Phylogenetic position of Toxarium, a pennate-like lineage within centric diatoms (Bacillariophyceae)

The diatom genus Toxarium Bailey has been treated as a pennate because of its elongate shape and benthic lifestyle (it grows attached to solid substrata in the marine sublittoral). Yet its valve face lacks all structures that would ally it with the pennates, such as apical labiate processes, a midrib (sternum) subtending secondary ribs and rows of pores extending perpendicularly out from the mid-rib, or a raphe system. Instead, pores are scattered irregularly over the valve face and only form two distinct rows along the perimeter of the valve face. In our nuclear small subunit rDNA phylogenies, Toxarium groups with bi- and multipolar centrics, as sister to Lampriscus A. Schmidt. Thus, the genus acquired a pennate-like shape and lifestyle independently from that of the true pennates. The two species known, T. hennedyanum Grunow and T. undulatum Bailey, differ only in a single feature: the valve perimeter of the former shows only a central expansion, whereas that of the latter possesses in addition a regular undulation. Yet both forms were observed in our monoclonal cultures, indicating that the two taxa represent extremes in a plasticity range. Toxarium resembles another elongate and supposedly araphid diatom, Ardissonea De Notaris, in being motile. Cells can move at speeds of up to 4 μm·s-1 through secretion of mucilage from the cell poles or they remain stationary for longer periods, when they form short polysaccharide stalks. Division during longer periods of quiescence leads to the formation of small colonies of linked or radiating cells.