A New Molecular Phylogeny and a New Genus, <i>Pendulorchis</i>, of the <i>Aerides–Vanda</i> Alliance (Orchidaceae: Epidendroideae)

<div><p>Background</p><p>The <i>Aerides–Vanda</i> alliance is a complex group in the subtribe Aeridinae (subfamily Epidendroideae, Orchidaceae). Some phylogenetic systems of this alliance have been previously proposed based on molecular and morphological analyses. However, several taxonomic problems within this alliance as well as between it and its allies remain unsolved.</p> <p>Methodology/Principal Findings</p><p>We utilized ITS and five plastid DNA regions in this phylogenetic analysis. Consensus trees strongly indicate that the <i>Aerides–Vanda</i> alliance is monophyletic, and the 14 genera of this alliance can be grouped into the following clades with 14 subclades: 1. <i>Aerides</i>, comprising two subclades: <i>Rhynchostylis</i> and <i>Aerides</i>; 2. <i>Ascocentropsis</i>; 3. <i>Papilionanthe</i>; 4. <i>Vanda</i>, comprising five subclades: <i>Neofinetia</i>, <i>Christensonia</i>, <i>Seidenfadenia</i>, <i>Ascocentrum</i>, and <i>Vanda–Trudelia</i>, in which <i>Vanda</i> and <i>Trudelia</i> form a subclade; 5. <i>Tsiorchis</i>, comprising three subclades: <i>Chenorchis</i>, <i>Tsiorchis</i>, and two species of <i>Ascocentrum</i>; 6. <i>Paraholcoglossum</i>; and 7. <i>Holcoglossum</i>. Among the 14 genera, only <i>Ascocentrum</i> is triphyletic: two species of the <i>Ascocentrum</i> subclade, an independent subclade <i>Ascocentrum</i> subclade in the <i>Tsiorchis</i> clade; the <i>Ascocentrum</i> subclade in the <i>Vanda</i> clade; and one species in the <i>Holcoglossum</i> clade. The <i>Vanda</i> and <i>Trudelia</i> species belong to the same subclade. The molecular conclusion is consistent with their morphological characteristics.</p> <p>Conclusions</p><p>We elucidate the relationship among the 14 genera of the <i>Aerides–Vanda</i> alliance. Our phylogenetic results reveal that the <i>Aerides–Vanda</i> alliance is monophyletic, but it can be divided into 14 genera. The data prove that <i>Ascocentrum</i> is triphyletic. Plants with elongate-terete leaves and small flowers should be treated as a new genus, <i>Pendulorchis</i>. <i>Saccolabium himalaicum</i> (<i>Ascocentrum himalaicum</i>) should be transferred to <i>Pendulorchis</i>. <i>Ascocentrum pumilum</i>, endemic to Taiwan, should be transferred to <i>Holcoglossum</i>. A new combination, <i>Holcoglossum pumilum</i>, was also established. <i>Trudelia</i> should not be recognized as an independent genus. Two new species, <i>Pendulorchis gaoligongensis</i> and <i>Holcoglossum singchianum</i>, were described as well.</p>