Littérature scientifique sur le sujet « Floral zygomorphy »

The floral development of Coris was investigated to clarify its controversial relationship with either Primulaceae (Primulales) or Lythraceae (Myrtales). We demonstrate that Coris is strongly related to the Primulaceae but differs in a few important features, such as the presence of an epicalyx and partial zygomorphy. The saccate calyx and epicalyx with unilateral development encloses an actinomorphic flower. The stamen–petal tube has two sections that arise through three growth processes: a lower common part for stamens and petals and an upper section representing a fused corolla. The central ovule-bearing part of the ovary arises separated from the carpel wall. The formation of ridges with teethlike appendages between the ovules suggests a derivation of the free-central placentation from an axile arrangement. Several characters support the monotypic family Coridaceae near the Primulaceae. Key words: Coridaceae, Primulaceae, Lythraceae, floral development, floral vasculature, epicalyx, free-central placentation, common primordium, zygomorphy.

Botanists have long identified bilaterally symmetrical (zygomorphic) flowers with more specialized pollination interactions than radially symmetrical (actinomorphic) flowers. Zygomorphic flowers facilitate more precise contact with pollinators, guide pollinator behaviour and exclude less effective pollinators. However, whether zygomorphic flowers are actually visited by a smaller subset of available pollinator species has not been broadly evaluated. We compiled 53 609 floral visitation records in 159 communities and classified the plants' floral symmetry. Globally and within individual communities, plants with zygomorphic flowers are indeed visited by fewer species. At the same time, zygomorphic flowers share a somewhat larger proportion of their visitor species with other co-occurring plants and have particularly high sharing with co-occurring plants that also have zygomorphic flowers. Visitation sub-networks for zygomorphic species also show differences that may arise from reduced visitor diversity, including greater connectance, greater web asymmetry and lower coextinction robustness of both plants and visitor species—but these changes do not necessarily translate to whole plant-visitor communities. These results provide context for widely documented associations between zygomorphy and diversification and imply that species with zygomorphic flowers may face a greater risk of extinction due to pollinator loss.

The sequence of events and morphology of structures were similar during early floral development of Agalinis tenuifolia and A. fasciculata. The lateral-posterior calyx primordia were initiated first, followed by the middle-posterior primordium, and lastly by the two anterior primordia. The corolla primordia arose in an anterior to posterior succession, then the four stamen primordia were initiated simultaneously. Later the gynoecium originated as an oval-shaped ridge. Two depressions within it became locules, and the two sides of the cleaved ridge separating them met and formed a septum. A placenta formed in each locule and numerous ovules were initiated on it. Zygomorphy was apparent in the calyx, corolla, and androecium during the primordial stage. Organogenesis in the calyx was rapid so that the calyx lobes and tube were well formed before organogenesis of other floral parts. Except for those of the calyx, floral character states distinctive for each species were manifested late in development of the floral bud.

Observations of floral development indicate that floral organ initiation in pentapetalous flowers more commonly results in a medially positioned abaxial petal (MAB) than in a medially positioned adaxial petal (MAD), where the medial plane is defined by the stem and the bract during early floral development. It was proposed that the dominant MAB petal initiation might impose a developmental constraint that leads to the evolution of limited patterns of floral zygomorphy in Asteridae, a family in which the floral zygomorphy develops along the medial plane and results in a central ventral (CV) petal in mature flowers. Here, I investigate whether the pattern of floral organ initiation may limit patterns of floral zygomorphy to evolve in pentapetalous angiosperms. I analyzed floral diagrams representing 405 species in 330 genera of pentapetalous angiosperms to reconstruct the evolution of floral organ initiation and the evolution of developmental processes that give rise to floral zygomorphy on a phylogenetic framework. Results indicate that MAB petal initiation is the most common; it occupies 86.2% of diversity and represents the ancestral state of floral organ initiation in pentapetalous angiosperms. The MAD petal initiation evolved 28 times independently from the ancestral MAB petal initiation. Among the 34 independent originations of floral zygomorphy, 76.5% of these clades represent MAB petal initiation, among which only 47% of the clades result a CV petal in mature flowers. The discrepancy is explained by the existence of developmental processes that result in floral zygomorphy along oblique planes of floral symmetry in addition to along the medial plane. Findings suggest that although the early floral organ initiation plays a constraining role to the evolution of patterns of floral zygomorphy, the constraint diverges along phylogenetically distantly related groups that allow the independent originations of floral zygomorphy through distinct development processes in pentapetalous angiosperms. In additional study, the butterfly-like flowers of Schizanthus are adapted to pollination by bees, hummingbirds, and moths. I investigated the genetic basis of the zygomorphic corolla, for which development is key to the explosive pollen release mechanism found in the species of Schizanthus adapted to bee pollinators. I examined differential gene expression profiles across the zygomorphic corolla of Schizanthus pinnatus, a bee-pollinated species, by analyzing RNA transcriptome sequencing (RNA- seq). Data indicated that CYC2 is not expressed in the zygomorphic corolla of Sc. pinnatus, suggesting CYC2 is not involved in the development of floral zygomorphy in Schizanthus (Solanaceae). The data also indicated that a number of genes are differentially expressed across the corolla.

In Antirrhinum majus (Scrophulariaceae), floral dorso-ventral asymmetry (monosymmetry) is controlled by two genes belonging to the TCP gene family, CYCLOIDEA and DICHOTOMA. My project was to investigate if putative orthologs of CYC and DICH have a conserved role in Schizanthus wisotonensis (Solanaceae), a species also bearing monosymmetrical flowers. To do so, TCP genes from S. wisotonensis were isolated and their expression pattern was characterised in both a wild type and a mutant plant with decreased dorso-ventral asymmetry. In the first chapter of results, a thorough morphological description of floral development in S. wisotonensis was carried out using the support of Scanning Electron Microscopy and quantitative analysis. The development of wild type S. wisotonensis was then compared to that of the mutant. In the second chapter of results, the cloning of 6 TCP genes in S. wisotonensis is reported (SCHCYCI-6). To identify the putative orthologs of CYC, a phylogenetic analysis was carried out using sequence information from other angiosperm TCP genes. This study showed that SCHCYCl, 2 and 3 are the most likely candidates to control dorso-ventral asymmetry in S. wisotonensis. Finally, in the third chapter of results, the study of expression patterns obtained for SCHCYCI-3 using RNA in situ hybridisation in both the wild type and the mutant is reported. The results obtained from the morphological description and the study of gene expression patterns suggest that (i) the establishment of dorso-ventral asymmetry in S. wisotonensis follows a different dynamic to that in A. majus (ii) SCHCYCI may have a function related to that of CYCLOIDEA and DICHOTOMA but unlike for A. majus, it is likely to be restricted to the androecium (iii) CYC-like genes (i.e. SCHCYC2-3) may have a function in the development of the inflorescence where they are expressed in overlapping domains.

La symétrie bilatérale (ou zygomorphie) est un caractère floral apparu de manière récurrente et indépendante au cours de l’évolution des angiospermes, à partir de formes ancestrales de fleurs à symétrie radiale. La zygomorphie est considérée comme une innovation clé des plantes à fleur et est souvent associée à des phénomènes de radiations adaptatives. Certains facteurs génétiques impliqués dans la mise en place de la zygomorphie sont désormais connus chez quelques espèces modèles d’eudicots dérivées et le gène CYCLOIDEA, identifié chez Antirrhinum majus (Plantaginaceae), joue un rôle clé dans le déterminisme de ce caractère. Cependant, les causes moléculaires de la zygomorphie restent encore mal connues chez les eudicots basales et les monocots et il demeure de ce fait impossible de déterminer dans quels groupes taxonomiques l’évolution répétée de la zygomorphie résulte d’une évolution parallèle ou convergente. L’apparition répétée de ce caractère homoplasique résulte aussi probablement de l’action de contraintes de développement, le canalisant au cours de l’évolution dans certaines directions privilégiées. Ce travail de thèse a tout d’abord consisté à identifier de potentielles contraintes de développement dans le groupe des Asteridae (core eudicots), grâce à des méthodes phylogénétiques comparatives. Dans ce groupe, une mérosité variable du périanthe et des étamines nombreuses se sont avérés être des facteurs limitant les transitions vers la zygomorphie. Une comparaison avec un travail similaire effectué sur les Ranunculales a montré que le développement des Asteridae était plus canalisé et moins apte à former des combinaisons variées de caractères. Ces résultats laissent supposer l’existence d’un contrôle génétique de la zygomorphie différent entre les eudicots dérivées et basales. Afin de tester cette hypothèse, les bases moléculaires de la zygomorphie ont été étudiées chez les Ranunculaceae (Ranunculales) par une approche évo-dévo de type gène candidat. La diversité ainsi que les profils d’expression des gènes homologues de CYCLOIDEA (RANACYL) ont été examinés dans différentes espèces et pour différents organes et stades de développement. Le développement des espèces étudiées a auparavant été décrit, en insistant particulièrement sur la mise en place de la zygomorphie. L’approche évo-dévo a montré que les gènes RANACYL ont subi une première duplication avant la divergence des Ranunculaceae et une seconde, cette fois-ci avant la divergence du seul clade à fleurs zygomorphes de la famille. La duplication spécifique aux Delphinieae est un argument en faveur d’un rôle des gènes RANACYL dans la zygomorphie, mais les profils d’expression, à ce stade de l’étude, n’apportent pas de véritable confirmation. La poursuite de ce travail nécessitera l’utilisation de techniques de génétique inverse (VIGS) et de biologie moléculaire (études fonctionnelles par hybridations in situ) pour déterminer si l’évolution de la zygomorphie chez les Ranunculaceae résulte du même type de mécanismes moléculaires que ceux identifiés chez les eudicots dérivées
Floral bilateral symmetry (zygomorphy) evolved several times independently in angiosperms, from radially symmetrical ancestors. The evolution of this floral trait is generally associated with adaptive radiation and high speciation rates. The genetic control of zygomorphy is well described in some core eudicot species and it was shown that the CYCLOIDEA gene, characterized in Antirrhinum majus (Plantaginaceae), is a major determinant of this character. However, molecular investigations of bilateral symmetry control are still patchy in basal eudicots and monocots and determining in which taxonomic group the repeated evolution of this homoplasious character is the result of convergent or parallel evolution is still a challenging question. Moreover, flower symmetry is an integrated phenotype, whose establishment during development can be controlled by historical associations among morphological traits, revealing either physical constraints or common genetic bases. The objectives of this study were to compare the morphological and phylogenetic contexts in which zygomorphy evolved in core and basal eudicots, and then to conduct an evo-devo approach combining a developmental study with the investigation of a candidate gene diversity and expression in Ranunculaceae. This work aimed first at identifying potential developmental constraints in Asteridae (core eudicots), using Phylogenetic Comparative Methods. The impact of perianth merism and stamen number on floral symmetry evolution was evidenced. A comparison with Ranunculales (basal eudicots) showed that in core eudicots there were fewer evolutionary trends associated with transitions toward zygomorphy and that development was more canalized. These results could indicate a different molecular control of zygomorphy between core eudicots and Ranunculales. The second step was to investigate the role of CYCLOIDEA homologs in shaping floral symmetry in Ranunculaceae (Ranunculales). The evo-devo approach showed that the lineage of CYCLOIDEA homologs of Ranunculaceae (RANACYL genes) was duplicated first before the divergence of Ranunculaceae and a second time likely in the lineage leading to the single zygomorphic clade of the family. The expression patterns of the members of the RANACYL multigenic family were uncovered in different species, organs and developmental stages. These were chosen after having described the development of our study species, giving a timing of the major landmarks and focusing on the establishment of zygomorphy. Even if a gene duplication specific to Delphinieae is an argument in favor of the involvement of RANACYL genes in controlling symmetry, the way these genes are expressed during development does not show clear evidence for this for the moment. This evo-devo study must be continued to assess whether the molecular pathways responsible for zygomorphy in Antirrhineae and Ranunculaceae are similar or not

This chapter considers pollination by bees, or melittophily. The bee flower syndrome involves flowers that have medium to long corolla tubes, often pendant, usually zygomorphic (i.e., bilaterally symmetrical rather than radial), commonly with a landing platform with complex texture or ridging so that a bee can hang on easily, and often arranged in spiked inflorescences. The flowers typically open in the early morning and offer their main rewards before midday, although a few are particularly rewarding in the evenings. The chapter begins with a discussion of the bee’s feeding apparatus and feeding methods, along with sensory systems and bee perception of flowers. It then examines the effects of sociality on bees’ flower-visiting patterns as well as behavior and learning in flower-visiting bees. Finally, it analyzes six melittophily types namely: solitary bees, carpenter bees, euglossine bees, bumblebees, stingless bees, and honeybees.

This chapter examines pollination syndromes, floral constancy, and pollinator effectiveness. Flowers show enormous adaptive radiation, but the same kind of flower reappears by convergent evolution in many different families. Thus many families produce rather similar, simple bowl-shaped flowers like buttercups; many produce similar zygomorphic tubular lipped flowers; and many produce fluffy flower heads of massed (often white) florets. These broad flower types are the basis of the idea of pollination syndromes—the flowers have converged on certain morphologies and reward patterns because they are exploiting the abilities and preferences of particular kinds of visitor. After providing an overview of pollination syndromes, the chapter explains why pollination syndromes can be defended. It then considers flower constancy, along with the distinction between flower visitors and effective pollinators. It concludes with some observations on how flower visitors can contribute to speciation of plants through specialization and through their constancy.