© 2021 The Linnean Society of London, Botanical Journal of the Linnean Society, 2021, 196, 540–555 540 Botanical Journal of the Linnean Society, 2021, 196, 540–555. With 4 figures. *Corresponding author. E-mail: gregory.anderson@uconn.edu A leaky dimorphic sexual system and breeding system characterize a successful island colonist: the reproductive biology of Plocama pendula (Rubiaceae) GREGORY J. ANDERSON 1, * , , JULIA PÉREZ DE PAZ 2 , MONA ANDERSON 3 , GABRIEL BERNARDELLO 4 and DAVID W. TAYLOR 5 1 Department of Ecology and Evolutionary Biology, University of Connecticut, 75 North Eagleville Road, Unit 3043, Storrs, CT 06269-3043, USA 2 Jardín Botánico Canario ‘Viera y Clavijo’, Unidad Asociada CSIC, Cabildo de Gran Canaria, Camino al Palmeral 15, 35017 Las Palmas de Gran Canaria, Spain 3 Department of Linguistics, University of Connecticut, Storrs, CT 06268-1145, USA 4 Universidad Nacional de Córdoba, Facultad de Ciencias Exactas, Físicas y Naturales–Instituto Multidisciplinario de Biología Vegetal (UNC-CONICET), C. C. 495, Córdoba, Argentina 5 Department of Biology, University of Portland, Swindells Hall 108, MSC 163, 5000 North Willamette Boulevard, Portland, OR 97203, USA Received 21 November 2020; revised 19 January 2021; accepted for publication 15 February 2021 Island plants provide special opportunities for the study of evolution and ecology. In field and greenhouse studies we characterized a model reproductive system for Plocama pendula, endemic to the Canary Islands. This species has a complicated and not immediately obvious reproductive system. Pollination is biotic, and all flowers are morphologically hermaphroditic, but half of the plants characteristically bear flowers with nectar, pistils with reflexed stigmatic lobes and pollen-less anthers (i.e. they are functionally female flowers). The other half bear nectar-less flowers with abundant pollen and full-sized pistils that mostly have un-reflexed stigmatic lobes (i.e. they are hermaphroditic flowers functioning mostly as males). However, experiments show these pollen-bearing flowers to be self-compatible. Thus, the functionally male flowers have a breeding system that allows selfing in limited circumstances, but the functionally male flowers produce far fewer fruits than do functionally female flowers. With morphologically gynodioecious, functionally largely dioecious flowers, sometimes capable of selfing, the reproductive system of this species could be labelled as ‘leaky’ in many respects. Thus, we propose that P. pendula has colonized new habitats and persists in substantial populations at least in part because it manifests a reproductive system that is a model for successfully balancing the often-conflicting evolutionary demands of colonization, establishment and persistence. ADDITIONAL KEYWORDS: Canary Islands – colonization – conservation – differential nectar production – dioecy – gynodioecy – islands – leaky breeding/sexual systems – reproductive systems – self-compatibility. INTRODUCTION Island biota provide special opportunities for the study of evolution and ecology. Charles Darwin’s first recorded doubts regarding the stability of species come from his notes, in 1836, on Galápagos birds during the Beagle voyage (Cambridge University Library, Darwin Online; Keynes, 2000). Carlquist’s comprehensive books on island biota (1965, 1974) and the treatise of MacArthur & Wilson from the same era (1967) laid out the theory for quantitative biogeographical interpretations of island organisms, energizing and re-directing studies of islands. These, and much research that followed, highlighted the ways in which island biota, instead of constituting just biological novelties, could be models and serve as natural laboratories (e.g. Whittaker et al., 2017; Crawford & Archibald, 2017) for study of dispersal, establishment and radiation. Furthermore, island landmasses are geo-datable, thus giving estimates of (maximum) Downloaded from https://academic.oup.com/botlinnean/article/196/4/540/6190341 by guest on 02 October 2022