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. subinfla ta and N. expecta. Dickieia cells contain one complex chloroplast per cell; this consists of two 'K'-shaped halves, linked by an isthmus con taining the single discoid pyrenoid. The pyrenoid lies near one side o f the girdle at the centre, while the nucleus lies opposite. Neither t he nucleus nor the chloroplast move significantly within the cell duri ng the cell cycle. The valves contain round poroids with hymenate occl usions. The raphe-sternum lacks accessory ribs and the centra 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 principa lly through expansion of the gametes until they meet and fuse. The ori entation 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. Dickie ia belongs within the Cymbellales and is closely related to the Rhoico spheniaceae and Anomoeoneidaceae.