Amphistemon and Thamnoldenlandia, two new genera of Rubiaceae (Spermacoceae) endemic to Madagascar INGE GROENINCKX 1 *, PETRA DE BLOCK 2 , ELMAR ROBBRECHT 2 , ERIK E. SMETS 1,3 and STEVEN DESSEIN 2 1 Laboratory of Plant Systematics, K.U.Leuven, Kasteelpark Arenberg 31, P.O. Box 2437, BE-3001 Leuven, Belgium 2 National Botanic Garden of Belgium, Domein van Bouchout, BE-1860 Meise, Belgium 3 National Herbarium of the Netherlands, Leiden University Branch, P.O. Box 9514, NL-2300 RA Leiden, the Netherlands Received 4 March 2010; revised 21 May 2010; accepted for publication 21 May 2010 Amphistemon and Thamnoldenlandia are described as two new genera of Rubiaceae endemic to Madagascar. The two novelties belong to the predominantly herbaceous tribe Spermacoceae. Amphistemon is unique within the tribe in having its stamens inserted at two levels in the corolla tube. The genus includes two species: the subshrub A. humbertii and the geoxylic herb A. rakotonasolianus. Thamnoldenlandia includes only one species, T. ambo- vombensis, which differs from most other Spermacoceae in being a medium-sized shrub with winged seeds. We present a detailed description of Amphistemon and Thamnoldenlandia, including observations of pollen and seeds. A molecular phylogenetic study based on atpB-rbcL, petD, rps16 and trnL-trnF sequences demonstrates that the two new genera belong to the Hedyotis–Oldenlandia complex of tribe Spermacoceae. Amphistemon and Tham- noldenlandia are sister taxa and share a common ancestor with the Madagascan endemic genus Astiella. A second, but not closely related, Madagascan clade includes the endemic genera Lathraeocarpa and Gomphocalyx and the Afro-Madagascan genus Phylohydrax. This indicates that the Madagascan endemic members of Spermacoceae are the result of at least two independent colonization events, most likely by long-distance dispersal from the African mainland. © 2010 The Linnean Society of London, Botanical Journal of the Linnean Society, 2010, 163, 447–472. ADDITIONAL KEYWORDS: Astiella delicatula – long-distance dispersal – molecular phylogenetics – scan- ning electron microscopy – taxonomy. INTRODUCTION Madagascar has a unique and spectacularly rich flora with a high percentage of endemism. The island is home to more than 10 000 plant species, 90% of which occur nowhere else in the world (Moat & Smith, 2007). With natural vegetation varying from rainfor- est to unique spiny forest, Madagascar shows a huge plant diversity, making it one of the most important biodiversity hotspots (Moat & Smith, 2007). On this island hotspot, Rubiaceae are the second- largest family of flowering plants with approximately 650 described species and c. 95 genera (Davis & Bridson, 2003). More than 30% of the genera (Davis & Bridson, 2003) and 91% of the species (Davis et al., 2009) are endemic. The inventory of the Madagascan Rubiaceae is advancing rapidly. In recent years, many genera have been studied taxonomically, including Bertiera Aubl. (Wittle & Davis, in press), Breonia A.Rich ex DC. (Razafimandimbison, 2002), Coffea L. (Davis & Rakotonasolo, 2001; Davis et al., 2006; Davis & Rakotonasolo, 2008), Coptosperma Hook.f. (De Block, Degreef & Robbrecht, 2002), Gyrostipula J.-F.Leroy (Emanuelsson & Razafimandimbison, 2007), Hymenodictyon Wall. with Paracorynanthe Capuron (Razafimandimbison & Bremer, 2006), Ixora L. (De Block, in press), Lathreaocarpa Bremek. *Corresponding author. E-mail: inge.groeninckx@bio.kuleuven.be Botanical Journal of the Linnean Society, 2010, 163, 447–472. With 9 figures © 2010 The Linnean Society of London, Botanical Journal of the Linnean Society, 2010, 163, 447–472 447 Downloaded from https://academic.oup.com/botlinnean/article/163/4/447/2532650 by guest on 06 June 2022