Morphological, biochemical and molecular evidence for the transfer of Gyrosigma nipkowii Meister to the genus Haslea (Bacillariophyta)

The diatom genus Haslea Simonsen was erected to accommodate spindle-shaped valves with acute apices, convex margins, straight raphe and approximate central pores, inconspicuous areas, and transapical striae crossed at a right angle by a longitudinal pattern with a slit apically oriented in each areola. Investigations on mudflats of the French Atlantic coast revealed the presence of a sigmoid diatom with characteristics of both Gyrosigma Hassall and Haslea. Following a thorough morphological study by light and scanning electron microscopy in addition to biochemical and molecular analyses, we propose the transfer of Gyrosigma nipkowii Meister to Haslea nipkowii (Meister) Poulin & Masse, comb. nov. Despite a deviant sigmoidicity, this taxon shows diagnostic features of Haslea, with longitudinal slits running externally from one pole to the other, all merging with a peripheral slit at the apex, and the presence of small areolae beyond the helictoglossa. Unique to H. nipkowii are the unequal T-shaped distal raphe fissures, the overlapping and deflected proximal raphe ends, and the overhanging axial costa on the primary side of the valve. This taxon synthesizes C-25 highly branched isoprenoids (HBIs) typical of the genus Haslea, with a double bond always present between C6 and C5. Phylogenetic analyses of the plastid-encoded 16S ribosomal gene for seven species of Haslea and two sigmoid taxa established the monophyly of the genus Haslea including H. nipkowii.