Reinstatement of Isotrema, a new generic delimitation of Aristolochia subgen. Siphisia (Aristolochiaceae)

Abstract

Aristolochia subgen. Siphisia, comprising 98 species, is mainly distributed in East and South Asia, with some scattered in North and Central America. It has often been recognized as one of three subgenera of Aristolochia and can be distinguished from both remaining subgenera (Aristolochia subgen. Aristolochia and Aristolochia subgen. Pararistolochia) by morphological synapomorphies such as a strongly curved perianth with a 3-lobed limb, and especially the 3-lobed gynostemium, anthers paired on the outer surface of each gynostemium segment. However, recent cladistic analyses based on morphological characters and molecular data suggested that Aristolochia s.l. should be divided into four genera in two subtribes. In this study, a comprehensive analysis of molecular data of two plastid genic spacers (rps16-trnK and petB-petD), and two nuclear genes (phyA and ITS2) with an extensive sampling of Asian and American species was conducted. The combined datasets confirmed that Aristolochia subgen. Siphisia should be treated as an independent genus. Then Isotrema is reinstated here with 87 new combinations for the most comprehensive enumeration.