Notable morphologies, molecular phylogenetics and taxonomy of the dinophyte Borghiella tenuissima (Borghiellaceae, Phytodiniales)

Mitotic division in dinophytes takes place either as desmoschisis or eleutheroschisis and usually, only one type is found in a species or group. Due to its big size, Borghiella tenuissima (Borghiellaceae) is a morphologically distinctive winter species. We collected plankton material in Germany and Poland, established unialgal strains and studied the morphology using light as well as scanning electron microscopy. We further obtained rRNA gene sequence data to embed them in molecular phylogenetics. Sequence data identified the strains as elements of the Borghiellaceae and showed no divergence among each other and to those of already published strains. The large flagellated cells had the characteristic, dorso-ventral flattened morphology of B. tenuissima, but many of them showed an undulated right margin of the hyposome. Contemporary material of B. tenuissima is remarkably smaller than stated in the protologue and other historical reports, which is why we have provisionally determined the studied material as B. cf. tenuissima. Moreover, B. cf. tenuissima exhibit different types of mitotic division (i.e., in the flagellated stage known as desmoschisis as well as in a deflagellated stage known as eleutheroschisis) at the same time, which might be more abundant among species of Borghiella than recognised before.