The ancestral flower was bisexual with multiple whorls (concentric cycles) of petal-like organs, in sets of threes.
Copyright: Hervé Sauquet/Jürg Schönenberger.
Flowering plants are by far the most diverse group of plants comprising some 300,000 different species. They were, however, late on the scene in evolutionary terms, only evolving some 140 million years ago.
This puts their evolution towards the end of the age of dinosaurs in the Cretaceous when the ancestors of mammals were still mammaliaform synapsids. The evolution of the first flowering plants is one of those enduring mysteries for evolution to explains, Darwin himself referring to it as "an abominable mystery".
Now a new study coordinated by Juerg Schoenenberger from the University of Vienna and Hervé Sauquet of the Université Paris-Sud has shed more than a little light on what the first flowers looked like. The same study has also reconstructed what flowers at the key divergence point looked like.
The entire study is an application of the Theory of Evolution - so much for creationists' constant assertion that the TOE is a 'theory in crisis', a major plank in the Discovery Institute's political program, the Wedge Strategy. In fact, the entire appearance and diversification of flowering plants can only be explained and understood in terms of evolutionary theory.
The results of the study were published open access a few days ago in Nature Communications:
The researchers also reconstructed what flowers looked like at all the key divergences in the flowering plant evolutionary tree.
Copyright: Hervé Sauquet/Jürg Schönenberger.
Abstract
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.
Introduction
Flowers are the reproductive structures of angiosperms (flowering plants), which represent ca. 90% of all living land plants and upon which most of terrestrial life depends, either directly or indirectly. However, flowers are a relatively recent evolutionary innovation on the geological timescale of plant diversification. The most recent common ancestor of all living angiosperms likely existed ∼140–250 million years ago1,2,3. In contrast, the most recent common ancestor of all extant seed plants (that is, angiosperms and gymnosperms) is estimated to have lived ∼310–350 million years ago4,5. A key question in evolutionary biology concerns the origin of the angiosperms and of their most important defining structure, the flower4,6,7,8,9,10,11,12. To address this problem, there are three complementary approaches7. The first is to study the fossil record and attempt to identify the closest extinct relatives of angiosperms4,6. The second is to seek answers in the growing body of evolutionary developmental genetic (evo-devo) studies on the reproductive structures of living angiosperms and gymnosperms8,11,13,14. The third approach, which we apply here using a massive new data set and state-of-the-art analytical methods, is to infer the structure of ancestral flowers using the distribution of floral traits among extant angiosperms, the latest estimates of their phylogeny and models of morphological evolution. This approach allows us to uncover important clues on the origin and subsequent diversification of the flower by providing estimates of what flowers were like at key points in time.
Previous attempts to reconstruct the ancestral flower using a modern phylogenetic framework of angiosperms15,16,17 have improved our understanding of ancestral floral traits, such as the ancestral structure of the carpel18. However, several essential aspects of the ancestral flower have so far remained unresolved, due to particularly confounding variation in floral structure among the earliest diverging lineages of angiosperms18,19,20. For instance, it was still unknown whether the ancestral flower was unisexual or bisexual21. Furthermore, although the reconstruction of the ancestral flower has received some attention, the more general question of its subsequent early evolution and diversification has been little addressed in recent years9,20,22. In addition, previous efforts were limited by taxon sampling and the lack of model-based approaches to address these questions.
Here we present the largest data set of floral traits ever assembled (13,444 referenced data points), sampling 792 species from 63 orders (98%) and 372 families (86%) of angiosperms. Using chronograms from molecular dating analyses calibrated with 136 fossil constraints1, we provide the first model-based reconstructions of ancestral flowers at the deepest nodes in the phylogeny of angiosperms. We infer ancestral states for 27 floral traits using three approaches: maximum parsimony (MP), maximum likelihood (ML) and a reversible-jump Markov Chain Monte Carlo (rjMCMC) Bayesian approach that allows simultaneous exploration of multiple models of morphological evolution. In addition, each analysis was replicated using alternative hypotheses for early angiosperm phylogeny (for example, whether Amborella alone or Amborella and Nymphaeales together are the sister group of all remaining angiosperms) and two alternative estimates for the age of the angiosperms, which remain highly debated topics (Supplementary Discussion)1,2,4,23. We found that our results are generally robust and unaffected by the choice of ancestral state reconstruction method, alternative phylogenies and different divergence time estimates. However, model-based methods (ML and Bayesian) resolve some long-standing questions where parsimony continues to give equivocal answers.
Sauquet, H. et al.
The ancestral flower of angiosperms and its early diversification.
Nat. Commun. 8, 16047 doi: 10.1038/ncomms16047 (2017)
© 2017 The authors. Published open access.
Reprinted under the terms of a Creative Commons Attribution 4.0 International Licence (CC-BY 4.0).
The international team spent six years analysing the largest data set of floral traits assembled to date, consisting of 13,444 referenced data points. Their samples were from 792 species from 63 orders (98%) and 372 families (86%) of angiosperms. In simple terms, using molecular dating analysis they looked for the most common (therefore likely to be the most primitive) traits across the sample data to arrive at the most likely form of the ancestral flower.
The results were surprising; the ancestral flower seems to have had an arrangement of parts not seen in modern flowers or that previously proposed. It was bisexual with both carpals and stamens and had multiple whorls of petal-like structures or tepals, arranged in layers of three. Close to but not the same as that seen in magnolias, buttercups and laurels. It was probably a reduction in the number of these tepals that allowed flowers to diversify into the huge array of forms we see today.
The team were also able to reconstruct the likely appearance of flowers at the major nodes in this diversification.
So, not much sign of a theory in crisis there to cheer up forlorn creationists still waiting for the entire body of evolutionary biology to disappear and for biologists to turn into Bible-literalist followers of Bronze Age superstitions.
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