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.