A Taxonomic Study of Astragalus sikokianus with a Disjunct Distribution between Northwestern China and the Korea-Japan Region

Abstract

Astragalus sikokianus has been known as endemic to the Korea-Japan region but is taxonomically confused with A. koraiensis from Korea and A. bhotanensis from Bhutan and western China. To clarify its taxonomic boundary and distribution, we examined external morphological characters and nrITS sequence data. Morphologically, A. koraiensis and A. sikokianus are hardly distinguishable from each other but they differ from A. bhotanensis of Bhutan and southwestern China in their leaves, leaflets, peduncles, corollas, calyx, and legumes. In contrast, the diagnostic characters of plants known as A. bhotanensis from northwestern China match those of A. sikokianus. Our TCS network and ML tree based on ITS sequence variations also recognized the plants from northwestern China, Korea, and Japan as being an independent evolutionary lineage that is distinct from the other group of A. bhotanensis from southwestern China. Therefore, this morphological and molecular evidence leads us to conclude that A. koraiensis is synonymized with A. sikokianus and, as delimited here, the latter species also includes plants from northwestern China. It is also possible that this disjunct distribution of newly circumscribed A. sikokianus can be explained by historical geographic changes and long-distance dispersal in East Asia.