The marine pennate diatom Phaeodactylum tricornutum has become a model for diatom biology, due to its ease of culture and accessibility to reverse genetics approaches. While several features underlying the molecular mechanisms of cell division have been described, morphological analyses are less advanced than they are in other diatoms. We therefore examined cell ultrastructure changes prior to and during cytokinesis. Following chloroplast division, cleavage furrows are formed at both longitudinal ends of the cell and are accompanied by significant vesicle transport. Although neither spindle nor microtubules were observed, the nucleus appeared to be split by the furrow after duplication of the Golgi apparatus. Finally, centripetal cytokinesis was completed by fusion of the furrows. Additionally, F-actin formed a ring structure and its diameter became smaller, accompanying the ingrowing furrows. To further analyse vesicular transport during cytokinesis, we generated transgenic cells expressing yellow fluorescent protein (YFP) fusions with putative diatom orthologs of small GTPase Sec4 and t-SNARE protein SyntaxinA. Time-lapse observations revealed that SyntaxinA-YFP localization expands from both cell tips toward the center, whereas Sec4-YFP was found in the Golgi and subsequently relocalizes to the future division plane. This work provides fundamental new information about cell replication processes in P. tricornutum.

There is increasing evidence that cryptic and pseudo-cryptic species are widespread in several diatom taxa. This is the case with the genus Pseudo-nitzschia, which includes chain-forming pennate diatoms that bloom both in coastal and open ocean waters. In the last years, up to 10 new species have been described on the basis of morphological and molecular investigations, and some of them are only distinguishable on the basis of minute ultrastructural features of their siliceous frustule. We here describe the morphology of the vegetative cell and sexual stages of Pseudo-nitzschia mannii sp. nov., and present an ITS2-based phylogeny to illustrate genetic relationships between P. mannii sp. nov. and the closest congeneric species. On the basis of ribosomal and chloroplastidial gene phylogenies, P. mannii is closely related to P. calliantha, from which it can be differentiated by its slightly wider cells and a different arrangement of the poroid sectors. The analysed strains did not produce the ASP-to...