New insights into the phylogenetic relationships of Japanese knotweed ( Reynoutriajaponica ) and allied taxa in subtribe Reynoutriinae (Polygonaceae).

Japanese knotweed ( Reynoutriajaponica ) is native to East Asia, but has been introduced to the West where it is a noxious invasive weed. Taxonomically, Japanese knotweed is placed within subtribe Reynoutriinae (Polygonaceae), which also contains the austral genus Muehlenbeckia (incl. Homalocladium ) and north temperate Fallopia . In the current study, we conducted a phylogenetic analysis using sequence data from six markers, two nuclear ( LEAFYi2 , ITS) and four plastid ( matK , rbcL , rps16-trnK and trnL-trnF ) to further resolve the evolutionary relationships within this group, using the widest sampling of in-group taxa to date. The results of this analysis confirmed that subtribe Reynoutriinae is a monophyletic group, characterised by the presence of extra-floral, nectariferous glands at the base of leaf petioles. Within the subtribe, four main clades were identified: Reynoutria , Fallopiasect.Parogonum, Fallopia s.s. (including Fallopia sects. Fallopia and Sarmentosae ) and Muehlenbeckia . The Fallopia s.s. and Muehlenbeckia clades are sister to one another, while the Fallopiasect.Parogonum clade is immediately basal to them and Reynoutria basal to all three. Fallopia , as currently circumscribed, is paraphyletic as Muehlenbeckia is nested within it. To resolve this, we propose that species of Fallopiasect.Parogonum should be treated as a new genus, Parogonum (Haraldson) Desjardins & J.P.Bailey, gen. et stat. nov. Within Reynoutria , the allied specific and infraspecific taxa that fall under the name Japanese knotweed s.l. form a monophyletic group and their taxonomic status is discussed.