And it was a success... like a flower!

It is the common ancestor of some 300,000 species of flowering plants that we know today. An international study coordinated by Hervé Sauquet a teacher-researcher at the Ecology, Systematics and Evolution Laboratory (Paris-Sud University/CNRS/AgroParisTech) has just released an image of the flower that is the ancestor of all flowering plants that would have appeared over the last 140 million years.

It is difficult to imagine a world without flowers. However, this is what has long proven to be the reality on Earth as flowering plants only arrived late on in the evolution of the plant world. Whilst the first plants appeared about 420 million years ago, flowering plants, also called Angiosperms (1), waited almost another 300 million years to unfurl their petals.

Although they may have arrived late, their appearance ultimately turned out to be a real success since they have spread to all the world’s continents and to the extent that they now account for almost 90% of all plants. All the more so since this colonisation of the planet was accompanied by extreme diversification as there are no fewer than 300,000 different species of flowering plants.

300,000 species but one common ancestor

The origin and the evolution of these flowering plants is one of biology’s great enigmas. To try to solve the mystery, a team of 36 researchers from 13 countries, whose work was coordinated by Hervé Sauquet, teacher-researcher at Paris-Sud University, set about drawing up a model of the first common ancestor of all flowering plants. Due the fragile nature of flowers, fossilised flowers are too rare to be used as truly reliable references, which makes it all the task all the more difficult.

Without any fossils for evidence, the researchers looked towards another means of research: analysing a large database that compiles phenotype information (i.e. the visual appearance) from species of flowers with the latest evolutionary tree of flowering plants, constructed from genetic information from living species. A long-term endeavour since this project, named eFLOWER, lasted almost six years.

The team sampled 800 species which were representative of the diversity of different Angiosperm families. This colossal task has made it possible to solve the enigma of the original flower and has provided a surprising answer: the very first flower, which was probably borne on a small tree or a shrub, was a hermaphrodite and covered in petaloid organs arranged in circles. But, this model doesn’t correspond to any of those put forward during the last 100 years! 

‘No one had really thought about the first stages in the evolution of flowers in this way, and yet a large part of diversity can now be explained in a simple way using the new scenario which has emerged from our models’ says Hervé Sauquet.

A new scenario to explain the evolutionary history of flowering plants

Therefore, the first surprise, according to the study, was that the ancestral flower was a hermaphrodite, in other words it has both female (carpels) and male parts (stamen). The separation of the two sexes in different flowers probably appeared later in their evolution. This issue has long been controversial because there are many unisexual flowers and the majority of non-flowering plants (such as conifers and cycas) typically have separate sexes in different structures. In any event, bisexuality gives the plant an advantage when colonising new environments.  

The researchers have also discovered that the ancestral flower has several whorls (concentric circles) of organs which resemble petals and are arranged in groups of three. But some of the very first surviving species in the genealogical tree of flowering plants have petals arranged in spirals. Therefore, the majority of biologists were convinced that the first flower would have its petals arranged in a similar fashion.

This model makes it possible to put forward a new scenario regarding the early diversification of flowers. These results suggest that the first flowers may have become more diverse, not by developing greater complexity, but by first becoming simpler.

‘These results will help to us to place the few flower fossils that we do have more accurately on the tree of life. They also make it possible for us to better understand the coevolution between flowers and pollinating insects’, said Hervé Sauquet.

1. Angiosperms (from the Greek meaning enclosed seeds) have in common having reproductive organs arranged on a short axis and surrounded by sterile organs (petals and sepals), forming the flower, and have their fertilised seeds protected inside a fruit.

Contact: Hervé Sauquet – Ecology, Systematics and Evolution Laboratory (Paris-Sud University/CNRS/AgroParisTech) – herve.sauquet @ u-psud.fr (herve.sauquet @ u-psud.fr)

Reference
The ancestral flower of angiosperms and its early diversification. Sauquet H, von Balthazar M, Magallón S, Doyle JA, Endress PK, Bailes EJ, Barroso de Morais E, Bull-Hereñu K, Carrive L, Chartier M, Chomicki G, Coiro M, Cornette R, El Ottra JHL, Epicoco C, Foster CSP, Haevermans A, Haevermans T, Hernández R, Jabbour F, Little SA, Löfstrand S, Luna JA, Massoni J, Nadot S, Pamperl S, Prieu C, Reyes E, dos Santos P, Schoonderwoerd KM, Sontag S, Soulebeau A, Städler Y, Tschan GF, Wing-Sze Leung A, Schönenberger J. 2017. Nature Communications: In press