Molecular systematics of Cardamine and allied genera (Brassicaceae): ITS and non-coding chloroplast DNA

Representatives of the genera Cardamine, Dentaria, Nasturtium, Rorippa and Armoracia (Brassicaceae) were analyzed to elucidate their phylogenetic relationships based on nuclear (ITS) and non-coding chloroplast (cp) DNA sequences. Dentaria seems to be polyphyletic. The two studied Dentaria species group with different Cardamine clades, and it is argued that D. bulbifera is an allopolyploid originating from a hybridization between a Cardamine and a Dentaria species. In the ITS tree, Nasturtium and Rorippa form well supported clades but their relationship to Cardamine and Armoracia remains unresolved. In the cpDNA tree, Nasturtium groups together with Cardamine. Hybridization events apparently played a role in the evolution of Nasturtium. The Cardamine/Nasturtium clade is separated from a clade placing Rorippa and Armoracia together. Armoracia is closely related to Rorippa. Analyses of the 19 Cardamine species studied revealed three main groupings, a northern hemispheric and two southern hemispheric groups. Within the northern hemisphere taxa the C. pratensis complex forms a well supported clade which seems to be closely related to C. amara, C. raphanifolia and C. flexuosa. The positions of C. hirsuta and C. impatiens are uncertain. The two southern hemisphere clades consist of New Guinean species and south-eastern Australian/Tasmanian and subantarctic species, respectively. They may reflect migration routes from the northern to the southern hemisphere, but further studies are necessary to fully understand the evolution of the bihemispheric distribution pattern of Cardamine.