Cathetus yuanjiangensis (Phyllanthaceae, Phyllantheae), a New Species from Southwest China, with a New Combination and an Updated Key to C. Subgen. Cathetus

Cathetus yuanjiangensis Huan C. Wang, Feng Yang et Chao Chen (Phyllantheae, Phyllanthaceae), a new species from the dry-hot valleys of Southwest China, is described and illustrated herein. Morphologically, the new species can be easily distinguished from its congeners by the following combination of characters: leaves 1.5–4.5 cm long, 1–2 cm wide; staminate flowers sepals spreading 1.5–2.0 mm long; filaments connate into a 2–3 mm long column; styles 3, ca. 1.6 mm long, connate into a ca. 0.5 mm long column at base; stigmas spreading, slender, oblong, each bifid to 1/2, the lobes linear. Phylogenetic analyses based on two nuclear (ITS and PHYC) and three chloroplast (matK, accD-psaI and trnS-trnG) DNA makers recover C. yuanjiangensis as the sister clade to C. fasciculate among the sampled taxa. Detailed morphological descriptions, photographs, pollen morphology, and complete plastid genome characteristics of the new species are provided, along with a distribution map and a preliminary conservation assessment. Additionally, we propose a new combination, Cathetus beillei (Hutch.) Huan C. Wang et Feng Yang, based on the basionym Phyllanthus beillei Hutch., which was originally published in Flora of Tropical Africa. An updated identification key for all recognized species of C. subgen. Cathetus is provided.