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
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.
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
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
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
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
Background and Aims
Preliminary field observations in 2001 and 2002 suggested that Kingdonia uniflora (Circaeasteraceae, Ranunculales) exhibits heterodichogamy, an unusual kind of reproductive heteromorphy, hitherto unreported in Ranunculales and known from only one other genus in basal eudicots.
During several subsequent years flowers were observed in the field. Flowers were fixed in FAA and studied with microtome sections series and with the scanning electron microscope.
The flowers proved to be heterodichogamous, with protandrous and protogynous morphs, which have a 1 : 1 ratio. Both morphs equally set fruit. Each year a single flower is formed at the tip of a rhizome or more rarely two flowers. The flowers are already open when they appear at the soil surface, before they are receptive and before pollen is dispersed. In both floral morphs the styles elongate early and the stigmas are positioned above the anthers before anthesis begins. In protogynous flowers the stigmas become receptive in this position; later the styles become reflexed and then the anthers dehisce. In contrast, in protandrous flowers the stamen filaments elongate during early anthesis such that the dehiscing anthers come to lie above the (still unreceptive) stigmas; after dehiscence of all anthers in a flower the styles begin to elongate and become receptive.
This is the first record of heterodichogamy in a representative of Ranunculales, in an herbaceous eudicot, and in a plant with uniflorous ramets. The occurrence of heterodichogamy in Kingdonia in which clonal reproduction appears to be dominant might be an adaptation to avoid mating between the ramets from a common mother individual (genet).
Kingdonia; Circaeasteraceae; Ranunculales; heterodichogamy; reproductive heteromorphy
Positive selection is recognized as the prevalence of nonsynonymous over synonymous substitutions in a gene. Models of the functional evolution of duplicated genes consider neofunctionalization as key to the retention of paralogues. For instance, duplicate transcription factors are specifically retained in plant and animal genomes and both positive selection and transcriptional divergence appear to have played a role in their diversification. However, the relative impact of these two factors has not been systematically evaluated. Class B MADS-box genes, comprising DEF-like and GLO-like genes, encode developmental transcription factors essential for establishment of perianth and male organ identity in the flowers of angiosperms. Here, we contrast the role of positive selection and the known divergence in expression patterns of genes encoding class B-like MADS-box transcription factors from monocots, with emphasis on the family Orchidaceae and the order Poales. Although in the monocots these two groups are highly diverse and have a strongly canalized floral morphology, there is no information on the role of positive selection in the evolution of their distinctive flower morphologies. Published research shows that in Poales, class B-like genes are expressed in stamens and in lodicules, the perianth organs whose identity might also be specified by class B-like genes, like the identity of the inner tepals of their lily-like relatives. In orchids, however, the number and pattern of expression of class B-like genes have greatly diverged.
The DEF-like genes from Orchidaceae form four well-supported, ancient clades of orthologues. In contrast, orchid GLO-like genes form a single clade of ancient orthologues and recent paralogues. DEF-like genes from orchid clade 2 (OMADS3-like genes) are under less stringent purifying selection than the other orchid DEF-like and GLO-like genes. In comparison with orchids, purifying selection was less stringent in DEF-like and GLO-like genes from Poales. Most importantly, positive selection took place before the major organ reduction and losses in the floral axis that eventually yielded the zygomorphic grass floret.
In DEF-like genes of Poales, positive selection on the region mediating interactions with other proteins or DNA could have triggered the evolution of the regulatory mechanisms behind the development of grass-specific reproductive structures. Orchidaceae show a different trend, where gene duplication and transcriptional divergence appear to have played a major role in the canalization and modularization of perianth development.
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
The genus Aquilegia is an emerging model system in plant evolutionary biology predominantly because of its wide variation in floral traits and associated floral ecology. The anatomy of the Aquilegia flower is also very distinct. There are two whorls of petaloid organs, the outer whorl of sepals and the second whorl of petals that form nectar spurs, as well as a recently evolved fifth whorl of staminodia inserted between stamens and carpels.
We designed an oligonucleotide microarray based on EST sequences from a mixed tissue, normalized cDNA library of an A. formosa x A. pubescens F2 population representing 17,246 unigenes. We then used this array to analyze floral gene expression in late pre-anthesis stage floral organs from a natural A. formosa population. In particular, we tested for gene expression patterns specific to each floral whorl and to combinations of whorls that correspond to traditional and modified ABC model groupings. Similar analyses were performed on gene expression data of Arabidopsis thaliana whorls previously obtained using the Ath1 gene chips (data available through The Arabidopsis Information Resource).
Our comparative gene expression analyses suggest that 1) petaloid sepals and petals of A. formosa share gene expression patterns more than either have organ-specific patterns, 2) petals of A. formosa and A. thaliana may be independently derived, 3) staminodia express B and C genes similar to stamens but the staminodium genetic program has also converged on aspects of the carpel program and 4) staminodia have unique up-regulation of regulatory genes and genes that have been implicated with defense against microbial infection and herbivory. Our study also highlights the value of comparative gene expression profiling and the Aquilegia microarray in particular for the study of floral evolution and ecology.
Background and Aims
Balsaminaceae consist of two genera, the monospecific Hydrocera and its species-rich sister Impatiens. Although both genera are seemingly rather similar in overall appearance, they differ in ecology, distribution range, habitat preference and morphology. Because morphological support for the current molecular phylogenetic hypothesis of Impatiens is low, a developmental study is necessary in order to obtain better insights into the evolutionary history of the family. Therefore, the floral development of H. triflora and I. omeiana was investigated, representing the most early-diverged lineage of Impatiens, and the observations were compared with the literature.
Flowers at all developmental stages were examined using scanning electron microscopy and light microscopy.
In Hydrocera, two whorls of five free perianth primordia develop into a less zygomorphic perianth compared with its sister genus. The androecial cap originates from five individual stamen primordia. Post-genital fusion of the upper parts of the filaments result in a filament ring below the anthers. The anthers fuse forming connivent anther-like units. The gynoecium of Hydrocera is pentamerous; it is largely synascidiate in early development. Only then is a symplicate zone formed resulting in style and stigmas. In I. omeiana, the perianth is formed as in Hydrocera. Five individual stamen primordia develop into five stamens, of which the upper part of the filaments converge with each other. The gynoecium of I. omeiana is tetramerous; it appears annular in early development.
Comparison of the present results with developmental data from the literature confirms the perianth morphocline hypothesis in which a congenital fusion of the parts of the perianth results in a shift from pentasepalous to trisepalous flowers. In addition, the development of the androecial cap and the gynoecium follows several distinct ontogenetic sequences within the family.
Balsaminaceae; androecium; floral development; gynoecium; Hydrocera triflora; Impatiens omeiana
The nearly 30 000 species of orchids produce flowers of unprecedented diversity. However, whether specific genetic mechanisms contributed to this diversity is a neglected topic and remains speculative. We recently published a theory, the ‘orchid code’, maintaining that the identity of the different perianth organs is specified by the combinatorial interaction of four DEF-like MADS-box genes with other floral homeotic genes.
Here the developmental and evolutionary implications of our theory are explored. Specifically, it is shown that all frequent floral terata, including all peloric types, can be explained by monogenic gain- or-loss-of-function mutants, changing either expression of a DEF-like or CYC-like gene. Supposed dominance or recessiveness of mutant alleles is correlated with the frequency of terata in both cultivation and nature. Our findings suggest that changes in DEF- and CYC-like genes not only underlie terata but also the natural diversity of orchid species. We argue, however, that true changes in organ identity are rare events in the evolution of orchid flowers, even though we review some likely cases.
The four DEF paralogues shaped floral diversity in orchids in a dramatic way by modularizing the floral perianth based on a complex series of sub- and neo-functionalization events. These genes may have eliminated constraints, so that different kinds of perianth organs could then evolve individually and thus often in dramatically different ways in response to selection by pollinators or by genetic drift. We therefore argue that floral diversity in orchids may be the result of an unprecedented developmental genetic predisposition that originated early in orchid evolution.
Orchidaceae; orchid evolution; evo-devo; perianth; class B genes; DEFICIENS; subfunctionalization; neofunctionalization; gene duplication; peloria; modularization
Background and Aims
Bisexual flowers of Carica papaya range from highly regular flowers to morphs with various fusions of stamens to the ovary. Arabidopsis thaliana sup1 mutants have carpels replaced by chimeric carpel–stamen structures. Comparative analysis of stamen to carpel conversions in the two different plant systems was used to understand the stage and origin of carpeloidy when derived from stamen tissues, and consequently to understand how carpeloidy contributes to innovations in flower evolution.
Floral development of bisexual flowers of Carica was studied by scanning electron microscopy and was compared with teratological sup mutants of A. thaliana.
In Carica development of bisexual flowers was similar to wild (unisexual) forms up to locule initiation. Feminization ranges from fusion of stamen tissue to the gynoecium to complete carpeloidy of antepetalous stamens. In A. thaliana, partial stamen feminization occurs exclusively at the flower apex, with normal stamens forming at the periphery. Such transformations take place relatively late in development, indicating strong developmental plasticity of most stamen tissues. These results are compared with evo-devo theories on flower bisexuality, as derived from unisexual ancestors. The Arabidopsis data highlight possible early evolutionary events in the acquisition of bisexuality by a patchy transformation of stamen parts into female parts linked to a flower axis-position effect. The Carica results highlight tissue-fusion mechanisms in angiosperms leading to carpeloidy once bisexual flowers have evolved.
We show two different developmental routes leading to stamen to carpel conversions by late re-specification. The process may be a fundamental aspect of flower development that is hidden in most instances by developmental homeostasis.
Arabidopsis; Carica papaya; bisexual flowers; carpeloidy; ectopic ovules; evo-devo; feminization; floral development; sex conversion; sup1 mutants
Background and Aims
Within Chenopodioideae, Atripliceae have been distinguished by two bracteoles enveloping the female flowers/fruits, whereas in other tribes flowers are described as ebracteolate with persistent perianth. Molecular phylogenetic hypotheses suggest ‘bracteoles’ to be homoplastic. The origin of the bracteoles was explained by successive inflorescence reductions. Flower reduction was used to explain sex determination. Therefore, floral ontogeny was studied to evaluate the nature of the bracteoles and sex determination in Atripliceae.
Inflorescences of species of Atriplex, Chenopodium, Dysphania and Spinacia oleracea were investigated using light microscopy and scanning electron microscopy.
The main axis of the inflorescence is indeterminate with elementary dichasia as lateral units. Flowers develop centripetally, with first the formation of a perianth primordium either from a ring primordium or from five individual tepal primordia fusing post-genitally. Subsequently, five stamen primordia originate, followed by the formation of an annular ovary primordium surrounding a central single ovule. Flowers are either initially hermaphroditic remaining bisexual and/or becoming functionally unisexual at later stages, or initially unisexual. In the studied species of Atriplex, female flowers are strictly female, except in A. hortensis. In Spinacia, female and male flowers are unisexual at all developmental stages. Female flowers of Atriplex and Spinacia are protected by two accrescent fused tepal lobes, whereas the other perianth members are absent.
In Atriplex and Spinacia modified structures around female flowers are not bracteoles, but two opposite accrescent tepal lobes, parts of a perianth persistent on the fruit. Flowers can achieve sexuality through many different combinations; they are initially hermaphroditic, subsequently developing into bisexual or functionally unisexual flowers, with the exception of Spinacia and strictly female flowers in Atriplex, which are unisexual from the earliest developmental stages. There may be a relationship between the formation of an annular perianth primordium and flexibility in floral sex determination.
Atriplex; Atripliceae; bract/bracteole; Chenopodiaceae; Chenopodioideae; Chenopodium; Dysphania; floral ontogeny; floral sex determination; perianth modification; SEM/LM; Spinacia
Evolution of unisexual flowers entails one of the most extreme changes in plant development. Cultivated spinach, Spinacia oleracea L., is uniquely suited for the study of unisexual flower development as it is dioecious and it achieves unisexually by the absence of organ development, rather than by organ abortion or suppression. Male staminate flowers lack fourth whorl primordia and female pistillate flowers lack third whorl primordia. Based on theoretical considerations, early inflorescence or floral organ identity genes would likely be directly involved in sex-determination in those species in which organ initiation rather than organ maturation is regulated. In this study, we tested the hypothesis that sexual dimorphism occurs through the regulation of B class floral organ gene expression by experimentally knocking down gene expression by viral induced gene silencing.
Suppression of B class genes in spinach resulted in the expected homeotic transformation of stamens into carpels but also affected the number of perianth parts and the presence of fourth whorl. Phenotypically normal female flowers developed on SpPI-silenced male plants. Suppression of the spinach C class floral organ identity gene, SpAG, resulted in loss of reproductive organ identity, and indeterminate flowers, but did not result in additional sex-specific characteristics or structures. Analysis of the genomic sequences of both SpAP3 and SpPI did not reveal any allelic differences between males and females.
Sexual dimorphism in spinach is not the result of homeotic transformation of established organs, but rather is the result of differential initiation and development of the third and fourth whorl primordia. SpAG is inferred to have organ identity and meristem termination functions similar to other angiosperm C class genes. In contrast, while SpPI and SpAP3 resemble other angiosperms in their essential functions in establishing stamen identity, they also appear to have an additional function in regulating organ number and identity outside of the third whorl. We present a model for the evolution of dioecy in spinach based on the regulation of B class expression.
Background and Aims
Individual flowers of the monocot Curculigo racemosa (Hypoxidaceae, Asparagales) are regularly polyandrous. To evaluate the significance of this almost unique character among Asparagales for flower evolution of asparagoid monocots, flowers of C. racemosa were studied comparatively.
Anthetic flowers as well as early floral developmental stages were studied by light and scanning electron microscopy.
Despite the polyandry, floral development is similar to that of other Asparagales with a developmental gradient from adaxial to abaxial. Stamens initiate simultaneously and the diameter of staminal primordia is about half of that in species with six anthers. The number of stamens is not fixed (12–26) and varies within the same inflorescence. Surprisingly, the gynoecium can be four- or six-locular, besides the normal trimerous state.
The discovery of a polyandrous Curculigo reveals plasticity of stamen number at the base of Asparagales. Orchidaceae – sister to all other Asparagales – has a reduced stamen number (three, two or one), whereas in Hypoxidaceae – part of the next diverging clade – either the normal monocot stamen number (six), polyandry (this study) or the loss of three anthers (Pauridia) occurs. However, at present it is impossible to decide whether the flexibility in stamen number is autapomorphic for each group or whether it is a synapomorphy. The small size of stamen primordia of Curculigo is conspicuous. It allows more space for additional androecial primordia. Stamens are initiated as independent organs, and filaments are not in bundles, hence C. racemosa is not secondarily polyandrous as may be the case in the distantly related Gethyllis of asparagoid Amaryllidaceae. The increase in carpel number is a rare phenomenon in angiosperms. A possible explanation for the polyandry of C. racemosa is that it is a natural SUPERMAN-deficient mutant, which shows an increase of stamens, or ULTRAPETALA or CARPEL FACTORY mutants, which are polyandrous and changed in carpel number.
Monocots; lower Asparagales; floral development; SEM; floral mutants; androecium; gynoecium; Orchidaceae
Floral organs usually take on the characteristics of petals in ginger plants. In Canna indica, the most ornamental parts of the flowers are considered to be staminode, which means rudimentary and sterile stamen. However, the precise nature of these petaloid organs is yet to be determined. Two floral organ identity genes GLOBOSA (GLO) and AGAMOUS (AG) are isolated from Canna indica. Their expression patterns suggest that petaloid staminodes and labellum are of androecium identity, in agreement with their position within the flower. But the current molecular, morphological and anatomical data, are still not sufficient to explain the distinct morphology observed in staminodes and fertile stamen in Canna indica.
Floral organs that take on the characteristics of petals can occur in all whorls of the monocot order Zingiberales. In Canna indica, the most ornamental or ‘petaloid’ parts of the flowers are of androecial origin and are considered staminodes. However, the precise nature of these petaloid organs is yet to be determined. In order to gain a better understanding of the genetic basis of androecial identity, a molecular investigation of B- and C-class genes was carried out. Two MADS-box genes GLOBOSA (GLO) and AGAMOUS (AG) were isolated from young inflorescences of C. indica by 3′ rapid amplification of cDNA ends polymerase chain reaction (3′-RACE PCR). Sequence characterization and phylogenetic analyses show that CiGLO and CiAG belong to the B- and C-class MADS-box gene family, respectively. CiAG is expressed in petaloid staminodes, the labellum, the fertile stamen and carpels. CiGLO is expressed in petals, petaloid staminodes, the labellum, the fertile stamen and carpels. Expression patterns in mature tissues of CiGLO and CiAG suggest that petaloid staminodes and the labellum are of androecial identity, in agreement with their position within the flower and with described Arabidopsis thaliana expression patterns. Although B- and C-class genes are important components of androecial determination, their expression patterns are not sufficient to explain the distinct morphology observed in staminodes and the fertile stamen in C. indica.
ABC model; Canna indica; floral organ identity; MADS-box gene; phylogenetic analysis; real-time PCR.
Benzylisoquinoline alkaloids (BIAs) are a diverse group of biologically active specialized metabolites produced mainly in four plant families. BIA metabolism is likely of monophyletic origin and involves multiple enzymes yielding structurally diverse compounds. Several BIAs possess defensive properties against pathogenic microorganisms and herbivores. Opium poppy (Papaver somniferum: Papaveraceae) has emerged as a model system to investigate the cellular localization of BIA biosynthesis. Although alkaloids accumulate in the laticifer cytoplasm (latex) of opium poppy, corresponding biosynthetic enzymes and gene transcripts are localized to proximal sieve elements and companion cells, respectively. In contrast, BIA metabolism in the non-laticiferous meadow rue (Thalictrum flavum; Ranunculaceae) occurs independent of the phloem. Evidence points toward the adoption of diverse strategies for the biosynthesis and accumulation of alkaloids as defensive compounds. Recruitment of cell types involved in BIA metabolism, both within and external to the phloem, was likely driven by selection pressures unique to different taxa. The biochemistry, cell biology, ecophysiology, and evolution of BIA metabolism are considered in this context.
benzylisoquinoline alkaloid; phloem; laticifer; sieve element; plant defense