Abstract: It has been debated whether two Polygonum species, P. cyanandrum Diels and P. forrestii Diels, should be transferred to Koenigia. In the present paper, nucleotide variations of the chloroplast atpB-rbcL noncoding spacer were examined for these two species and related representatives from the Polygonaceae. The atpB-rbcL spacer region varies among sampled species with a range from 753 to 902 bp, and the great variation in this region provides enough information for evaluating the phylogenetic relationships of the two investigated species. In contrast to the view that these two species belong to Polygonum, our phylogenetic analyses based on these molecular data showed that P. cyanandrum and P. forrestii were nested within a well supported clade including Koenigia islandica; the other three Polygonum species grouped with species of different genera. These results suggest that the two species should be transferred to Koenigia from Polygonum, and that phylogenetic relationships among Polygonum and related genera need to be further investigated with increased taxon sampling and gene markers. The resulting phylogeny is consistent with the previous suggestions based on pollen and chromosome investigations. The generic circumscription between Koenigia and Polygonum is also discussed in light of these results in combination with gross morphology.

Key words: Polygonaceae, atpB-rbcL, Polygonum cyanandrum, Polygonum forrestii, Koenigia