Taxonomy of Exochaenium and Lagenias: Two Resurrected Genera of Tribe Exaceae (Gentianaceae)

Morphological and molecular evidence has recently revealed the paraphyly of Sebaea (Gentianaceae: Exaceae), and support the establishment of three genera corresponding to highly supported clades. Consequently, the reinstatement of both Exochaenium and Lagenias as genera segregated from Sebaea is proposed here. Exochaenium consists of 22 species distributed in tropical continental Africa that are characterized by papillate stigmas and cubical seeds with star-shaped testa cells. Lagenias is a monotypic genus endemic to the Western Cape Province of South Africa, which differs from Exochaenium and Sawn by its medifixed anthers, the insertion of the filaments at the base of the corolla tube, and cubical seeds with polygonal testa cells. The synapomorphies of Sebaea s. s. are the presence of secondary stigmas along the style and seeds with rectangular testa cells arranged in parallel rows. Differences with the other genera of tribe Exaceae (Exacum, Gentianothammus, Klackenbergia, Ornichia, and Tachiadenus) are discussed. In addition, diagnostic characters as well as keys to the genera of the tribe and species of Exochaenium are provided. To account for the new circumscription of Exochaenium, 12 new combinations are also published here (Exochaenium (datum, E. caudatum, E. clavatum, E. dimidiatum, E. fernandesianum, E. hockii, E. lineariforme, E. macropterum, E. lipid:hum, E. perparvum, E. rotundifolium, and E. wildemanianum).