Polygonum sect. Echinocaulon (Polygonaceae) is a highly variable and taxonomically difficult taxon consisting of 21 species. The main objectives of this study were to infer phylogenetic relationships of the taxa within the section, and to provide a hypothesis on the origin of disjunct distribution pattern of the taxa based on molecular data. We have examined the ITS and trnL-trnF sequences from 18 taxa of sect. Echinocaulon. In all ITS and trnL-trnF trees, the taxa previously included in the section failed to form a monophyletic group: P. bungeanum was placed as sister to sect. Persicaria, suggesting that P. bungeanum should be transferred from sect. Echinocaulon to sect. Persicaria. The consensus tree obtained from the combined molecular data sets revealed the presence of five major lineages in the section; these include (1) P. perfoliatum-P. senticosum var. senticosum clade, (2) P. dissitiflorum clade, (3) P. sagittatum clade, (4) P. dichotomum-P. muricatum clade (seven taxa), and (5) P. arifolium-P. biconvexum clade (six taxa). Relationships of the taxa depicted in the consensus tree are, in general, in agreement with those previously suggested by the comparative flavonoid chemistry. In addition, the results strongly suggest that P. sagittatum may once have had a more continuous range in the northern Hemisphere and fragmented into two widely separated disjunct populations of eastern Asia and eastern North America during Pleistocene glaciation.

Key words: Disjunct distribution pattern, ITS, Molecular phylogeny, Polygonum sect. Echinocaulon, trnL-trnF