The palm family (Arecaceae): a microcosm of sexual system evolution SOPHIE NADOT 1 *, ELODIE ALAPETITE 1 , WILLIAM J. BAKER 3 , JAMES W. TREGEAR 4 and ANDERS S. BARFOD 1,2 1 Laboratoire Ecologie, Syst  ematique et Evolution UMR 8079, Univ. Paris-Sud, CNRS, AgroParisTech, Universit  e Paris-Saclay, Orsay Cedex 91405, France 2 Department of Bioscience, Aarhus University, Ny Munkegade 114, Aarhus C DK-8000, Denmark 3 Royal Botanic Gardens, Kew, Richmond, Surrey TW9 3AB, UK 4 Institut de Recherche pour le D eveloppement (IRD), UMR DIADE, 911 Avenue Agropolis BP 64501, Montpellier Cedex 5, 34394, France Received 5 January 2016; revised 2 March 2016; accepted for publication 19 April 2016 Among the 416 angiosperm families, palms (Arecaceae) are striking in possessing almost all possible combinations of hermaphroditic and/or unisexual flowers, making them a particularly interesting subject for studies of the evolution of plant sexuality. The purpose of this review is to highlight the amazing diversity of sexual expression in palms with a view to proposing scenarios to explain the evolution of this character, drawing on the numerous advances that have been made over the last 20 years in palm systematics, ecology, developmental biology, phylogenetics and genomics. We provide an overview of the variability of sexual expression in palms, with illustrations of the associated morphological diversity and its significance to reproduction. We discuss the evolution of sexual systems using the most recent phylogenetic framework available for palms. Finally, we review advances made towards unravelling the genetic basis of sexual expression in palms. Our survey opens new perspectives for understanding how plant sexuality has evolved in angiosperms as a whole. © 2016 The Linnean Society of London, Botanical Journal of the Linnean Society, 2016, 182, 376–388 ADDITIONAL KEYWORDS: dioecy – floral traits – Palmae. INTRODUCTION The diversity of reproductive structures and strate- gies in flowering plants has long fascinated botanists and evolutionary biologists. Reproductive systems in plants may relate to pollination, sexual expression in space and time, and mating (or breeding) systems, i.e. outcrossing vs. selfing (Barrett, 2010, 2013). The term ‘sexual systems’ (or sometimes sex systems) is employed to describe the spatial arrangement of sexes (both sexes in the same flower, in different flowers borne on the same plant or each sex on a dif- ferent plant) (e.g. (Barrett, 1998; Charlesworth, 2006). Sexual expression in flowering plants is classi- cally broken down into three main categories (her- maphroditism, dioecy and monoecy), with an additional minor category, often termed as ‘polygamy’ that defines various intermediate states between the first three groups. Hermaphroditism, in which all flowers bear functional male and female organs, is the most common situation in angios- perms. Dioecy, where male and female flowers are borne on different plants, is found in 6% of angios- perm species distributed across half of all families (Renner & Ricklefs, 1995). Monoecy, with male and female flowers borne on the same plant, has received less attention than hermaphroditism and dioecy. In a survey of various temperate and tropical floras, monoecy was estimated to occur in 3–19% of angios- perm species (de Jong, Shmida & Thuijsman, 2008). Darwin (1877) considered monoecy to be an adapta- tion to favour outcrossing due to greater spatial sep- aration between pollen and stigmas, whereas dioecy guarantees that selfing will never occur. Renner & Ricklefs (1995) noted that monoecy, like dioecy, is often associated with abiotic pollination. In *Corresponding author. E-mail: sophie.nadot@u-psud.fr 376 © 2016 The Linnean Society of London, Botanical Journal of the Linnean Society, 2016, 182, 376–388 Botanical Journal of the Linnean Society, 2016, 182, 376–388. With 4 figures Downloaded from https://academic.oup.com/botlinnean/article-abstract/182/2/376/2707825 by guest on 04 June 2020