A knot of hybrids: Differentiating Asian knotweeds in North-Eastern France using genetic, cytological, and morphological data

The two invasive Reynoutria species, Reynoutria japonica var. japonica and Reynoutria sachalinensis, and their hybrid Reynoutria x bohemica are often misidentified by managers and nonspecialists. The taxonomic confusions are all the more exacerbated by the infraspecific variability of introduced populations in terms of morphology, genetic diversity, and ploidy level. We resolved the identity of North-Eastern French invasive populations using 4582 single-nucleotide polymorphisms (SNPs) from a RADseq analysis, DNA contents estimated by flow cytometry, and 12 vegetative morphometric variables. The SNPs supported only one single genotype for R. japonica over 11 localities, while the nine localities of Reynoutria x bohemica were represented by one genotype each. Estimation of genome size using DAPI staining and flow cytometry revealed only octoploid cytotypes for R. japonica and hexaploid cytotypes for R. x bohemica, whereas R. sachalinensis was represented by tetraploid and hexaploid cytotypes. Among morphometric variables, no single one allows for a clear differentiation of the three taxa. We propose a combination of characters to easily and quickly identify these three invasive taxa based on six vegetative criteria including leaf and apex length, as well as leaf shape, leaf base, and apex shape, and the extrafloral nectaries on the node.