Growth of dinoflagellates,Ceratium furca and Ceratium fusus in Sagami Bay,Japan: The role of vertical migration and cell division

To better understand the mechanism underlying the bloom outbreaks of dinoflagellates, Ceratium furca, and Ceratium fusus in the temperate coastal area of Sagami Bay, we investigated the diel changes of vertical migration, swimming speed, cell volume, and cell division. Our results from both the field and laboratory indicate that C. furca and C. fusus can migrate vertically between surface and sub-surface layers to avoid strong sunlight (>1000 μmol m⁻² s⁻¹). Diel vertical migration (DVM) of C. furca was observed in the laboratory, while that of C. fusus was not observed. C. furca demonstrated a constant DVM rhythm, i.e., their cells began to descend from the surface before the light was extinguished, and ascended into the surface before the light was turned on. The downward and upward migrations of the cells occurred at every 3 h before turning on and off the light, suggesting that the DVM pattern was independent of nutrient concentration. The swimming speeds of C. furca (avg. 250 μm s⁻¹) were always faster than those of C. fusus (avg. 75 μm s⁻¹). In addition, the speeds of C. furca during light periods were faster than those during dark periods, whereas the speeds of C. fusus remained relatively constant. A higher proportion of dividing cells was recorded near dawn (05:00–07:00 h). Cell volumes of C. furca and C. fusus did not markedly change between 12:00 and 21:00 h, but gradually increased until 03:00 h and then sharply decreased. Furthermore, the cell volume of the two Ceratium species was significantly shifted to the temporal pattern of cell division. Combined with the DVM manner of two Ceratium and cell division timing, only C. furca divided at the bottom, and then moved toward the surface shortly before the dark to light transition. Based on our observations, C. furca has an ecological advantage due to their DVM activity, since nutrients can be obtained well in the near bottom layers, while during the daytime, light present in nutrient-depleted surface water can be obtained using their high swimming speed. On the other hand, C. fusus stimulated by low salinity conditions, might be dependent on external environmental conditions such as additional nutrients following freshwater discharge by heavy rainfall because they may not perform active DVM due to a slow swimming ability. Our findings support that specific characteristics, including the DVM behavior in C. furca, yield a competitive advantage over C. fusus in Sagami Bay.     

Cell division in leaves ofNicotiana

Summary Young leaves ofNicotiana tabacum were fixed in glutaraldehyde-formaldehyde followed by osmium tetroxide. The fine structure of dividing cells was studied. Before prophase a band of microtubules was observed between the nucleus and the cell wall at a position judged as the future plane of division. The microtubules in the band are 4–6 units deep and relatively closely packed, giving sections of the band a characteristic appearance. Micro-tubules of the mitotic spindle, the phragmoplast, and the preprophase band are morphologically similar. Some of the microtubules of the mitotic spindle and the phragmoplast have an undulate appearance. It is suggested that the undulate microtubules may have been fixed at a time when microwaves were traveling along them. The cell plate is formed by a fusion of small smooth surfaced vesicles and small coated vesicles. Fusion of small vesicles results first in larger vesicles and then in a meshwork of new cell-wall material surrounded by new regions of plasma membrane. Most of the vesicles are derived from dictyosomes and may be produced before and during prophase as well as during later stages of division. The ER may also contribute some vesicles to the cell plate.     

Cell division orientation in animals

Cell division orientation during animal development can serve to correctly organize and shape tissues, create cellular diversity or both. The underlying cellular mechanism is regulated spindle orientation. Depending on the developmental context, extrinsic signals or intrinsic cues control the correct orientation of the mitotic spindle. Cell geometry has been known to be another determinant of spindle orientation and recent results have shed new light?on the link between cellular shape and cell division orientation. The importance of controlling spindle orientation is manifested in neurodevelopmental defects such as?microcephaly, tumor initiation as well as defects in tissue architecture and cell fate misspecification. Here, we summarize the role of oriented cell division during animal development and also outline the cellular and molecular mechanisms in selected invertebrate and vertebrate systems.     

Cell division in caffeine-induced binucleate protonemal cells ofAdiantum. I. Asynchronous mitosis of two nuclei in a single cell

Coordination of karyokinesis of two nuclei in individual filamentous binucleate cells of the fern,Adiantum capillus-veneris was investigated. To induce binucleate cells, the protonemata were treated with caffeine, which is known as an inhibitor of plant cytokinesis, during the first synchronous division of cells that was induced by blue light (BL). The next synchronous division of cells in the resultant binucleate cells was analysed. In most cases, the two nuclei were associated with each other and were located in the apical region of the long protonemal cells (approximately 400–600 μm in length, 20 μm in width). In some cells, one nucleus was located in the apical region and the other was located in the middle of the cylinderical region. In such cells, karyokinesis of the apical nucleus preceded that of the basal nucleus, even though karyokinesis of associated nuclei progressed synchronously. Mitotic binucleate cells were centrifuged in order to gather two dissociated heterophasic nuclei. Progression of karyokinesis in the re-associated nuclei became coordinated within 1 h in most cells. These results suggest that mitosis-regulating factor(s) may diffuse to only limited distances inAdiantum protonemata.     

Cell shape and division in Escherichia coli: experiments with shape and division mutants.

Double mutants which carry mutations in genes (rodA, pbpA) required for cell elongation (i.e., maintenance of rod shape) in combination with mutations in genes (ftsA, ftsI, ftsQ, or ftsZ) required for septation were constructed. Such mutants were able to grow for about two mass doublings at a normal rate at the restrictive temperature (42 degrees C). The morphology of the cells formed under these conditions was interpreted by assuming the existence of a generalized system for peptidoglycan growth together with two additional systems which modify the shape of the growing peptidoglycan layer. The results also showed that different fts genes probably control different stages in septation. ftsZ (sulB or sfiB) appears to be required for the earliest step in septation, ftsQ and ftsI (pbpB or sep) are required for a later step or steps, and ftsA is required only for the latest stages in septation.