title: Floral development of Sabia (Sabiaceae): evidence for the derivation of pentamery from a trimerous ancestry.
abstract: •
  
  
  OBJECTIVE
  Flowers of Sabiaceae diverge from basal eudicots in combining pentamery with superposed whorls of sepals, petals, and stamens and are therefore crucial in understanding origins of core eudicot flowers. Different hypotheses are tested using floral developmental evidence, whether the pentamerous flower is derived from a spiral, trimerous, or dimerous progenitor.•
  
  
  METHODS
  The floral development of two species of Sabia was investigated with the scanning electron microscope to understand their unusual floral morphology and the origin of pentamery.•
  
  
  RESULTS
  The species show major developmental differences in their inflorescence morphology and organ initiation sequence. In S. limoniacea, flowers are subtended by a pherophyll preceding two prophylls, one of which encloses a younger flower; floral organs arise in a continuous spiral sequence without interruption between different organs. The ovary is oriented in an oblique-median position. In S. japonica, one prophyll replaces one of the sepals, and there is a disruption in the spiral sequence. As a result, the ovary is inserted more or less transversally.•
  
  
  CONCLUSIONS
  The flower of Sabiaceae is structurally best interpreted as derived from a trimerous progenitor, and a derivation from a dimerous or spiral progenitor is less likely. One organ of each median adaxial whorl is interpreted as lost (from K3+3 C3+3 A3+3 G3 to K3+2 C3+2 A3+2 G2). The number of sepals is variable as pherophylls, prophylls, and sepals cannot be distinguished by shape and intergrade with each other. The floral organization of Sabia is reminiscent of trimerous Ranunculales and supports an earlier divergence of Sabiaceae relative to Proteales.
authors.affiliation: Royal Botanic Garden Edinburgh, 20A Inverleith Row, Edinburgh EH3 5LR, UK.
authors.forename: Louis P
authors.initials: LP
authors.lastname: Ronse De Craene
authors.affiliation: Rheinische Friedrich-Wilhelms-Universität Bonn, Nees-Institut für Biodiversität der Pflanzen (NEES), Germany.
authors.forename: Dietmar
authors.initials: D
authors.lastname: Quandt
authors.affiliation: Swedish Museum of Natural History, Stockholm, Box 50007 SE-104 05 Stockholm, Sweden.
authors.forename: Livia
authors.initials: L
authors.lastname: Wanntorp
mesh_terms: D005075:Biological Evolution; D035264:Flowers / Q000254:growth & development* / Q000648:ultrastructure; D019684:Magnoliopsida / Q000254:growth & development* / Q000648:ultrastructure; D008855:Microscopy, Electron, Scanning
keywords: Proteales; Ranunculales; Sabiaceae; dimery; floral development; pentamery; phyllotaxis; phylogeny; trimery