A taxonomic, cytological and genetic survey of Japanese knotweed s.l. in New Zealand indicates multiple secondary introductions from Europe and a direct introduction from Japan

Japanese knotweed sensu lato is a group of invasive plants, including Reynoutria japonica (var. japonica, var. compacta), R. sachalinensis, R. × bohemica, F2s and any backcrosses. Outside of its native East Asia, Japanese knotweed s.l. is an intractable weed, spreading by massive clonal reproduction, and causing significant ecological damage and economic cost. Japanese knotweed s.l. has been naturalised in New Zealand for at least a century and it is now locally abundant, particularly on the West Coast of the South Island, and poses a substantial threat, with large parts of the country being susceptible to colonisation. However, little is known of the taxonomic, cytological and genetic diversity of the clones present in New Zealand, nor of the biogeographical origin of their source populations. In the current study, Japanese knotweed s.l. was sampled from New Zealand, and each specimen characterised in detail using morphological characters, diagnostic microsatellite markers, somatic chromosome counts and PCR-RFLP on three chloroplast regions to create multiprimer haplotypes (MPHs), all of which were compared with pre-existing global databases. On the North Island, Japanese knotweed s.l. is exclusively represented by the notorious female, octoploid var. japonica clone, which is widespread in Europe. While the South Island is more diverse, in addition to R. japonica var. japonica, there is a tetraploid var. compacta clone and several R. × bohemica clones (tetraploid & hexaploid). The majority of these are also of European stock and represent further secondary introductions from the existing adventive range. However, there is also evidence for tetraploid Japanese R. × bohemica clones growing on the West Coast that have never been recorded in Europe, or elsewhere in the adventive range, which represent independent introductions from the native range, possibly arriving with Asian workers in the New Zealand gold rush of the nineteenth century.