Meiosis, nuclear cyclosis, and auxospore formation in Navicula sensu stricto (Bacillariophyta)

The planktonic diatom Fragilariopsis kerguelensis plays an important role in the biogeochemical cycles of the Southern Ocean, where remains of its frustules form the largest deposit of biogenic silica anywhere in the world. We assessed the genetic identity of 26 strains, from cells collected at various sites in the Southern Ocean, using three molecular markers, LSU and ITS rDNA and rbcL. The LSU sequences were identical among the tested strains, ITS sequences were highly similar, and only one base pair difference was detected among the rbcL sequences. These results, together with a large number of successful mating experiments demonstrated that the strains belong to a single biological species. We investigated the mating system and life cycle traits of F. kerguelensis. Cell size diminished gradually in clonal strains. Gamete formation only occurred when strains of opposite mating type -within a cell size range of 7-36 µm -were mixed together. Two binucleate gametes were formed in each gametangium and gamete conjugation produced a zygote that had four nuclei and was surrounded by thin siliceous scales. Two out of the four nuclei subsequently degenerated and the zygote expanded to form an auxospore surrounded by a transverse and a longitudinal perizonium. Staining with the fluorochrome PDMPO provided for the first time a clear demonstration that the longitudinal perizonium is formed after auxospore expansion is complete. Initial cells produced within the mature auxospores were 78-101 µm in length. Various authors have shown that the average valve size of F. kerguelensis varies in sediment samples collected in regions and seasons with different primary production regimes and this parameter has thus been proposed as a biological proxy for palaeoproductivity. A better understanding of the life cycle of F. kerguelensis should help the design of future investigations aimed at testing the link between cell size distribution in the natural environment and the role that environmental factors might have in the regulation of population cell size.

Section: Transition From the Vegetative To The Sexual Phase And Formamentioning

confidence: 86%

Genetic characterization and life cycle of the diatomFragilariopsis kerguelensis

Fuchs

Scalco

Kooistra

et al. 2013

“…In addition, the epitheca contains several narrow girdle bands, each bearing one or two rows of tiny round poroids (Hanic & Lobban, 1979;Cox, 1985), whereas in Navicula there are fewer bands and the first band is not only much wider than the others but non-porous (Round et al, 1990). The observations reported here show, moreover, that in N. ulvacea the gametangia are not aligned and secrete a large, robust capsule of mucilage and the superfluous nuclei quickly become pycnotic after meiosis II, again unlike Navicula (Mann & Stickle, 1989).…”

Section: Discussionmentioning

confidence: 87%

“…51). There is no sign of extra structures at the ends of the expanding auxospores (Figs 49-51), in contrast, for example, to Navicula oblonga (Ktitzing) Kfitzing (Mann & Stickle, 1989), where prominent caps of organic material are present, representing the ruptured wall of the zygote. There is no fixed relationship between the orientations of auxospores and gametangia (Figs 27,49,52).…”

Section: Allogamous Auxospore Formationmentioning

confidence: 99%

“…12, lower cell) and subsequent dispersal of the paired homologues following pachytene ( Fig. 11) occurs as in Cymbella (Geitler, 1927a) and Navicula (Mann & Stickle, 1989). While the synizetic knot is moving around within the nucleus, structures on the outer surface of the nuclear envelope are also translocated, suggesting that the movements of both may be generated in the envelope itself.…”

Section: Allogamous Auxospore Formationmentioning

confidence: 99%

See 1 more Smart Citation

Auxospore formation, reproductive plasticity and cell structure inNavicula ulvaceaand the resurrection of the genusDickieia(Bacillariophyta)

Mann

1994

Self Cite

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 forraation 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.

“…enlargement of the nucleus (apparently corresponding to prophase of meiosis I, by analogy with other raphid pennate diatoms, e.g. Navicula: Mann & Stickle, 1989), translocation of the nucleus towards the epivalve, and simultaneous displacement of the chloroplasts (which by then had divided) towards the hypovalve (Figs 29, 30).…”

Section: Sexual Reproduction and Auxospore Formationmentioning

confidence: 99%

Congruence of morphological, reproductive and ITS rDNA sequence data in some AustralasianEunotia bilunaris(Bacillariophyta)

Vanormelingen¹,

Chepurnov²,

Mann³

et al. 2007

Self Cite

Clones identified as Eunotia bilunaris were isolated from two freshwater habitats in New Zealand and one in Tasmania. Experimental studies of sexual reproduction showed that the mating system was strictly heterothallic. All clones were morphologically similar and, in mixtures of compatible clones, were able to produce a viable F 1 , regardless of geographical origin. There was nevertheless some geographically related variation in ITS rDNA sequences, suggesting that dispersal and concerted evolution are unable to prevent allopatric divergence. Isolates from two of the three localities also showed significant ITS polymorphisms. Auxosporulation followed a similar pattern to that in other Eunotia species, being isogamous with a single functional gamete per gametangium; plasmogamy was effected via two fusing papillae. Details of auxospore structure, initial cell size, and vegetative valve metrics (width, striation densities) show that conspecificity with a recently studied European E. bilunaris is doubtful. The first SEM observations of the perizonium of Eunotia are presented and show that the central transverse band is apparently a split ring as in araphid pennate diatoms, in contrast to other raphid diatoms, except some Bacillariaceae.