Phylogeny of five species of Nusuttodinium gen. nov. (Dinophyceae), a genus of unarmoured kleptoplastidic dinoflagellates.

Cells of five unarmoured kleptoplastidic dinoflagellates, Amphidinium latum, Amphidinium poecilochroum, Gymnodinium amphidinioides, Gymnodinium acidotum and Gymnodinium aeruginosum were observed under light and/or scanning electron microscopy and subjected to single-cell PCR. The SSU rDNA and the partial LSU rDNA of all the examined species were sequenced, and the SSU rDNA of G. myriopyrenoides was sequenced. Phylogenetic analyses revealed that the unarmoured kleptoplastidic species formed a monophyletic clade within the Gymnodinium-clade sensu Daugbjerg et al. (2000). The sister taxa for this clade were Gymnodinium palustre and Spiniferodinium galeiforme, both of which possess brown-coloured chloroplasts. The results indicated that acquisition of kleptoplastidy in these unarmoured dinoflagellates was a single event and that these unarmoured kleptoplastidic dinoflagellates may have evolved from a form with permanent chloroplasts. Molecular trees suggested that the acquisition of kleptoplastidy took place in a marine habitat and later some species colonized the freshwater habitat. Because these unarmoured kleptoplastidic dinoflagellates are monophyletic and characterized by distinct morphological and cytological features (including the presence of the same type of apical groove, absence of nuclear chambers in the nuclear envelope, absence of genuine chloroplasts, and the possession of kleptochloroplasts), we propose the establishment of a new genus, Nusuttodinium, to accommodate all these dinoflagellates.