Phylogenetic placement of Psilopeganum, a rare monotypic genus of Rutaceae (the citrus family) endemic to China

Psilopeganum (Rutaceae) is a rare monotypic genus endemic to the vicinity of the Yangtze River valley in Chongqing, Hubei, Sichuan and Guizhou provinces in China. It differs from most Rutaceae taxa by its herbaceous habit and has been treated as a member of the tribe Ruteae. Our study is the first attempt to place Psilopeganum in a phylogenetic context and our results show that the genus belongs to a clade with Boenninghausenia, Ruta and Thamnosma, which are part of Ruteae. Within this group, the position of Psilopeganum remains unclear because the Boenninghausenia-Thamnosma clade, Psilopeganum and Ruta form a trichotomy in most analyses. The ITS dataset placed Psilopeganum as sister to the Mediterranean and Canarian genus Ruta, which is corroborated by morphological similarities. Our studies support that Ruteae is paraphyletic with respect to Aurantioideae and that Dictamnus does not belong to Ruteae. The Indian, Sri Lankan, and Malagasy genus Chloroxylon is sister to the Boenninghausenia-Psilopeganum-Ruta-Thamnosma clade, despite its traditional placement in the subfamily Flindersioideae. The placement of Chloroxylon is consistent with an origin of the group of Chloroxylon, Boenninghausenia, Psilopeganum, Ruta and Thamnosma in southern Asia. The rapid uplifts of the Himalayas could account for one or two vicariance events splitting the lineages of the Boenninghausenia-Psilopeganum-Ruta-Thamnosma clade, and may explain the short branch length and low support for the relationships among Psilopeganum, Ruta, and the Boenninghausenia-Thamnosma clade.