Molecular Phylogeny Determined Using Chloroplast DNA Inferred a New Phylogenetic Relationship of Rorippa aquatica (Eaton) EJ Palmer & Steyermark (Brassicaceae)—Lake Cress

Abstract

North American lake cress, Rorippa aquatica (Eaton) EJ Palmer & Steyermark (Brassicaceae), is listed as an endangered or threatened species. Lake cress shows heterophyllic changes in leaf form in response to the surrounding environment. Therefore, this species has received considerable attention from ecological and morphological perspectives. However, its phylogenetic position and taxonomic status have long been a subject of debate. To analyze the phylogenetic relationship of lake cress, we investigated chloroplast DNA sequences from 17 plant species. The results of phylogenetic reconstruction performed using trnL intron, trnG (GCC)-trnM (CAU), and psbC-trnS (UGA) indicated that lake cress is a member of Rorippa. Moreover, we found that the chromosome number of lake cress is 2n = 30. This result indicated that lake cress might have originated from aneuploidy of triploid species or via intergeneric crossing. Taken together, our results suggest an affinity between lake cress and Rorippa at the molecular level, indicating that lake cress should be treated as Rorippa aquatica (Eaton) EJ Palmer & Steyermark.