Evolutionary studies in subtribe Reynoutriineae (Polygonaceae). With contributions to the study of hybridisation in Helosciadium and Berula (Apiaceae) included as appendices.

Subtribe Reynoutriineae is a monophyletic group within the Polygonaceae, containing three genera: Reynoutria, Muehlenbeckia and Fallopia. The clade is strongly supported by molecular data and is characterised by the presence of a nectariferous gland at the base of leaf petioles, which secretes a sweet, sugary exudate. In this thesis the evolutionary relationships within the subtribe were investigated, and a revised classification is presented that amalgamates all three genera under Fallopia, which has priority. This treatment is supported by phylogenetic analysis, and the detection of intergeneric hybridisation between Reynoutria and Muehlenbeckia. A phylogenetic analysis of molecular sequence data from six gene regions (two nuclear & four plastid) was conducted with the widest sampling of ingroup taxa for this clade to date, in particular being the first to include species from Fallopia sect. Parogonum. The results of this analysis revealed four main clades within the subtribe: a Reynoutria clade, a Fallopia sect. Parogonum clade, a Fallopia s.s. clade (including Fallopia sects. Fallopia & Sarmentosae), and a Muehlenbeckia clade. The Fallopia s.s. and Muehlenbeckia clades are sister to one another, while the Fallopia sect. Parogonum clade is immediately basal. Fallopia, as currently circumscribed, appears to be paraphyletic as species of Muehlenbeckia are nested within it. The parentage of putative hybrids collected as open-pollinated seed from Japanese knotweed s.l. (Reynoutria) in New Zealand, where Muehlenbeckia was the nearest identifiable pollen source, was investigated using: chromosome counts, fluorescent in situ hybridisation with labelled total genomic probes (GISH), and sequence data from three gene regions, one plastid (matK) and two nuclear (the ITS & LEAFYi2). The hybrids were found to be the result of intergeneric hybridisation between Reynoutria and Muehlenbeckia, which supports the amalgamation of the genera. The diversity and geographical origin of Japanese knotweed s.l. in Australasia was also investigated, using: morphology, chromosome counts and PCR-RFLP derived chloroplast haplotypes. The majority of clones match those found in Europe, but there is evidence for an independent introduction from Japan.