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
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.
Background and Aims
Floral symmetry presents two main states in angiosperms, namely polysymmetry and monosymmetry. Monosymmetry is thought to have evolved several times independently from polysymmetry, possibly in co-adaptation with specialized pollinators. Monosymmetry commonly refers to the perianth, even though associated androecium modifications have been reported. The evolution of perianth symmetry is examined with respect to traits of flower architecture in the Ranunculales, the sister group to all other eudicots, which present a large diversity of floral forms.
Characters considered were perianth merism, calyx, corolla and androecium symmetry, number of stamens and spurs. Character evolution was optimized on a composite phylogenetic tree of Ranunculales using maximum parsimony.
The ancestral state for merism could not be inferred because the basalmost Eupteleaceae lack a perianth and have a variable number of stamens. The Papaveraceae are dimerous, and the five other families share a common trimerous ancestor. Shifts from trimery to dimery (or reverse) are observed. Pentamery evolved in Ranunculaceae. Ranunculales except Eupteleaceae, present a polysymmetric ancestral state. Monosymmetry evolved once within Papaveraceae, Ranunculaceae and Menispermaceae (female flowers only). Oligandry is the ancestral state for all Ranunculales, and polyandry evolved several times independently, in Papaveraceae, Menispermaceae, Berberidaceae and Ranunculaceae, with two reversions to oligandry in the latter. The ancestral state for androecium symmetry is ambiguous for the Ranunculales, while polysymmetry evolved immediately after the divergence of Eupteleaceae. A disymmetric androecium evolved in Papaveraceae. The ancestral state for spurs is none. Multiple spurs evolved in Papaveraceae, Berberidaceae and Ranunculaceae, and single spurs occur in Papaveraceae and Ranunculaceae.
The evolution of symmetry appears disconnected from changes in merism and stamen number, although monosymmetry never evolved in the context of an open ground plan. In bisexual species, monosymmetry evolved coincidently with single spurs, allowing us to propose an evolutionary scenario for Papaveraceae.
Floral symmetry; Ranunculales; perianth; androecium; stamen; spur; merism; evo-devo
Background and Aims
Studies of evolutionary diversification in the basal eudicot family Papaveraceae, such as the transition from actinomorphy to zygomorphy, are hampered by the lack of comparative functional studies. So far, gene silencing methods are only available in the actinomorphic species Eschscholzia californica and Papaver somniferum. This study addresses the amenability of Cysticapnos vesicaria, a derived fumitory with zygomorphic flowers, to virus-induced gene silencing (VIGS), and describes vegetative and reproductive traits in this species.
VIGS-mediated downregulation of the C. vesicaria PHYTOENE DESATURASE gene (CvPDS) and of the FLORICAULA gene CvFLO was carried out using Agrobacterium tumefaciens transfer of Tobacco rattle virus (TRV)-based vectors. Wild-type and vector-treated plants were characterized using reverse transcription–PCR (RT–PCR), in situ hybridization, and macroscopic and scanning electron microscopic imaging.
Cysticapnos vesicaria germinates rapidly, can be grown at high density, has a short life cycle and is self-compatible. Inoculation of C. vesicaria with a CvPDS-VIGS vector resulted in strong photobleaching of green parts and reduction of endogenous CvPDS transcript levels. Gene silencing persisted during inflorescence development until fruit set. Inoculation of plants with CvFLO-VIGS affected floral phyllotaxis, symmetry and floral organ identities.
The high penetrance, severity and stability of pTRV-mediated silencing, including the induction of meristem-related phenotypes, make C. vesicaria a very promising new focus species for evolutionary–developmental (evo–devo) studies in the Papaveraceae. This now enables comparative studies of flower symmetry, inflorescence determinacy and other traits that diversified in the Papaveraceae.
Agrobacterium tumefaciens; basal eudicots; Cysticapnos vesicaria; FLORICAULA; Papaveraceae; PHYTOENE DESATURASE; Ranunculales; Tobacco rattle virus; VIGS, zygomorphy
Molecular sequence data show that the three species oDelphinium subg. Staphisagria (J. Hill) Peterm. form the sister clade to Aconitum L., Aconitella SpachConsolida (DC.) S.F. Gray, and all remaining species of Delphinium L. To account for this finding we resurrect Staphisagria J. Hill (1756). Names in Staphisagria are available for two of the species. We here make the required new combination for the third species, Staphisagria picta (Willd.) F. Jabbour, provide a key to the species, and illustrate one of them.
Aconitum; Delphinium; Mediterranean region; molecular phylogeny; nomenclature; Staphisagria
The genetic basis of floral symmetry is a topic of great interest because of its effect on pollinator behavior and, consequently, plant diversification. The Asteraceae, which is the largest family of flowering plants, is an ideal system in which to study this trait, as many species within the family exhibit a compound inflorescence containing both bilaterally symmetric (i.e., zygomorphic) and radially symmetric (i.e., actinomorphic) florets. In sunflower and related species, the inflorescence is composed of a single whorl of ray florets surrounding multiple whorls of disc florets. We show that in double-flowered (dbl) sunflower mutants (in which disc florets develop bilateral symmetry), such as those captured by Vincent van Gogh in his famous nineteenth-century sunflower paintings, an insertion into the promoter region of a CYCLOIDEA (CYC)-like gene (HaCYC2c) that is normally expressed specifically in WT rays is instead expressed throughout the inflorescence, presumably resulting in the observed loss of actinomorphy. This same gene is mutated in two independent tubular-rayed (tub) mutants, though these mutations involve apparently recent transposon insertions, resulting in little or no expression and radialization of the normally zygomorphic ray florets. Interestingly, a phylogenetic analysis of CYC-like genes from across the family suggests that different paralogs of this fascinating gene family have been independently recruited to specify zygomorphy in different species within the Asteraceae.
The evolution of flower shape and symmetry is of great interest to plant biologists, because it can affect pollinator behavior. Species in the flowering plant family Asteraceae exhibit flower heads that can contain both bilaterally and radially symmetric flowers. In this study, we identify a CYCLOIDEA-like gene that is responsible for determining flower symmetry in sunflower. Mis-expression of this gene causes a double-flowered phenotype, similar to those captured in Vincent van Gogh's famous nineteenth-century paintings, whereas loss of gene function causes radialization of the normally bilaterally symmetric ray florets. Interestingly, this gene is not orthologous to the CYCLOIDEA-like gene responsible for floral symmetry in other members of the Asteraceae, providing evidence of the parallel recruitment of different members of the same gene family for the same function.
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
ECE-CYC2 clade genes known in patterning floral dorsoventral asymmetry (zygomorphy) in Antirrhinum majus are conserved in the dorsal identity function including arresting the dorsal stamen. However, it remains uncertain whether the same mechanism underlies abortion of the ventral stamens, an important morphological trait related to evolution and diversification of zygomorphy in Lamiales sensu lato, a major clade of predominantly zygomorphically flowered angiosperms. Opithandra (Gesneriaceae) is of particular interests in addressing this question as it is in the base of Lamiales s.l., an early representative of this type zygomorphy.
We investigated the expression patterns of four ECE-CYC2 clade genes and two putative target cyclinD3 genes in Opithandra using RNA in situ hybridization and RT-PCR. OpdCYC gene expressions were correlated with abortion of both dorsal and ventral stamens in Opithandra, strengthened by the negatively correlated expression of their putative target OpdcyclinD3 genes. The complement of OpdcyclinD3 to OpdCYC expressions further indicated that OpdCYC expressions were related to the dorsal and ventral stamen abortion through negative effects on OpdcyclinD3 genes.
These results suggest that ECE-CYC2 clade TCP genes are not only functionally conserved in the dorsal stamen repression, but also involved in arresting ventral stamens, a genetic mechanism underlying the establishment of zygomorphy with abortion of both the dorsal and ventral stamens evolved in angiosperms, especially within Lamiales s.l.
Vicia sativa (the common vetch) possesses a predominant zygomorphic flower and belongs to the subfamily Papilionoideae, which is related to Arabidopsis thaliana in the eurosid II clade of the core eudicots. Each vetch flower consists of 21 concentrically arranged organs: the outermost five sepals, then five petals and ten stamens, and a single carpel in the center.
We explored the floral transcriptome to examine a genome-scale genetic model of the zygomorphic flower of vetch. mRNA was obtained from an equal mixture of six floral organs, leaves and roots. De novo assembly of the vetch transcriptome using Illumina paired-end technology produced 71,553 unigenes with an average length of 511 bp. We then compared the expression changes in the 71,553 unigenes in the eight independent organs through RNA-Seq Quantification analysis. We predominantly analyzed gene expression patterns specific to each floral organ and combinations of floral organs that corresponded to the traditional ABC model domains. Comparative analyses were performed in the floral transcriptomes of vetch and Arabidopsis, and genomes of vetch and Medicago truncatula.
Our comparative analysis of vetch and Arabidopsis showed that the vetch flowers conform to a strict ABC model. We analyzed the evolution and expression of the TCP gene family in vetch at a whole-genome level, and several unigenes specific to three different vetch petals, which might offer some clues toward elucidating the molecular mechanisms underlying floral zygomorphy. Our results provide the first insights into the genome-scale molecular regulatory network that controls the evolution and development of the zygomorphic flower in Papilionoideae.
Background and Aims
The TCP family is an ancient group of plant developmental transcription factors that regulate cell division in vegetative and reproductive structures and are essential in the establishment of flower zygomorphy. In-depth research on eudicot TCPs has documented their evolutionary and developmental role. This has not happened to the same extent in monocots, although zygomorphy has been critical for the diversification of Orchidaceae and Poaceae, the largest families of this group. Investigating the evolution and function of TCP-like genes in a wider group of monocots requires a detailed phylogenetic analysis of all available sequence information and a system that facilitates comparing genetic and functional information.
The phylogenetic relationships of TCP-like genes in monocots were investigated by analysing sequences from the genomes of Zea mays, Brachypodium distachyon, Oryza sativa and Sorghum bicolor, as well as EST data from several other monocot species.
All available monocot TCP-like sequences are associated in 20 major groups with an average identity ≥64 % and most correspond to well-supported clades of the phylogeny. Their sequence motifs and relationships of orthology were documented and it was found that 67 % of the TCP-like genes of Sorghum, Oryza, Zea and Brachypodium are in microsyntenic regions. This analysis suggests that two rounds of whole genome duplication drove the expansion of TCP-like genes in these species.
A system of classification is proposed where putative or recognized monocot TCP-like genes are assigned to a specific clade of PCF-, CIN- or CYC/tb1-like genes. Specific biases in sequence data of this family that must be tackled when studying its molecular evolution and phylogeny are documented. Finally, the significant retention of duplicated TCP genes from Zea mays is considered in the context of balanced gene drive.
TCP; tb1; CYCLOIDEA; PCF1; Zea mays; Oryza sativa; Brachypodium distachyon; Sorghum bicolor; gene family; monocot; whole genome duplication
Background and Aims
Ranunculaceae has a prominent phylogenetic position in Ranunculales which appears at the base of eudicots. The aims of the present paper are to reveal the features of ovule morphogenesis in different taxa and gain a better understanding of the systematics of Ranunculaceae.
Flowers of 17 species from three subfamilies, nine tribes and 16 genera of Ranunculaceae, at successive developmental stages, were collected in the wild and studied with a scanning electron microscope.
The integuments in the unitegmic ovules in Helleborus, Ranunculus and Oxygraphis, as well as the inner integuments in the bitegmic genera, initiate annularly and eventually become cup-shaped. However, the integuments in the unitegmic ovules in Anemone and Clematis, as well as the outer integuments in the bitegmic genera, arise semi-annularly and eventually become hood-shaped. Different kinds of appendages appear on the ovules during development. In Coptis of subfamily Coptidoideae, a wrap-shaped appendage arises outside the ovule and envelopes the ovule entirely. In the genera of subfamily Thalictroideae and tribe Anemoneae of subfamily Ranunculoideae, appendages appear on the placenta, the funicle or both. In tribe Helleboreae of subfamily Ranunculoideae, an alary appendage is initiated where the integument and the funicle join and becomes hood-shaped.
Ovule morphogenesis characteristics are significant in classification at the levels of subfamilies and tribes. The initiation patterns of the integuments and the development of appendages show diversity in Ranunculaceae. The present observations suggest that the bitegmic, hood-shaped outer integument and endostomic micropyle are primitive while the unitegmic, cupular-shaped outer integument and bistomic micropyle are derivative.
Ranunculaceae; ovule; morphogenesis; systematics
Background and Aims
The pollinator-mediated stabilizing selection hypothesis suggests that the specialized pollination system of zygomorphic flowers might cause stabilizing selection, reducing their flower size variation compared with actinomorphic flowers. However, the degree of ecological generalization and of dependence on pollinators varies greatly among species of both flower symmetry types and this may also affect flower size variation.
Data on 43 species from two contrasting communities (one alpine and one lowland community) were used to test the relationships and interactions between flower size phenotypic variation, floral symmetry, ecological pollination generalization and species' dependence on pollinators.
Contrary to what was expected, higher flower size variation was found in zygomorphic than in actinomorphic species in the lowland community, and no difference in flower size variation was found between symmetry types in the alpine community. The relationship between floral symmetry and flower size variation depended on ecological generalization and species' dependence on pollinators, although the influence of ecological generalization was only detected in the alpine community. Zygomorphic species that were highly dependent on pollinators and that were ecologically specialized were less variable in flower size than ecologically generalist and selfing zygomorphic species, supporting the pollinator-mediated stabilizing selection hypothesis. However, these relationships were not found in actinomorphic species, probably because they are not dependent on any particular pollinator for efficient pollination and therefore their flower size always shows moderate levels of variation.
The study suggests that the relationship between flower size variation and floral symmetry may be influenced by population-dependent factors, such as ecological generalization and species' dependence on pollinators.
Bilaterally symmetrical species; pollinator-mediated stabilizing selection hypothesis; radially symmetrical species; specialization; floral symmetry; zygomorphy; actinomorphy; entomophilous herbs
Malpighiaceae possess flowers with a unique bilateral symmetry (zygomorphy), which is a hypothesized adaptation associated with specialization on neotropical oil bee pollinators. Gene expression of two representatives of the CYC2 lineage of floral symmetry TCP genes, CYC2A and CYC2B, demarcate the adaxial (dorsal) region of the flower in the characteristic zygomorphic flowers of most Malpighiaceae. Several clades within the family, however, have independently lost their specialized oil bee pollinators and reverted to radial flowers (actinomorphy) like their ancestors. Here, we investigate CYC2 expression associated with four independent reversals to actinomorphy. We demonstrate that these reversals are always associated with alteration of the highly conserved CYC2 expression pattern observed in most New World (NW) Malpighiaceae. In NW Lasiocarpus and Old World (OW) Microsteria, the expression of CYC2-like genes has expanded to include the ventral region of the corolla. Thus, the pattern of gene expression in these species has become radialized, which is comparable to what has been reported in the radial flowered legume clade Cadia. In striking contrast, in NW Psychopterys and OW Sphedamnocarpus, CYC2-like expression is entirely absent or at barely detectable levels. This is more similar to the pattern of CYC2 expression observed in radial flowered Arabidopsis. These results collectively indicate that, regardless of geographic distribution, reversals to similar floral phenotypes in this large tropical angiosperm clade have evolved via different genetic changes from an otherwise highly conserved developmental program.
CYC2-like genes; development; floral symmetry; Malpighiaceae; reversals
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
Flower bilateral symmetry (zygomorphy) has evolved multiple times independently across angiosperms and is correlated with increased pollinator specialization and speciation rates. Functional and expression analyses in distantly related core eudicots and monocots implicate independent recruitment of class II TCP genes in the evolution of flower bilateral symmetry. Furthermore, available evidence suggests that monocot flower bilateral symmetry might also have evolved through changes in B-class homeotic MADS-box gene function.
In order to test the non-exclusive hypotheses that changes in TCP and B-class gene developmental function underlie flower symmetry evolution in the monocot family Commelinaceae, we compared expression patterns of teosinte branched1 (TB1)-like, DEFICIENS (DEF)-like, and GLOBOSA (GLO)-like genes in morphologically distinct bilaterally symmetrical flowers of Commelina communis and Commelina dianthifolia, and radially symmetrical flowers of Tradescantia pallida.
Expression data demonstrate that TB1-like genes are asymmetrically expressed in tepals of bilaterally symmetrical Commelina, but not radially symmetrical Tradescantia, flowers. Furthermore, DEF-like genes are expressed in showy inner tepals, staminodes and stamens of all three species, but not in the distinct outer tepal-like ventral inner tepals of C. communis.
Together with other studies, these data suggest parallel recruitment of TB1-like genes in the independent evolution of flower bilateral symmetry at early stages of Commelina flower development, and the later stage homeotic transformation of C. communis inner tepals into outer tepals through the loss of DEF-like gene expression.
B class genes; Commelinaceae; CYCLOIDEA; homeotic change; monocots; tepals; teosinte branched1
Floral traits within plants can vary with flower position or flowering time. Within an inflorescence, sexual allocation of early produced basal flowers is often female-biased while later produced distal flowers are male-biased. Such temporal adjustment of floral resource has been considered one of the potential advantages of modularity (regarding a flower as a module) in hermaphrodites. However, flowers are under constraints of independent evolution of a given trait. To understand flower diversification within inflorescences, here we examine variation and covariation in floral traits within racemes at the individual and the maternal family level respectively in an alpine herb Aconitum gymnandrum (Ranunculaceae).
We found that floral traits varied significantly with flower position and among families, and position effects were family-specific. Most of the variance of floral traits was among individuals rather than among flowers within individuals or among families. Significant phenotypic correlations between traits were not affected by position, indicating trait integration under shared developmental regulation. In contrast, positive family-mean correlations in floral traits declined gradually from basal to distal flowers (nine significant correlations among floral traits in basal flowers and only three in distal flowers), showing position-specificity. Therefore, the pattern and magnitude of genetic correlations decreased with flower position.
This finding on covariation pattern in floral reproductive structures within racemes has not been revealed before, providing insights into temporal variation and position effects in floral traits within plants and the potential advantages of modularity in hermaphrodites.
Members of the euasterid angiosperm family Solanaceae have been characterized as remarkably diverse in terms of flower morphology and pollinator type. In order to test the relative contribution of phylogeny to the pattern of distribution of floral characters related to pollination, flower form and pollinators have been mapped onto a molecular phylogeny of the family. Bilateral flower symmetry (zygomorphy) is prevalent in the basal grades of the family, and more derived clades have flowers that are largely radially symmetric, with some parallel evolution of floral bilateralism. Pollinator types (‘syndromes’) are extremely homoplastic in the family, but members of subfamily Solanoideae are exceptional in being largely bee pollinated. Pollinator relationships in those genera where they have been investigated more fully are not as specific as flower morphology and the classical pollinator syndrome models might suggest, and more detailed studies in some particularly variable genera, such as Iochroma and Nicotiana, are key to understanding the role of pollinators in floral evolution and adaptive radiation in the family. More studies of pollinators in the field are a priority.
adaptive radiation; flower morphology; phylogeny; Solanaceae; pollination syndrome; homoplasy
Background and Aims
Molecular phylogenies have suggested a new circumscription for Fabales to include Leguminosae, Quillajaceae, Surianaceae and Polygalaceae. However, recent attempts to reconstruct the interfamilial relationships of the order have resulted in several alternative hypotheses, including a sister relationship between Quillajaceae and Surianaceae, the two species-poor families of Fabales. Here, floral morphology and ontogeny of these two families are investigated to explore evidence of a potential relationship between them. Floral traits are discussed with respect to early radiation in the order.
Floral buds of representatives of Quillajaceae and Surianaceae were dissected and observed using light microscopy and scanning electron microscopy.
Quillajaceae and Surianaceae possess some common traits, such as inflorescence morphology and perianth initiation, but development and organization of their reproductive whorls differ. In Quillaja, initiation of the diplostemonous androecium is unidirectional, overlapping with the petal primordia. In contrast, Suriana is obdiplostemonous, and floral organ initiation is simultaneous. Independent initiation of five carpels is common to both Quillaja and Suriana, but subsequent development differs; the antesepalous carpels of Quillaja become fused proximally and exhibit two rows of ovules, and in Suriana the gynoecium is apocarpous, gynobasic, with antepetalous biovulate carpels.
Differences in the reproductive development and organization of Quillajaceae and Surianaceae cast doubt on their potential sister relationship. Instead, Quillaja resembles Leguminosae in some floral traits, a hypothesis not suggested by molecular-based phylogenies. Despite implicit associations of zygomorphy with species-rich clades and actinomorphy with species-poor families in Fabales, this correlation sometimes fails due to high variation in floral symmetry. Studies considering specific derived clades and reproductive biology could address more precise hypotheses of key innovation and differential diversification in the order.
Fabales; Leguminosae; eurosids I; floral ontogeny; Polygalaceae; Quillajaceae; Surianaceae; floral symmetry
The lower eudicot genus Aquilegia, commonly known as columbine, is currently the subject of extensive genetic and genomic research aimed at developing this taxon as a new model for the study of ecology and evolution. The ability to perform functional genetic analyses is a critical component of this development process and ultimately has the potential to provide insight into the genetic basis for the evolution of a wide array of traits that differentiate flowering plants. Aquilegia is of particular interest due to both its recent evolutionary history, which involves a rapid adaptive radiation, and its intermediate phylogenetic position between core eudicot (e.g., Arabidopsis) and grass (e.g., Oryza) model species.
Here we demonstrate the effective use of a reverse genetic technique, virus-induced gene silencing (VIGS), to study gene function in this emerging model plant. Using Agrobacterium mediated transfer of tobacco rattle virus (TRV) based vectors, we induce silencing of PHYTOENE DESATURASE (AqPDS) in Aquilegia vulgaris seedlings, and ANTHOCYANIDIN SYNTHASE (AqANS) and the B-class floral organ identity gene PISTILLATA in A. vulgaris flowers. For all of these genes, silencing phenotypes are associated with consistent reduction in endogenous transcript levels. In addition, we show that silencing of AqANS has no effect on overall floral morphology and is therefore a suitable marker for the identification of silenced flowers in dual-locus silencing experiments.
Our results show that TRV-VIGS in Aquilegia vulgaris allows data to be rapidly obtained and can be reproduced with effective survival and silencing rates. Furthermore, this method can successfully be used to evaluate the function of early-acting developmental genes. In the future, data derived from VIGS analyses will be combined with large-scale sequencing and microarray experiments already underway in order to address both recent and ancient evolutionary questions.
• 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 repeated origin of similar phenotypes is invaluable for studying the underlying genetics of adaptive traits; molecular evidence, however, is lacking for most examples of such similarity. The floral morphology of neotropical Malpighiaceae is distinctive and highly conserved, especially with regard to symmetry, and is thought to result from specialization on oil-bee pollinators. We recently demonstrated that CYCLOIDEA2–like genes (CYC2A and CYC2B) are associated with the development of the stereotypical floral zygomorphy that is critical to this plant–pollinator mutualism. Here, we build on this developmental framework to characterize floral symmetry in three clades of Malpighiaceae that have independently lost their oil bee association and experienced parallel shifts in their floral morphology, especially in regard to symmetry. We show that in each case these species exhibit a loss of CYC2B function, and a strikingly similar shift in the expression of CYC2A that is coincident with their shift in floral symmetry. These results indicate that similar floral phenotypes in this large angiosperm clade have evolved via parallel genetic changes from an otherwise highly conserved developmental program.
Background and Aims
The legume flower is highly variable in symmetry and differentiation of petal types. Most papilionoid flowers are zygomorphic with three types of petals: one dorsal, two lateral and two ventral petals. Mimosoids have radial flowers with reduced petals while caesalpinioids display a range from strongly zygomorphic to nearly radial symmetry. The aims are to characterize the petal micromorphology relative to flower morphology and evolution within the family and assess its use as a marker of petal identity (whether dorsal, lateral or ventral) as determined by the expression of developmental genes.
Petals were analysed using the scanning electron microscope and light microscope. A total of 175 species were studied representing 26 tribes and 89 genera in all three subfamilies of the Leguminosae.
The papilionoids have the highest degree of variation of epidermal types along the dorsiventral axis within the flower. In Loteae and genistoids, in particular, it is common for each petal type to have a different major epidermal micromorphology. Papillose conical cells are mainly found on dorsal and lateral petals. Tabular rugose cells are mainly found on lateral petals and tabular flat cells are found only in ventral petals. Caesalpinioids lack strong micromorphological variation along this axis and usually have only a single major epidermal type within a flower, although the type maybe either tabular rugose cells, papillose conical cells or papillose knobby rugose cells, depending on the species.
Strong micromorphological variation between different petals in the flower is exclusive to the subfamily Papilionoideae. Both major and minor epidermal types can be used as micromorphological markers of petal identity, at least in papilionoids, and they are important characters of flower evolution in the whole family. The molecular developmental pathway between specific epidermal micromorphology and the expression of petal identity genes has yet to be established.
Epidermis; Fabaceae; Papilionoideae; Caesalpinioideae; Mimosoideae; petal surface; scanning electron microscopy; papillose conical cells; tabular rugose cells; tabular flat cells; organ identity
Flowering is a critical transition in plant development, the timing of which can have considerable fitness consequences. Until recently, research into the genetic control of flowering time and its associated developmental changes was focused on core eudicots (for example, Arabidopsis) or monocots (for example, Oryza). Here we examine the flowering response of Aquilegia formosa, a member of the eudicot order Ranunculales that is emerging as an important model for the investigation of plant ecology and evolution.
We have determined that A. formosa has a strong vernalization requirement but little or no photoperiod response, making it a day neutral (DN) plant. Consistent with this, the Aquilegia homolog of FLOWERING LOCUS T (AqFT) is expressed in both long and short days but surprisingly, the locus is expressed before the transition to flowering. In situ hybridizations with homologs of several Arabidopsis Floral Pathway Integrators (FPIs) do not suggest conserved functions relative to Arabidopsis, the potential exceptions being AqLFY and AqAGL24.2.
In Aquilegia, vernalization is critical to flowering but this signal is not strictly required for the transcriptional activation of AqFT. The expression patterns of AqLFY and AqAGL24.2 suggest a hypothesis for the development of Aquilegia's determinate inflorescence whereby their differential expression controls the progression of each meristem from inflorescence to floral identity. Interestingly, none of the Aquilegia expression patterns are consistent with a function in floral repression which, combined with the lack of a FLC homolog, means that new candidate genes must be identified for the control of vernalization response in Aquilegia.
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
Perennial woodland herbs in the genus Thalictrum exhibit high diversity of floral morphology, including four breeding and two pollination systems. Their phylogenetic position, in the early-diverging eudicots, makes them especially suitable for exploring the evolution of floral traits and the fate of gene paralogs that may have shaped the radiation of the eudicots. A current limitation in evolution of plant development studies is the lack of genetic tools for conducting functional assays in key taxa spanning the angiosperm phylogeny. We first show that virus-induced gene silencing (VIGS) of a PHYTOENE DESATURASE ortholog (TdPDS) can be achieved in Thalictrum dioicum with an efficiency of 42% and a survival rate of 97%, using tobacco rattle virus (TRV) vectors. The photobleached leaf phenotype of silenced plants significantly correlates with the down-regulation of endogenous TdPDS (P<0.05), as compared to controls. Floral silencing of PDS was achieved in the faster flowering spring ephemeral T. thalictroides. In its close relative, T. clavatum, silencing of the floral MADS box gene AGAMOUS (AG) resulted in strong homeotic conversions of floral organs. In conclusion, we set forth our optimized protocol for VIGS by vacuum-infiltration of Thalictrum seedlings or dormant tubers as a reference for the research community. The three species reported here span the range of floral morphologies and pollination syndromes present in Thalictrum. The evidence presented on floral silencing of orthologs of the marker gene PDS and the floral homeotic gene AG will enable a comparative approach to the study of the evolution of flower development in this group.