Recent advances in molecular phylogenetics and a series of important palaeobotanical discoveries have revolutionized our understanding of angiosperm diversification. Yet, the origin and early evolution of their most characteristic feature, the flower, remains poorly understood. In particular, the structure of the ancestral flower of all living angiosperms is still uncertain. Here we report model-based reconstructions for ancestral flowers at the deepest nodes in the phylogeny of angiosperms, using the largest data set of floral traits ever assembled. We reconstruct the ancestral angiosperm flower as bisexual and radially symmetric, with more than two whorls of three separate perianth organs each (undifferentiated tepals), more than two whorls of three separate stamens each, and more than five spirally arranged separate carpels. Although uncertainty remains for some of the characters, our reconstruction allows us to propose a new plausible scenario for the early diversification of flowers, leading to new testable hypotheses for future research on angiosperms.

Angiosperm flowers have diversified in adaptation to pollinators, but are also shaped by developmental and genetic histories. The relative importance of these factors in structuring floral diversity remains unknown. We assess the effects of development, function and evolutionary history by testing competing hypotheses on floral modularity and shape evolution in Merianieae (Melastomataceae). Merianieae are characterized by different pollinator selection regimes and a developmental constraint: tubular anthers adapted to specialized buzz-pollination. Our analyses of tomography-based 3-dimensional flower models show that pollinators selected for functional modules across developmental units and that patterns of floral modularity changed during pollinator shifts. Further, we show that modularity was crucial for Merianieae to overcome the constraint of their tubular anthers through increased rates of evolution in other flower parts. We conclude that modularity may be key to the adaptive success of functionally specialized pollination systems by making flowers flexible (evolvable) for adaptation to changing selection regimes.

New computed tomography-based tools allow plant reproductive investment to be quantified, and their use is demonstrated by studying the differences in pollen/ovule ratios across inflorescences of deceptive versus rewarding orchid species.

The flower is a bisexual reproductive unit where both genders compete for resources.Counting pollen and ovules in flowers is essential to understand how much is invested in each gender. Classical methods to count very numerous pollen grains and ovules are inefficient when pollen grains are tightly aggregated, and when fertilization rates of ovules are unknown. We thus established novel, Computed-Tomography-based counting techniques. In order to display the potential of our methods in very difficult cases, we counted pollen and ovules across inflorescences of deceptive and rewarding species of European orchids, which possess both very large numbers of pollen grains (tightly aggregated) and ovules. Pollen counts did not significantly vary across inflorescences and pollination strategies, whereas deceptive flowers had significantly more ovules than rewarding flowers. The within inflorescence variance of pollen to ovule ratios in rewarding flowers was four times higher than in deceptive flowers, possibly demonstrating differences in the constraints acting on both pollination strategies. We demonstrate the inaccuracies and limitations of previously established methods, and the broad applicability of our new techniques: they allow measurement of reproductive investment without restriction on object number or aggregation, and without specimen destruction.

Ranunculaceae comprise ca. 2,500 species (ca. 55 genera) that display a broad range of floral diversity, particularly at the level of the perianth. Petals, when present, are often referred to as “elaborate” because they have a complex morphology. In addition, the petals usually produce and store nectar, which gives them a crucial functional role in the interaction with pollinators. Its morphological diversity and species richness make this family a particularly suitable model group for studying the evolution of complex morphologies. Our aims are (1) to reconstruct the ancestral form of the petal and evolutionary stages at the scale of Ranunculaceae, (2) to test the hypothesis that there are morphogenetic regions on the petal that are common to all species and that interspecific morphological diversity may be due to differences in the relative proportions of these regions during development. We scored and analyzed traits (descriptors) that characterize in detail the complexity of mature petal morphology in 32 genera. Furthermore, we described petal development using high resolution X-Ray computed tomography (HRX-CT) in six species with contrasting petal forms (Ficaria verna, Helleborus orientalis, Staphisagria picta, Aconitum napellus, Nigella damascena, Aquilegia vulgaris). Ancestral state reconstruction was performed using a robust and dated phylogeny of the family, allowing us to produce new hypotheses for petal evolution in Ranunculaceae. Our results suggest a flat ancestral petal with a short claw for the entire family and for the ancestors of all tribes except Adonideae. The elaborate petals that are present in different lineages have evolved independently, and similar morphologies are the result of convergent evolution.