Auxospore formation, reproductive plasticity and cell structure in Navicula ulvacea and the resurrection of the genus Dickieia (Bacillariophyta)
The genus Dickieia is re-established for Navicula ulvacea, N. subinflata and N. expecta. Dickieia cells contain one complex chloroplast per cell; this consists of two K’-shaped halves, linked by an isthmus containing the single discoid pyrenoid. The pyrenoid lies near one side of the girdle at the centre, while the nucleus lies opposite. Neither the nucleus nor the chloroplast move significantly within the cell during the cell cycle. The valves contain round poroids with hymenate occlusions. The raphe-sternum lacks accessory ribs and the central internal raphe endings are simple and straight. The epitheca contains several narrow, porous girdle bands. Auxosporulation is unusually variable in D. ulvacea and can be allogamous (with isogamy) or automictic, though the gametangia are always paired. Plasmogamy is brought about principally through expansion of the gametes until they meet and fuse. The orientation of the auxospores bears no fixed relation to the gametangia. Haploid parthenogenesis and the formation of polyploid auxospores and double’ gametes are reported and their significance discussed. Dickieia belongs within the Cymbellales and is closely related to the Rhoicospheniaceae and Anomoeoneidaceae.