The ancestral flower of angiosperms and its early diversification

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.

Total evidence phylogeny of Pontederiaceae (Commelinales) sheds light on the necessity of its recircumscription and synopsis of Pontederia L.

“…In Haemodoraceae, there is much variation in the shape of the perianth lobes (Simpson 1990, 1998). Nonetheless, equal perianth lobes seem to be plesiomorphic in the monocots (Sauquet et al 2017; Stevens 2001–onwards) and dominant in the family, being recorded for 11 out of 14 genera (Pellegrini, pers. observ.).…”

Key questions and challenges in angiosperm macroevolution

“…A recent study has confirmed that the most recent common ancestor (i.e. crown node) of all living angiosperms possessed bisexual flowers with a perianth (Sauquet et al ., ). The most recent common ancestor of seed plants (i.e.…”

“…On the one hand, morphospace approaches have allowed the quantification of differences among taxa or sets of traits (modules) in general integration using multidimensional statistics (Chartier et al ., , ). On the other hand, correlations among key floral traits have been tested using comparative methods that take the phylogeny explicitly into account (O'Meara et al ., ; Sauquet et al ., ).…”

“…This new confidence in a robust phylogenetic framework has provided the foundation for a blossoming number of macroevolutionary analyses seeking to answer old and new questions about deep‐time angiosperm evolution. For instance, recent work has focused on the estimation of divergence times (Magallón et al ., ), reconstruction of the mode and tempo of angiosperm diversification (Silvestro et al ., ; Tank et al ., ), reconstruction of ancestral traits (Sauquet et al ., ) and testing of the correlates of speciation and extinction rates (O'Meara et al ., ). However, vexingly, many of these important questions still remain largely open.…”

Recent advances in understanding the roles of whole genome duplications in evolution

“…An A/E gymnosperm pro-orthologue gave rise to angiosperm A and E genes, and further duplicated A genes were also retained after the ε WGD 19, 21– 24 . These duplicated and diversified gene sets organised to generate the first now-extinct flowers, and recent reconstructions suggest that these were bisexual with petal-like tepals and pollen-bearing stamens arranged in multiple concentric whorls, and female carpels in a central spiral 25 . Among living angiosperms, the Amborella lineage evolved a ‘fading borders’ programme, such that the whole flower is a spiral that gradually transitions from bracts to outer then inner tepals (specified by ABc combinations), from inner tepals to stamens (aBC) then carpels (abC), in which upper case indicates functions of greatest influence in the respective organs 26 .…”