Phytotaxa 327 (3): 201–222 http://www.mapress.com/j/pt/ Copyright © 2017 Magnolia Press Article PHYTOTAXA ISSN 1179-3155 (print edition) ISSN 1179-3163 (online edition) Accepted by Marcus Lehnert: 14 Oct. 2017; published: 10 Nov. 2017 https://doi.org/10.11646/phytotaxa.327.3.1 201 Taxonomic study of the fern genera Abrodictyum C.Presl and Trichomanes L. (Hymenophyllaceae, Polypodiidae) in the western Indian Ocean, and description of a new Abrodictyum species for Madagascar JEAN-YVES DUBUISSON 1 , LUCIE BAURET 1,2 , AURELIE GRALL 3 , BRUNO SENTERRE 4,5 , AHAMADA H. SAID 2 , KERSLEY PYNEE 6 , ATSUSHI EBIHARA 7 & SABINE HENNEQUIN 1 1 Institut Systématique Evolution Biodiversité (ISYEB), Sorbonne Universités, UPMC Univ Paris 06, MNHN, CNRS, EPHE, 57 rue Cuvier, CP48, 75005 Paris, France. E-mail: dubuisson@mnhn.fr. 2 Institut Systématique Evolution Biodiversité (ISYEB), Sorbonne Universités, MNHN, CNRS, UPMC, EPHE, Herbier National, 57 rue Cuvier, CP39, 75005 Paris, France. 3 Royal Botanic Gardens, Kew, Richmond, Surrey, TW9 3AE, United Kingdom. 4 Island Biodiversity & Conservation Centre, associated with the University of Seychelles, Anse Royale, Mahé, P.O. Box 1348, Sey- chelles. 5 Evolutionary Biology & Ecology - CP 160/12, Université Libre de Bruxelles, 50 Av. F. Roosevelt, 1050 Bruxelles, Belgium. 6 The Mauritius Herbarium, Agricultural Services, Agronomy Division, Ministry of Agro-Industry and Food Security, Réduit, Mauritius. 7 Department of Botany, National Museum of Nature and Science, 4-1-1 Amakubo, Tsukuba, 305-0005, Japan Abstract Recent studies on both genera Abrodictyum and Trichomanes (Hymenophyllaceae) in western Indian Ocean (Madagascar and neighbouring islands) resolved many taxonomic problems and led to the identification of new species. We propose here to finalize the taxonomic revision of both genera for the region, by completing and combining an expanded rbcL molecular phylogeny with morphological investigations and field observations on all the recognized species. Our results confirm the distinction of the supposedly six species of Abrodictyum and three species of Trichomanes. They also reveal the polyphyly of the highly polymorphic A. meifolium. Additional investigations on A. meifolium suggest the existence of at least two dis- tinct taxa: the true highly variable and widespread A. meifolium which is renamed A. parviflorum comb. nov. and the newly described A. cylindratum sp. nov. restricted to Madagascar. A key and updated descriptions (including new diagnostic char- acters for A. tamarisciforme) are provided for the seven Abrodictyum and three Trichomanes species present in the western Indian Ocean. Introduction As introduced in one of the most recent works on the family in western Indian Ocean (Dubuisson et al. 2016), Hymenophyllaceae or filmy ferns are quite diversified in the region with at least 58 species mostly found as epiphytes (or epilithes) in the understory of rainforests from sea level to high elevations (above 2,000 m). Nevertheless, some terrestrial taxa are also found. Three terrestrial species belong to the genus Trichomanes Linné (1753: 1097) and four belong to the genus Abrodictyum Presl (1843: 20), according to the classification of Ebihara et al. (2006). It should be noted that the individual epiphytic A. tamarisciforme (von Jacquin 1789: 285) Ebihara & Dubuisson (2006: 244) mostly grows on tree ferns (Alsophila spp.) while all the other epiphytic filmy ferns are forming colonies and usually observed on diverse substrates or phorophytes (Dubuisson et al. 2003a, 2011, 2013; Tardieu-Blot 2008; Grangaud 2010). However, the same individual epiphytic habitat is also observed for the questionable A. angustimarginatum (Bonaparte 1925: 12, 13) J.P. Roux (2009: 37) as we will discuss here-after. All the Trichomanes and Abrodictyum species share, at least in the western Indian Ocean, the same gross morphology (short-creeping to erect rhizome bearing tufted erect large simply-pinnate to highly divided fronds). This explains the numerous confusions in the field and in collections, especially for species with similar highly divided fronds (Bauret et al. 2015). Recent morphological and molecular studies have thus been conducted in order to precisely discriminate some problematic species (Bauret et al. 2015; Dubuisson et al. 2016). Those studies shed light upon the status and identification of the Seychellois Trichomanes cupressoides Desvaux (1827: 330) and led to the combination