In places where multiple related taxa are invasive and known to hybridize, it is important to have correct identifications to enable an appropriate legal, ecological, and management understanding of each kind of invader. Invasive knotweeds in the genus Reynoutria Houtt. are noxious weeds in Europe, North America, Africa, and Oceania, where they disrupt native plant communities and negatively impact human activities. Two species (R. japonica Houtt., R. sachalinensis (F.Schmidt) Nakai) and their hybrid (known as R. × bohemica Chrtek & Chrtková) have similar invasive tendencies, although there are some noted differences among them in their reproduction potential, ecological tolerance, and effect on native communities. Prior studies demonstrated that there is not only one kind of interspecific hybrid, but in fact at least four kinds that differ in the sequence variants they possess from each parent. Thus, in addition to identifying plants as hybrids, it may become important to distinguish each kind of hybrid when considering control or treatment strategies. In the current study, we expand the available genetic information for invasive knotweeds by providing expanded DNA sequence data for the low-copy nuclear gene LEAFY, which has become important for characterizing hybrids. Our methods recover the same LEAFY genotypes that were identified previously for the commonly sequenced second intron, and we also provide sequence data for the first intron and second exon of the gene.

Knotweed (Fallopia spp.) is an herbaceous perennial from East Asia that was brought to Europe and North America and, despite control efforts, subsequently spread aggressively on both continents. Data are available on knotweed’s modes of sexual and asexual spread, historical spread, preferred habitat, and ploidy levels. Incomplete information is available on knotweed’s current global geographic distribution and genetic diversity. The chemical composition of knotweed leaves and rhizomes has been partially discovered as related to its ability to inhibit growth and germination of neighboring plant communities via phytochemicals. There is still critical information missing. There are currently no studies detailing knotweed male and female fertility. Specifically, information on pollen viability would be important for further understanding sexual reproduction as a vector of spread in knotweed. This information would help managers determine the potential magnitude of knotweed sexual reproduction and the continued spread of diverse hybrid swarms. The potential range of knotweed and its ability to spread into diverse habitats makes studies on knotweed seed and rhizome cold tolerance of utmost importance, yet to date no such studies have been conducted. There is also a lack of genetic information available on knotweed in the upper Midwest. Detailed genetic information, such as ploidy levels and levels of genetic diversity, would answer many questions about knotweed in Minnesota, including understanding its means of spread, what species are present in what densities, and current levels of hybridization. This literature review summarizes current literature on knotweed to better understand its invasiveness and to highlight necessary future research that would benefit and inform knotweed management in the upper Midwest.

Knotweed species in the genus Reynoutria are native to eastern Asia but have become noxious weeds in Europe and North America. In the United States, invasive populations of Japanese knotweed (Reynoutria japonica Houtt.), giant knotweed [Reynoutria sachalinensis (F. Schmidt) Nakai], and their interspecific hybrid known as Bohemian knotweed (R. × bohemica Chrtek & Chrtková) continue to expand their ranges. Although these plants are among the most invasive terrestrial species, there are relatively few molecular tools for identifying the parental species, the F1 hybrid, or subsequent hybrids or introgressed individuals. We studied Reynoutria populations in Wisconsin, a state where all three taxa grow, to determine whether molecular data would be useful for distinguishing species and identifying hybrids. We obtained DNA sequence data from the plastid matK gene and the nuclear LEAFY gene and compared these to previously published sequences. Data from the uniparentally inherited matK region included haplotypes attributable to R. japonica and R. sachalinensis. Nuclear data indicated that R. sachalinensis plants are most similar to native plants in Japan, with two Wisconsin accessions exhibiting a monomorphic genotype for the LEAFY gene. Three Wisconsin accessions of R. japonica were each characterized by having three distinct kinds of LEAFY sequence. Most plants in our study were found to possess two or three phylogenetically distinct copies of the LEAFY gene, with the copies being most closely related to R. japonica and R. sachalinensis, respectively, and these were inferred to be interspecific hybrids. Altogether, five kinds of interspecific hybrids were identified, reflecting various combinations of LEAFY sequence types from the parental species. The widespread existence of hybrid plants in Wisconsin, many of which are morphologically identifiable as R. japonica, indicates a cryptic genetic diversity that should be examined more broadly in North America using molecular tools.