Molecular phylogeny and morphology of the marine diatom Talaroneis posidoniae gen. et sp. nov. (Bacillariophyta) advocate the return of the Plagiogrammaceae to the pennate diatoms

A new genus, Talaroneis Kooistra & De Stefano, gen. nov. and a new species T. posidoniae Kooistra & De Stefano, sp. nov. are described. The elongate cells, which were found on a leaf of Posidonia oceanica, formed star-, zigzag- or ribbon-shaped colonies in culture. The distinctive character of T. posidoniae was the presence of two silica wings flanking a furrow near each apical pore field. Uniseriate, transverse striae were arranged perpendicular to the apical axis; those on one side of the valve offset with respect to those on the other side. Valve poroids were occluded by perforated rotae. These morphological characteristics placed the taxon unambiguously within the supposedly centric family Plagiogrammaceae. The taxon would be excluded from the morphologically similar araphid family Rhaphoneidaceae because it lacked apical labiate processes and a clearly defined sternum. Nonetheless, a phylogeny inferred from nuclear-encoded SSU ribosomal DNA sequences of a range of diatoms unambiguously recovered T. posidoniae within the araphid pennates, close to Rhaphoneis cf. belgica (Rhaphoneidaceae). Apparently, labiate processes were lost secondarily in T. posidoniae and possibly in the common ancestor of all Plagiogrammaceae. Valve ultrastructural features of T. posidoniae would allow placement of the new species in Dimeregrammopsis but this genus was invalidly described. We transferred the single species of this genus to T. furcigerum (Grunow) Sterrenburg, comb. nov. and designated it as the type species of Talaroneis.