back to radial symmetry) have occurred. From developmental studies, we know that a genetic programme largely dependent on the functional action of the CYCLOIDEA gene is necessary for differentiation along the snapdragon dorsoventral flower axis. Bringing these two lines of inquiry together has provided surprising insights into both the parallel recruitment of a CYC-dependent developmental programme during independent transitions to bilateral flower symmetry, and the modifications to this programme in transitions back to radial flower symmetry, during flowering plant evolution.
flower symmetry; CYCLOIDEA; parallel evolution; character evolution; evolution of development
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
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
The Orchidaceae have a history of recurring convergent evolution in floral function as nectar production has evolved repeatedly from an ancestral nectarless state. However, orchids exhibit considerable diversity in nectary type, position and morphology, indicating that this convergence arose from alternative adaptive solutions. Using the genus Disa, this study asks whether repeated evolution of floral nectaries involved recapitulation of the same nectary type or diversifying innovation. Epidermis morphology of closely related nectar-producing and nectarless species is also compared in order to identify histological changes that accompanied the gain or loss of nectar production.
The micromorphology of nectaries and positionally equivalent tissues in nectarless species was examined with light and scanning electron microscopy. This information was subjected to phylogenetic analyses to reconstruct nectary evolution and compare characteristics of nectar-producing and nectarless species.
Two nectary types evolved in Disa. Nectar exudation by modified stomata in floral spurs evolved twice, whereas exudation by a secretory epidermis evolved six times in different perianth segments. The spur epidermis of nectarless species exhibited considerable micromorphological variation, including strongly textured surfaces and non-secreting stomata in some species. Epidermis morphology of nectar-producing species did not differ consistently from that of rewardless species at the magnifications used in this study, suggesting that transitions from rewardlessness to nectar production are not necessarily accompanied by visible morphological changes but only require sub-cellular modification.
Independent nectary evolution in Disa involved both repeated recapitulation of secretory epidermis, which is present in the sister genus Brownleea, and innovation of stomatal nectaries. These contrasting nectary types and positional diversity within types imply weak genetic, developmental or physiological constraints in ancestral, nectarless Disa. Such functional convergence generated by morphologically diverse solutions probably also underlies the extensive diversity of nectary types and positions in the Orchidaceae.
Disa; Disinae; Orchidaceae; orchid; deceit pollination; modified stoma; nectar; nectary; reward; rewardless; evolution; functional convergence
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
Ruta, which belongs to tribe Ruteae, is the type genus of the subfamily Rutoideae and the family Rutaceae. Molecular systematic studies have shown that the genera in Ruteae are closer related to Aurantioideae than to most other genera of Rutoideae, some of the genera traditionally placed in Ruteae have been shown to be nested within the Aurantioideae clade, but the diagnostic characters for determining new patterns in the relationship are poor. In this study, we investigated the floral development of Boenninghausenia in Ruteae (sensu stricto), Haplophyllum in the basal position of Aurantioideae and Murraya in traditional Aurantioideae using scanning electron microscopy. The androecium of Boenninghausenia is obdiplostemony. As androecia in other genera within Ruteae (s.s.) are also obdiplostemonous, reconstruction of the ancestral state indicates that obdiplostemony is an ancestral character in this clade. Because the androecia of Haplophyllum and Murraya are also obdiplostemonous, obdiplostemony is also an ancestral character in Aurantioideae clade. The ancestral state reconstruction indicates this character can serve as a synapomorphy of the Ruteae-Aurantioideae clade. The results of our work also shed light on the evolution of the androecium in Rutaceae, as the obdiplostemony of this group is clearly derived from haplostemony in the ancestral genera in Rutaceae and has develop into polyandry by increasing antepetalous stamens.
In the co-evolution between insects and plants, the establishment of floral monosymmetry was an important step in angiosperm development as it facilitated the interaction with insect pollinators and, by that, likely enhanced angiosperm diversification. In Antirrhinum majus, the TCP transcription factor CYCLOIDEA is the molecular key regulator driving the formation of floral monosymmetry. Although most Brassicaceae form a polysymmetric corolla, six genera develop monosymmetric flowers with two petal pairs of unequal size. In the monosymmetric crucifer Iberis amara, formation of the different petal pairs coincides with a stronger expression of the CYC-homolog IaTCP1 in the small, adaxial petals.
In this study, RNA-Seq was employed to reconstruct the petal transcriptome of the non-model species Iberis amara. About 9 Gb of sequence data was generated, processed and re-assembled into 18,139 likely Iberis unigenes, from which 15,983 showed high sequence homology to Arabidopsis proteins. The transcriptome gives detailed insight into the molecular mechanisms governing late petal development. In addition, it was used as a scaffold to detect genes differentially expressed between the small, adaxial and the large, abaxial petals in order to understand the molecular mechanisms driving unequal petal growth. Far more genes are expressed in adaxial compared to abaxial petals implying that IaTCP1 activates more genes than it represses. Amongst all genes upregulated in adaxial petals, a significantly enhanced proportion is associated with cell wall modification and cell-cell signalling processes. Furthermore, microarrays were used to detect and compare quantitative differences in TCP target genes in transgenic Arabidopsis plants ectopically expressing different TCP transcription factors.
The increased occurrences of genes implicated in cell wall modification and signalling implies that unequal petal growth is achieved through an earlier stop of the cell proliferation phase in the small, adaxial petals, followed by the onset of cell expansion. This process, which forms the monosymmetric corolla of Iberis amara, is likely driven by the enhanced activity of IaTCP1 in adaxial petals.
Electronic supplementary material
The online version of this article (doi:10.1186/s12870-014-0285-4) contains supplementary material, which is available to authorized users.
Brassicaceae; Monosymmetry; CYC; TCP1; RNA-Seq; Microarray
Background and Aims
Icacinaceae sensu stricto consist of a group of early branching lineages of lamiids whose relationships are not yet resolved and whose detailed floral morphology is poorly known. The most bizarre flowers occur in Emmotum: the gynoecium has three locules on one side and none on the other. It has been interpreted as consisting of three fertile and two sterile carpels or of one fertile carpel with two longitudinal septa and two sterile carpels. This study focused primarily on the outer and inner morphology of the gynoecium to resolve its disputed structure, and ovule structure was also studied. In addition, the perianth and androecium were investigated.
Flowers and floral buds of two Emmotum species, E. harleyi and E. nitens, were collected and fixed in the field, and then studied by scanning electron microscopy. Microtome section series were used to reconstruct their morphology.
The gynoecium in Emmotum was confirmed as pentamerous, consisting of three fertile and two sterile carpels. Each of the three locules behaves as the single locule in other Icacinaceae, with the placenta of the two ovules being identical, which shows that three fertile carpels are present. In addition, it was found that the ovules are bitegmic, which is almost unique in lamiids, and that the stamens have monosporangiate thecae, which also occurs in the closely related family Oncothecaceae, but is not known from any other Icacinaceae sensu lato so far.
The flowers of Emmotum have unique characters at different evolutionary levels: the pseudotrimerous gynoecium at angiosperm level, the bitegmic ovules at lamiid level and the monosporangiate thecae at family or family group level. However, in general, the floral morphology of Emmotum fits well in Icacinaceae. More comparative research on flower structure is necessary in Icacinaceae and other early branching lineages of lamiids to better understand the initial evolution of this large lineage of asterids.
Asterids; Emmotaceae; Emmotum harleyi; E. nitens; Garryales; Icacinaceae; early branching lamiids; anthers; floral morphology; gynoecium; ovules
Background and Aims
The order Piperales has the highest diversity of growth forms among the earliest angiosperm lineages, including trees, shrubs, climbers and herbs. However, within the perianth-bearing Piperales (Asarum, Saruma, Lactoris, Hydnora, Prosopanche, Thottea and Aristolochia), climbing species only occur in the most species-rich genus Aristolochia. This study traces anatomical and morphological traits among these lineages, to detect trends in growth form evolution and developmental processes.
Transverse stem sections of different developmental stages of representatives of Asarum, Saruma, Lactoris, Hydnora, Thottea and Aristolochia were compared and anatomical traits were linked to growth form evolution. Biomechanical properties of representative climbers were determined in three-point bending tests and are discussed based on the anatomical observations. Growth form evolution of the perianth-bearing Piperales was reconstructed by ancestral character state reconstruction using Mesquite.
While species of Asarum and Saruma are exclusively herbaceous, species of the remaining genera show a higher diversity of growth habit and anatomy. This growth form diversity is accompanied by a more complex stem anatomy and appropriate biomechanical properties. The ancestral growth form of the perianth-bearing Piperales is reconstructed with either a shrub-like or herbaceous character state, while the following three backbone nodes in the reconstruction show a shrub-like character state. Accordingly, the climbing habit most probably evolved in the ancestor of Aristolochia.
Since the ancestor of the perianth-bearing Piperales has been reconstructed with a herb- or shrub-like habit, it is proposed that the climbing habit is a derived growth form, which evolved with the diversification of Aristolochia, and might have been a key feature for its diversification. Observed anatomical synapomorphies, such as the perivascular fibres in Lactoris, Thottea and Aristolochia, support the phylogenetic relationship of several lineages within the perianth-bearing Piperales. In addition, the hypothesis that the vegetative organs of the holoparasitic Hydnoraceae are most probably rhizomes is confirmed.
Aristolochia; Thottea; Lactoris; Hydnora; Asarum; Saruma; growth form; anatomy; biomechanics; secondary woodiness; heterochrony; perianth-bearing Piperales
Background and Aims
Homeotic transitions are usually dismissed by population geneticists as credible modes of evolution due to their assumed negative impact on fitness. However, several lines of evidence suggest that such changes in organ identity have played an important role during the origin and subsequent evolution of the angiosperm flower. Better understanding of the performance of wild populations of floral homeotic varieties should help to clarify the evolutionary potential of homeotic mutants. Wild populations of plants with changes in floral symmetry, or with reproductive organs replacing perianth organs or sepals replacing petals have already been documented. However, although double-flowered varieties are quite popular as ornamental and garden plants, they are rarely found in the wild and, if they are, usually occur only as rare mutant individuals, probably because of their low fitness relative to the wild-type. We therefore investigated a double-flowered variety of lesser periwinkle, Vinca minor flore pleno (fl. pl.), that is reported to have existed in the wild for at least 160 years. To assess the merits of this plant as a new model system for investigations on the evolutionary potential of double-flowered varieties we explored the morphological details and distribution of the mutant phenotype.
The floral morphology of the double-flowered variety and of a nearby population of wild-type plants was investigated by means of visual inspection and light microscopy of flowers, the latter involving dissected or sectioned floral organs.
The double-flowered variety was found in several patches covering dozens of square metres in a forest within the city limits of Jena (Germany). It appears to produce fewer flowers than the wild-type, and its flowers are purple rather than blue. Most sepals in the first floral whorl resemble those in the wild-type, although occasionally one sepal is broadened and twisted. The structure of second-whorl petals is very similar to that of the wild-type, but their number per flower is more variable. The double-flowered character is due to partial or complete transformation of stamens in the third whorl into petaloid organs. Occasionally, ‘flowers within flowers’ also develop on elongated pedicels in the double-flowered variety.
The flowers of V. minor fl. pl. show meristic as well as homeotic changes, and occasionally other developmental abnormalities such as mis-shaped sepals or loss of floral determinacy. V. minor fl. pl. thus adds to a growing list of natural floral homeotic varieties that have established persistent populations in the wild. Our case study documents that even mutant varieties that have reproductive organs partially transformed into perianth organs can persist in the wild for centuries. This finding makes it at least conceivable that even double-flowered varieties have the potential to establish new evolutionary lineages, and hence may contribute to macroevolutionary transitions and cladogenesis.
Double-flowered variety; homeosis; lesser periwinkle; macroevolution; Vinca minor fl. pl
• 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
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
The order Zingiberales comprises ∼2500 species of tropical to subtropical plants, including agriculturally (e.g. banana, ginger) and horticulturally (e.g. cannas, heliconias, bird-of-paradise) important plants. Throughout the evolution of this order, the stamens have been modified from the ancestral filamentous structures that produce pollen (seen in Banana flowers) to petal-like structures that no longer bear pollen sacs (seen in Canna flowers). This results in a reduction of pollen, but an effective increase in the overall size of the floral display and perhaps in the efficacy of specialized pollinators by converting stamens into ‘petals’. This study investigates the genetic mechanisms that are involved in making petal-like structures in place of pollen-producing stamens.
Flowers of the order Zingiberales demonstrate a remarkable trend of reduction in the number of fertile stamens; from five or six fertile, filamentous stamens bearing two thecae each in Musaceae and Strelitziaceae to just a single petaloid stamen bearing a single theca in Cannaceae and Marantaceae. As one progresses from ancestral to derived floral forms, 5–6 fertile stamens are replaced by 4–5 petaloid staminodes. In Cannaceae and Costaceae, all members of the androecial whorls exhibit petaloidy, including the fertile stamen. In Costaceae, a single fertile stamen develops two thecae embedded on a broad petaloid appendage, while in Cannaceae the single fertile stamen is further reduced to a single theca with a prominent, expanded petaloid appendage. Whether petaloidy of the fertile stamen is a synapomorphy of the entire ginger clade (including Cannaceae, Costaceae, Zingiberaceae and Marantaceae), or the result of independent convergent evolution in Cannaceae, Costaceae, and some Zingiberaceae, is unclear. We combine a developmental series of the formation of the petaloid fertile stamen in Canna indica with data on the expression of B- and C-class floral organ identity genes to elucidate the organogenetic identity of the petaloid stamen and staminodes. Our data indicate that the single fertile theca in C. indica and its petaloid appendage are derived from one-half of the primordium of a single stamen, with no contribution from the remaining part of the stamen (i.e. the second theca primordium) which aborts early in development. The petaloid appendage expands later, and develops from the position of the filament/connective of the developing theca. Floral identity gene expression shows that petal identity genes (i.e. B-class genes) are expressed in all floral organs studied while C-class gene AG-1 is expressed in an increasing gradient from sepals to gynoecium, and AG-2 is expressed in all floral organs except the petals. The canonical model for molecular specification of floral organ identity is not sufficient to explain petaloidy in the androecial whorl in Canna sp. Further studies understanding the regulation of gene networks are required.
Canna; evo-devo; floral development; MADS-box genes; petaloid stamens; petaloidy; Zingiberales
• 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
The ABC model of flower development describes the molecular basis for specification of floral organ identity in model eudicots such as Arabidopsis and Antirrhinum. According to this model, expression of C-class genes is linked to stamen and gynoecium organ identity. The Zingiberales is an order of tropical monocots in which the evolution of floral morphology is characterized by a marked increase in petaloidy in the androecium. Petaloidy is a derived characteristic of the ginger families and seems to have arisen in the common ancestor of the ginger clade. We hypothesize that duplication of the C-class AGAMOUS (AG) gene followed by divergence of the duplicated AG copies during the diversification of the ginger clade lineages explains the evolution of petaloidy in the androecium. In order to address this hypothesis, we carried out phylogenetic analyses of the AG gene family across the Zingiberales and investigated patterns of gene expression within the androecium.
Phylogenetic analysis supports a scenario in which Zingiberales-specific AG genes have undergone at least one round of duplication. Gene duplication was immediately followed by divergence of the retained copies. In particular, we detect positive selection in the third alpha-helix of the K domain of Zingiberales AGAMOUS copy 1 (ZinAG-1). A single fixed amino acid change is observed in ZinAG-1 within the ginger clade when compared to the banana grade. Expression analyses of AG and APETALA1/FRUITFULL (AP1/FUL) in Musa basjoo is similar to A- and C-class gene expressions in the Arabidopsis thaliana model, while Costus spicatus exhibits simultaneous expression of AG and AP1/FUL in most floral organs. We propose that this novel expression pattern could be correlated with the evolution of androecial petaloidy within the Zingiberales.
Our results present an intricate story in which duplication of the AG lineage has lead to the retention of at least two diverged Zingiberales-specific copies, ZinAG-1 and Zingiberales AGAMOUS copy 2 (ZinAG-2). Positive selection on ZinAG-1 residues suggests a mechanism by which AG gene divergence may explain observed morphological changes in Zingiberales flowers. Expression data provides preliminary support for the proposed mechanism, although further studies are required to fully test this hypothesis.
AGAMOUS; Androecial morphogenesis; Gene duplication; K domain; Petaloidy; Positive selection; Protein divergence; Zingiberales