SEXUAL REPRODUCTION AND SYSTEMATICS OF PLACONEIS (BACILLARIOPHYTA)

Sexual reproduction and protoplast structure are used to check the cla ssification of the naviculoid genus Placoneis Mereschkowsky (Bacillari ophyta). The gametangia are closely associated within a robust mucilag e capsule throughout auxosporulation, and pairing is +/-random with re spect to asymmetries of the frustule and protoplast. Two gametes are p roduced per gametangium, which are morphologically identical but diffe rentiated into active and passive (physiological anisogamy). The super numerary nuclei from meiosis II begin to degenerate soon after telopha se II, but can still be detected in the initial cells. The gametes bec ome rearranged within the gametangia after meiosis II and plasmogamy t akes place via a single, simple aperture. The auxospores expand parall el to the apical axes of the gametangia. The classification of Placone is in the Cymbellales, as the sister group to the cymbelloid and gomph onemoid diatoms, is supported by valve and protoplast structure, the m ethod of chloroplast division, and by sexual reproduction and auxospor e development. Placoneis seems to occupy an intermediate position in t he Cymbellales between the cymbelloid-gomphonemoid cluster and more an cient lineages that include Anomoeoneis Pfitzer, Dickieia Berkeley ex Kutzing, Rhoicosphenia Grunow and Campylopyxis Medlin. No features can be found that are common to Placoneis and Navicula Bory (from which P laconeis was separated) except for the bilateral symmetry of the valve and the central raphe system, which would be synapomorphies only at a much higher taxonomic level.