MOLECULAR SYSTEMATICS AND TAXONOMY OF LAKE CRESS (NEOBECKIA-AQUATICA,BRASSICACEAE), AN IMPERILED AQUATIC MUSTARD

The North American lake cress, Neobeckia aquatica (Eaten) Greene, is a rare, aquatic member of the mustard family, Brassicaceae. The phyloge netic position and taxonomic status of this species have been uncertai n due to the considerable overlap of morphological characters that exi sts among the five genera of aquatic cresses: Armoracia, Cardamine, Na sturtium, Neobeckia, and Rorippa. In this study, DNA sequence data fro m the chloroplast gene rbcL were compared for these aquatic cress gene ra and were used to construct a phylogenetic tree depicting their evol utionary interrelationships. Molecular data support a natural relation ship of the aquatic cresses, but specifically indicate that Neobeckia is more closely related to Rorippa than it is to Armoracia. This analy sis indicates that the incomplete fruit septa of Neobeckia and Armorac ia are parallelisms which should not be used to justify the merger of these genera. DNA sequence data also indicate a close relationship bet ween Cardamine and Nasturtium, and their distinctness from the other a quatic cress genera. A relatively high level of DNA sequence divergenc e separates Neobeckia from all other aquatic cress genera. Neobeckia a nd Rorippa share similar vegetative and seed features, but differ by h abit, petal color, and fruit characteristics. The taxonomic decision i s made to retain Neobeckia as a monotypic genus rather than to merge i t with Rorippa.