Symmetry and sex in Bacillariaceae (Bacillariophyta), with descriptions of three new <i>Nitzschia</i> species

The large diatom genus <i>Nitzschia</i> and the morphologically similar <i>Hantzschia</i> are currently distinguished on the basis of cell symmetry and division. In <i>Hantzschia</i>, the eccentrically placed raphe systems of the two valves are always on the same side of the frustule (‘hantzschioid’ symmetry), never diagonally opposite (‘nitzschioid’ symmetry). Although some <i>Nitzschia</i> species produce hantzschioid cells, it has previously been thought that these cells never breed true but divide to give one nitzschioid daughter and one hantzschioid daughter. Sublittoral marine epipelon from Loch Goil, W Scotland, contained three undescribed species of Bacillariaceae whose cells were always hantzschioid, but which possessed a valve structure (silica flaps on either side of the raphe, no central raphe endings, a slit-like entrance to the raphe canal that narrows towards the poles, and discrete bar-like fibulae) linking them to <i>N. spathulata</i> and the type species of <i>Nitzschia, N. sigmoidea</i>. They also agreed with these species in chloroplast morphology and auxosporulation and are therefore assigned to <i>Nitzschia</i> as <i>N. dicrogramma, N. brachygramma</i> and <i>N. parkii, spp. nov</i>. Consequently the only type of cell division found in all <i>Hantzschia</i> species (in which a hantzschioid parent cell divides to give two hantzschioid daughters) is not unique to <i>Hantzschia</i> but also occurs in at least one lineage of <i>Nitzschia</i>. ‘Core’ <i>Hantzschia</i> species may nevertheless comprise a monophyletic group, characterized by an unusual kind of internal central raphe endings. Four symmetry categories are now known in <i>Hantzschia</i> and <i>Nitzschia</i>: (1) all cells hantzschioid; (2) hantzschioid and nitzschioid cells in a 2 : 1 ratio; (3) hantzschioid and nitzschioid cells in a 1 : 1 ratio; and (4) all cells nitzschioid. The division pattern seems to be constant within species and may therefore be useful as a taxonomic character above the species level. <i>Hantzschia segmentalis</i> is given its first written description.