Molecular systematics and taxonomy of lake cress ( Neobeckia aquatica; Brassicaceae), an imperiled aquatic mustard

Abstract

The North American lake cress Neobeckia aquatica (eaton) Greene, is rare, aquatic member of the mustard family, Brassicaceae. The phylogenetic position and taxonomic status of this species have been uncertain due to the considerable overlap of morphological characters that exists among the five genera of aquatic cresses: Armoracia, Cardamine, Nasturtium, Neobeckia, and Rorippa. In this study, DNA sequence data from the chloroplast gene rbcL were compared for these aquatic cress genera and were used to construct a phylogenetic tree depicting their evolutionary interrelationships. Molecular data support a natural relationship of the aquatic cresses, but specifically indicate that Neobeckia is more closely related to Rorippa than it is to Armoracia. This analysis indicates that the incomplete fruit septa of Neobeckia and Armoracia are parallelisms which should not be used to justify the merger of these genera. DNA sequence data also indicate a close realtionship between Cardamine and Nasturtium, and their distinctness from the other aquatic cress genera. A relatively high level of DNA sequence divergence separates Neobeckia from all other aquatic cress genera. Neobeckia and Rorippa share similar vegetative and seed features, but differ by habit, petal color, and fruit characteristics. The taxonomic decision is made to retain Neobeckia as a monotypic genus rather than to merge it with Rorippa.