High pollinator specificity and the potential for simple genetic changes to affect pollinator attraction make sexually deceptive orchids an ideal system for the study of ecological speciation, in which change of flower odour is likely important. This study surveys reproductive barriers and differences in floral phenotypes in a group of four closely related, co-flowering sympatric Ophrys species, and uses a genotyping by sequencing (GBS) approach to obtain information on the proportion of the genome that is differentiated between species. Ophrys species were found to effectively lack post-pollination barriers, but are strongly isolated by their different pollinators (floral isolation), and to a smaller extent, by shifts in flowering time (temporal isolation). Although flower morphology and perhaps labellum coloration may contribute to floral isolation, reproductive barriers may largely be due to differences in flower odour chemistry. GBS revealed shared polymorphism throughout the Ophrys genome, with very little population structure between species. Genome scans for FST outliers identified few markers that are highly differentiated between species and repeatable in several populations. These genome scans also revealed highly differentiated polymorphisms in genes with putative involvement in floral odour production, including a previously identified candidate gene thought to be involved in the biosynthesis of pseudo-pheromones by the orchid flowers. Taken together, these data suggest that ecological speciation associated with different pollinators in sexually deceptive orchids has a genic rather than a genomic basis, placing these species at an early phase of genomic divergence within the 'speciation continuum'. This article is protected by copyright. All rights reserved.
Bird pollination has evolved repeatedly among flowering plants but is almost exclusively characterized by passive transfer of pollen onto the bird and by nectar as primary reward [1, 2]. Food body rewards are exceedingly rare among eudicot flowering plants and are only known to occur on sterile floral organs . In this study, we report an alternative bird pollination mechanism involving bulbous stamen appendages in the Neotropical genus Axinaea (Melastomataceae). We studied the pollination process by combining pollination experiments, video monitoring, and detailed analyses of stamen structure and metabolomic composition. We show that the bulbous stamen appendages, which are consumed by various species of passerines (Thraupidae, Fringillidae), are bifunctional during the pollination process. First, the appendages work as bellows organs in a unique pollen expulsion mechanism activated by the passerines. As the birds seize an appendage with their beaks in order to remove it from the flower for consumption, air contained in the appendage's aerenchymatous tissue is pressed into the hollow anther. The resulting air flow causes the expulsion of a pollen jet and the deposition of pollen on the bird's head and beak. Second, the stamen appendages provide a hexose-rich, highly nutritious (15,100 J/g) food body reward for the pollinating passerines. This discovery expands our knowledge of flowering plant pollination systems and provides the first report of highly specialized bellows organs for active pollen transfer in flowering plants. In addition, this is the only known case of a food body reward associated with reproductive structures in the eudicot clade of flowering plants.
The holoparasitic plant family Rafflesiaceae include the world's largest flowers. Despite their iconic status, relatively little is known about the morphology and development of their flowers. A recent study clarified the organization of the outer (sterile) floral organs, surprisingly revealing that their distinctive floral chambers arose via different developmental pathways in the two major genera of the family. Here, we expand that research to investigate the structure and development of the reproductive organs of Rafflesiaceae.
New associations have recently been discovered between arboreal ants that live on myrmecophytic plants, and different groups of fungi. Most of the - usually undescribed - fungi cultured by the ants belong to the order Chaetothyriales (Ascomycetes). Chaetothyriales occur in the nesting spaces provided by the host plant, and form a major part of the cardboard-like material produced by the ants for constructing nests and runway galleries. Until now, the fungi have been considered specific to each ant species. We focus on the three-way association between the plant Tetrathylacium macrophyllum (Salicaceae), the ant Azteca brevis (Formicidae: Dolichoderinae) and various chaetothyrialean fungi. Azteca brevis builds extensive runway galleries along branches of T. macrophyllum. The carton of the gallery walls consists of masticated plant material densely pervaded by chaetothyrialean hyphae. In order to characterise the specificity of the ant-fungus association, fungi from the runway galleries of 19 ant colonies were grown as pure cultures and analyzed using partial SSU, complete ITS, 5.8S and partial LSU rDNA sequences. This gave 128 different fungal genotypes, 78% of which were clustered into three monophyletic groups. The most common fungus (either genotype or approximate species-level OTU) was found in the runway galleries of 63% of the investigated ant colonies. This indicates that there can be a dominant fungus but, in general, a wider guild of chaetothyrialean fungi share the same ant mutualist in Azteca brevis.
The interrelationship of morphogenesis and metabolism is a poorly studied phenomenon. The main paradigm is that development is controlled by gene expression. The aim of the present study was to correlate metabolism to early and late stages of flower and fruit development in order to provide the basis for the identification of metabolic adjustment and limitations. A highly detailed picture of morphogenesis is achieved using nondestructive micro computed tomography. This technique was used to quantify morphometric parameters of early and late flower development in an Arabidopsis thaliana mutant with synchronized flower initiation. The synchronized flower phenotype made it possible to sample enough early floral tissue otherwise not accessible for metabolomic analysis. The integration of metabolomic and morphometric data enabled the correlation of metabolic signatures with the process of flower morphogenesis. These signatures changed significantly during development, indicating a pronounced metabolic reprogramming in the tissue. Distinct sets of metabolites involved in these processes were identified and were linked to the findings of previous gene expression studies of flower development. High correlations with basic leucine zipper (bZIP) transcription factors and nitrogen metabolism genes involved in the control of metabolic carbon : nitrogen partitioning were revealed. Based on these observations a model for metabolic adjustment during flower development is proposed.
Computed tomography remains strongly underused in plant sciences despite its high potential in delivering detailed 3D phenotypical information because of the low X-ray absorption of most plant tissues. Existing protocols to study soft tissues display poor performance, especially when compared to those used on animals. More efficient protocols to study plant material are therefore needed. Flowers of Arabidopsis thaliana and Marcgravia caudata were immersed in a selection of contrasting agents used to treat samples for transmission electron microscopy. Grayscale values for floral tissues and background were measured as a function of time. Contrast was quantified via a contrast index. The thick buds of Marcgravia were scanned to determine which contrasting agents best penetrate thick tissues. The highest contrast increase with cytoplasm-rich tissues was obtained with phosphotungstate, whereas osmium tetroxide and bismuth tatrate displayed the highest contrast increase with vacuolated tissues. Phosphotungstate also displayed the best sample penetration. Furthermore, infiltration with phosphotungstate allowed imaging of all plants parts at a high resolution of 3 µm, which approaches the maximum resolution of our equipment: 1.5 µm. The high affinity of phosphotungstate for vasculature, cytoplasm-rich tissue, and pollen causes these tissues to absorb more X-rays than the surrounding tissues, which, in turn, makes these tissues appear brighter on the scan data. Tissues with different brightness can then be virtually dissected from each other by selecting the bracket of grayscale to be visualized. Promising directions for the future include in silico phenotyping and developmental studies of plant inner parts (e.g., ovules, vasculature, pollen, and cell nuclei) via virtual dissection as well as correlations of quantitative phenotypes with omics datasets. Therefore, this work represents a crucial improvement of previous methods, allowing new directions of research to be undertaken in areas ranging from morphology to systems biology.
The rostellum, a projecting part of the gynostemium in orchid flowers, separates the anther(s) from the stigma and thus commonly prevents auto-pollination. Nonetheless, as a modified (usually distal) portion of the median stigma lobe, the rostellum has been frequently invoked of having re-gained a stigmatic function in rare cases of orchid auto-pollination. Here it is shown that a newly discovered selfing variant of Madagascan Bulbophyllumbicoloratum has evolved a modified rostellum allowing the penetration of pollen tubes from in situ pollinia.
This is a combination of review and original data on floral structure and diversity in the two earliest diverging lineages of the Ericales, i.e. the balsaminoids, comprising Balsaminaceae, Marcgraviaceae and Tetrameristaceae, and the polemonioids, comprising Fouquieriaceae and Polemoniaceae. Each clade is strongly supported in molecular studies, while structural synapomorphies have largely been lacking. For the balsaminoid families, we compare floral morphology, anatomy and histology among selected taxa and find that the entire clade is strongly supported by the shared presence of nectariferous tissue in the floral periphery, thread-like structures on anthers, truncate stigmas, secretion in the ovary, as well as mucilage cells, raphides and tannins in floral tissues. A possible sister group relationship between Balsaminaceae and Tetrameristaceae is supported by the shared presence of post-genital fusion of filaments and ovary and a star-shaped stylar canal. For polemonioids, we document unexpected diversity of floral features in Polemoniaceae, partly providing structural links to Fouquieriaceae. Features include cochlear and quincuncial corolla aestivation, connective protrusions, ventrifixed anthers and nectariferous tissue in the base of the ovary. In addition, we outline future directions for research on floral structure in the Ericales and briefly discuss the general importance of structural studies for our understanding of plant phylogeny and evolution.
Ericales are a major group of extant asterid angiosperms that are well represented in the Late Cretaceous fossil record, mainly by flowers, fruits and seeds. Exceptionally well preserved fossil flowers, here described as Glandulocalyx upatoiensis gen. & sp. nov., from the Santonian of Georgia, USA, yield new detailed evidence of floral structure in one of these early members of Ericales and provide a secure basis for comparison with extant taxa.
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