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The number of extant species of diatoms is estimated here to be at least 30,000 and probably ca. 100,000, by extrapolation from an eclectic sample of genera and species complexes. Available data, although few, indicate that the pseudocryptic species being discovered in many genera are not functionally equivalent. Molecular sequence data show that some diatom species are ubiquitously dispersed. A good case can be made that at least some diatom species and even a few genera are endemics, but many such claims are still weak. The combination of very large species numbers and relatively rapid dispersal in diatoms is inconsistent with some versions of the ubiquity hypothesis of protist biogeography, and appears paradoxical. However, population genetic data indicate geographical structure in all the (few) marine and freshwater species that have been examined in detail, sometimes over distances of a few tens of kilometres. The mode of speciation may often be parapatric, in the context of a constantly shifting mosaic of temporarily isolated (meta) populations, but if our intermediate dispersal hypothesis is true (that long-distance dispersal is rare, but not extremely rare), allopatric speciation could also be maximized.

Diatoms are a species-rich group of photosynthetic eukaryotes, with enormous ecological significance and great potential for biotechnology. During the last decade, diatoms have begun to be studied intensively using modern molecular techniques and the genomes of four diatoms have been wholly or partially sequenced. Although new insights into the biology and evolution of diatoms are accumulating rapidly due to the availability of reverse genetic tools, the full potential of these molecular biological approaches can only be fully realized if experimental control of sexual crosses becomes firmly established and widely accessible to experimental biologists. Here we discuss the issue of choosing new models for diatom research, by taking into account the broader context of diatom mating systems and the place of sex in relation to the intricate cycle of cell size reduction and restitution that is characteristic of most diatoms. We illustrate the results of our efforts to select and develop experimental systems in diatoms, using species with typical life cycle attributes, which could be used as future model organisms to complement existing ones

Thalassiosira species are common components of marine planktonic communities worldwide and are used intensively as model experimental organisms. However, data on life cycles and sexuality within the genus are fragmentary. A clone of the cosmopolitan marine diatom Thalassiosira punctigera Cleve emend. Hasle was isolated from the North Sea and oogamous sexual reproduction was observed in culture. Cells approximately 45 µm and smaller became sexualized. Oogonia were produced preferentially and spermatogenesis was infrequent. Unfertilized oogonia always aborted and their development was apparently arrested at prophase of meiosis I. Further progression through meiosis and auxospore formation occurred only after a sperm had penetrated into the oocyte. Many cells of the new large-celled generation (approximately 90–120 µm in size) immediately became sexualized again but only oogonia were produced. A few of the large oogonia became auxospores and produced initial cells 132–153 µm in diameter. The second step of auxosporulation probably involved fertilization of large-celled oocytes by the sperm of the small-celled spermatogonangia that were still present in the culture. An F1 clone obtained after selfing within the small-celled auxosporulation size range was investigated. Like the parent clone, the F1 clone was homothallic but no auxosporulation was observed: spermatogonangia were unable to produce viable sperm, apparently because of inbreeding depression. Aggregation and interaction of oogonia were documented, and may be relevant for understanding the mechanisms of signaling and recognition between sexualized cells and the evolution of sexuality in pennate diatoms.

The availability of extensive experimental data and remarkable intra- and interspecific variation in breeding behaviour make Achnanthes Bory sensu stricto an especially good model for studying the reproductive and population biology of pennate diatoms. In most Achnanthes species studied, auxospore formation is accompanied by biparental sexual reproduction, but we found uniparental auxosporulation in Achnanthes cf. subsessilis. Auxosporulation appears to be apomictic and follows contraction of the contents of unpaired cells and then a mitotic division, which is normally acytokinetic: one nucleus aborts before the cell develops into an auxospore. Rarely, both daughter nuclei survive and cytokinesis produces two auxospores (two auxospores per mother cell is highly unusual in pennate diatoms); one may abort. Expansion of auxospores is not accompanied by deposition of a transverse perizonium, but a longitudinal perizonium is produced and consists of a wide central strip (structurally similar to the araphid valve) and at least one narrow lateral strip. This newly discovered asexual lineage in Achnanthes is discussed in relation to other reproductive systems found in the genus, and also in relation to the 'sex clock' hypothesis concerning the adaptive significance of the diatom life cycle. Brief information on chloroplast division and nuclear dynamics over the cell cycle is also presented.

Chloroplast, mating behaviour, and auxosporulation are described for Amphora cf. proteus Gregory. The interphase chloroplast is butterfly-shaped and lies with its central part, containing a flat discoid pyrenoid, pressed against the ventral side of the girdle. Chloroplast division is longitudinal and begins and ends before mitosis and cytokinesis. Over the cell cycle, no rotations or lateral translocations of the chloroplast were noticed. A. cf. proteus is the first species of this genus in which sexual reproduction and breeding behaviour have been studied in clonal cultures. The cells isolated exhibited strictly dioecious behaviour, reproducing only in mixed cultures of sexually compatible clones. Sexual reproduction in A. cf. proteus is morphologically and behaviourally isogamous. Each gametangium forms two functional gametes, which do not rearrange within the gametangium, and two small aborted cells. The aborted cells are produced after an unequal cytokinesis accompanying meiosis II; the second cytokinesis during gametogenesis represents a phenomenon hitherto unknown in pennate diatoms. Auxospore development takes place by bipolar expansion. The expansion is unequal on the two long sides of the auxospore, so that the auxospore becomes curved and dorsiventral. The auxospore forms a transverse and longitudinal perizonium.

A previous study, leading to the redefinition of Seminavis ventricosa (Gregory) Garcia-Baptista and some related taxa, made it clear that a considerable number of species exist within Seminavis D.G. Mann. Many species are still classified under Amphora Ehrenberg, although they clearly share common morphological features with Seminavis. Some of these species are described as new or transferred here, whereas others are only mentioned because of insufficient evidence, lack of original material, or poor documentation, so that taxonomic changes would be unwise at present. The species S. basilica Danielidis sp. Nov., S. heidenii D.G. Mann sp. Nov., S. strigosa (Hustedt) Danielidis & Economou-Amilli comb, nov., S. latior (A. Schmidt) Danielidis & D.G. Mann comb, et stat. Nov., and S. obtusiuscula (Grunow) Danielidis & D.G. Mann comb. Nov. all share the characteristic valve shape, areola structure and raphe constraction of typical Seminavis. Furthermore, in those species where fresh material was available, two plate-like, girdle-appressed chloroplasts were evident, as in other Seminavis and in almost all members of the Naviculaceae Kützing emend. D.G. Mann. The identity of S. cymbelloides (Grunow) D.G. Mann is discussed, although original material has not been available to us and may not exist. Lectotypes for A. angusta var. angustissima Van Heurck, A. diducta A. Schmidt, S. latior (A. Schmidt) Danielidis & D. G. Mann, and S. obtusiuscula (Grunow) Danielidis & D.G. Mann are also designated. © 2003 Taylor & Francis Group, LLC.

Amphora angusta Gregory, Amphora ventricosa Gregory and Amphora macilenta Gregory have been misunderstood for the last 140 years. Gregory described these species from sublittoral sediments off the west coast of Scotland. Although the illustrations were excellent for their time (1857), it is impossible to be confident of identification using Gregory's paper, and subsequent authors have created such confusion that records of these taxa must be regarded as untrustworthy unless backed by photographic evidence. In this paper, the first to describe in detail the recently established genus Seminavis, we show that A. angusta is in fact a species of Amphora Ehrenberg, whereas A. Ventricosa sensu Gregory represents two independent species of the genus Seminavis D. G. Mann, namely Seminavis Ventricosa (Gregory) Garcia-Baptista (non S. ventricosa sensu Garcia-Baptista 1993) and Seminavis arranensis Danielidis & D. G. Mann, sp. nov. The form usually known as A. ventricosa is neither of these and requires a new name, Seminavis robusta Danielidis & D. G. Mann sp. nov. The long forgotten and misclassified species A. macilenta Gregory is shown to be yet another Seminavis species, requiring a new combination as S. macilenta (Gregory) Danielidis & D. G. Mann, comb. nov.; contrary to previous claims, it is separate from A. ergadensis Gregory, which is a true Amphora. Valve and girdle ultrastructure, plastid arrangement and auxosporulation are described for Seminavis and resemble those in the Naviculaceae sensu stricto.