Victor A. Chepurnov

Historical processes constrain patterns in global diatom diversity

There is a long-standing belief that microbial organisms have unlimited dispersal capabilities, are therefore ubiquitous, and show weak or absent latitudinal diversity gradients. In contrast, using a global freshwater diatom data set, we show that latitudinal gradients in local and regional genus richness are present and highly asymmetric between both hemispheres. Patterns in regional richness are explained by the degree of isolation of lake districts, while the number of locally coexisting diatom genera is highly constrained by the size of the regional diatom pool, habitat availability, and the connectivity between habitats within lake districts. At regional to global scales, historical factors explain significantly more of the observed geographic patterns in genus richness than do contemporary environmental conditions. Together, these results stress the importance of dispersal and migration in structuring diatom communities at regional to global scales. Our results are consistent with predictions from the theory of island biogeography and metacommunity concepts and likely underlie the strong provinciality and endemism observed in the relatively isolated diatom floras in the Southern Hemisphere.

Experimental Studies on Sexual Reproduction in Diatoms

The diatoms are the most speciose group of algae, having global ecological significance in the carbon and silicon cycles. They are almost unique among algae in being diplontic, and sexual reproduction is an obligate stage in the life cycle of most diatom species. It is unclear which are the principal factors that have fostered the evolutionary success of diatoms, but the unique life cycle (which is correlated with a curious wall structure and cell division mechanism) and size-dependent control of sexuality must have played an important part. Progress in understanding life cycle dynamics and their interrelationships with population biology and evolution will depend on how successfully sex can be initiated and manipulated experimentally, and our review provides a foundation for such work. Relevant data are scattered in time and come mostly from non-English publications, producing a false impression of diatoms as recalcitrant with respect to sexualization. Recent advances dependent on experimental cultures include the discovery of widespread heterothallism (including some complex types of behavior) in pennate diatoms, sexual diversity among clones of centric diatoms, more flexible size restitution strategies in centric diatoms than had been suspected, and use of reproductive isolation as a criterion in diatom taxonomy. We identify unsolved problems in the life history of diatoms, including aspects of sexualization, cell-cell recognition, sexual reproduction, and the development of the special expanding cell (the auxospore), which is crucial to morphogenesis in this group. Some of these problems are being addressed using modern molecular genetic tools, and progress will be facilitated when whole-genome sequences are published (e.g., for Thalassiosira pseudonana). Problems of culture maintenance and methods for manipulating the life cycle are discussed.

The Sellaphora pupula species complex (Bacillariophyceae): morphometric analysis, ultrastructure and mating data provide evidence for five new species

Abstract Morphometric shape analysis and ultrastructural data are provided for six genodemes of the Sellaphora pupula species complex that have been studied during the last 20 years from Blackford Pond, Edinburgh, UK. The demes have previously been shown to be separated by prezygotic reproductive barriers: cells of different demes do not pair, though they may show residual interactions when sexualized. A new morphometric method, contour segment analysis, which was recently developed for diatoms in relation to automated identification, gives a clear separation of all six genodemes and indicates no heterogeneity within each. Legendre shape analysis gives less separation in this instance. All other available data, including molecular sequence data, also support separation of the six demes at species level. Five new species are therefore described: S. auldreekie, S. blackfordensis, S. capitata, S. lanceolata and S. obesa. The identity of S. pupula sensu stricto is clarified through designation of epitypes.

Morphological, genetic and mating diversity within the widespread bioindicator Nitzschia palea (Bacillariophyceae)

Trobajo R., Clavero E., Chepurnov V.A., Sabbe K., Mann D.G., Ishihara S. and Cox E.J. 2009. Morphological, genetic and mating diversity within the widespread bioindicator Nitzschia palea (Bacillariophyceae). Phycologia 48: 443–459. DOI: 10.2216/08-69.1. Nitzschia palea (Kützing) W. Smith is believed to be a widely distributed diatom and is common in various lotic and lentic freshwater habitats. However, it is also taxonomically problematic. As part of a multidisciplinary study of this diatom, 25 clones identified morphologically as N. palea were isolated from different freshwater habitats around the world (Belgium, Brazil, Egypt, India, Japan, Paraguay, Spain, Sri Lanka and the United Kingdom). Morphological and genetic diversity (using the hypervariable D1–D2 domains of LSU rDNA) were investigated, and an almost complete set of interclonal crossing experiments was carried out. Results indicate that N. palea is not a simple, homogeneous taxon and that N. palea will probably have to be split into three or more species. Molecular and mating groups do not separate along the traditional morphological boundaries among N. palea varieties, in particular between vars palea and debilis, two taxa that are commonly used to discriminate between different degrees of water pollution. At least two of the putative species within the N. palea complex appear to be geographically widespread. Because of the complexity of variation revealed by the LSU, mating and morphometric data, it is clear that further work, using extra genetic markers and new isolates, will be needed to determine the full extent of cryptic and pseudocryptic speciation in N. palea and to investigate whether the segregate species are ecologically differentiated and have value as indicators.

A MOLECULAR SYSTEMATIC APPROACH TO EXPLORE DIVERSITY WITHIN THE SELLAPHORA PUPULA SPECIES COMPLEX (BACILLARIOPHYTA)1

The common and cosmopolitan freshwater benthic diatom Sellaphora pupula (Kütz.) Mereschk. is a model system for studying the nature of species in microalgae; the biological significance of morphological variation in this species complex has been widely demonstrated. The aim of this study was to establish a two‐gene phylogeny (18S rDNA and rbcL) for 23 Sellaphora taxa, including 19 S. pupula aggregate (agg.) demes or species, S. bacillum (Ehrenb.) D. G. Mann, and S. laevissima (Kütz.) D. G. Mann. A range of analyses on separate and combined data sets indicated that Sellaphora is a monophyletic group containing four major clades. Of the traditionally recognized species, S. bacillum and S. laevissima are natural groups, but S. pupula is paraphyletic or polyphyletic because S. bacillum groups with S. pupula“small lanceolate” and S. lanceolata. Thickened bars at the poles of valves within the core “pupula–bacillum” group may be a morphological synapomorphy; the fossil record suggests that this group is at least 12 million years old. Otherwise, there was no clear pattern in the distribution of different morphologies among the major clades; each clade was also heterogeneous with respect to mating system. More intensive and extensive sampling will doubtless uncover even greater diversity; the challenge lies in its interpretation. Our results demonstrate the limitations of paleoecological, ecological, and biogeographical research based on morphospecies.

SEXUAL REPRODUCTION, MATING SYSTEM, AND PROTOPLAST DYNAMICS OF SEMINAVIS (BACILLARIOPHYCEAE)1

Cell division, the mating system, and auxosporulation were studied in the marine epipelic diatom Seminavis cf. robusta Danielidis & D. G. Mann. The interphase protoplast contains two girdle‐appressed chloroplasts, each with an elongate bar‐like pyrenoid, and also a central nucleus, located in a bridge between two vacuoles. Before cell division, the chloroplasts divide transversely and translocate onto the valves. The nucleus relocates to the ventral side for mitosis. After cytokinesis and valve formation, the chloroplasts move back to the girdle, showing a constant clockwise movement relative to the epitheca of the daughter cell. Seminavis cf. robusta is dioecious, and sexual reproduction is possible once cells are less than 50 μm. In crosses of compatible clones, gametangia pair laterally, without the formation of a copulation envelope, and produce two gametes apiece. The intensity of sexualization increases as cells reduce further in size below the 50‐μm threshold. At plasmogamy, the gametangia dehisce fully and the gametes, which were morphologically and behaviorally isogamous, fuse in the space between the gametangial thecae. The auxospore forms a transverse and longitudinal perizonium. After expansion is complete, there is an unequal contraction of the protoplast within the perizonium, creating the asymmetrical shape of the vegetative cell. Apart from this last feature, almost all characteristics exhibited by the live cell and auxospores of Seminavis agree with what is found in Navicula sensu stricto, supporting the classification of both in the Naviculaceae. Haploid parthenogenesis and polyploid auxospores were found, lending support to the view that change in ploidy may be a significant mechanism in diatom evolution.

SEXUALITY, INCOMPATIBILITY, SIZE VARIATION, AND PREFERENTIAL POLYANDRY IN NATURAL POPULATIONS AND CLONES OF SELLAPHORA PUPULA (BACILLARIOPHYCEAE)

The capitate and rectangular demes of the freshwater epipelic diatom Sellaphora pupula (Kütz.) Mereschk. are dioecious, the first such report for any freshwater diatom. Sexual differentiation, which is probably determined genetically, involves recognition at the cell surface as well as differences in gamete behavior (one gametangium produces an active “male” gamete, the other a passive “female” gamete). In culture, successful sexual reproduction occurs only when compatible clones are mixed. All cells of a clone behave identically in interclonal crosses, being either male or female, regardless of the stage of the life cycle, in contrast to the sequential hermaphroditism of centric diatoms. Males and females have identical frustule morphology. As in other diatoms, there is an upper size threshold for sexual reproduction, below which cells become progressively easier to sexualize. In culture, sexual interactions occur in cells much smaller than those ever seen in natural populations, so that in nature the sexual size range is effectively open. Natural populations almost always contain sexualizable cells; often, most of the cells are below the upper sexual size threshold. Male gametangia are, on average, slightly larger than females in the capitate deme, which may be produced by preferential polyandry, depleting the population of males and making them younger at mating. Rarely, selfing occurs producing zygotes, but these abort before producing initial cells. The sizes of the gametangia and initial cells are correlated but this does not invalidate the use of “cardinal points” of the life cycle in taxonomy. No interbreeding occurs between the rectangular and capitate demes. However, when males of one deme are mixed with females of the other, there is a stimulation of activity, as during the early stages of pairing in compatible intrademic crosses.

Sexual reproduction, mating system, chloroplast dynamics and abrupt cell size reduction in Pseudo-nitzschia pungens from the North Sea (Bacillariophyta)

Clonal cultures of Pseudo-nitzschia pungens were isolated at various times from seven sites in the North Sea. During the mitotic cell cycle, the two plate-shaped chloroplasts were girdle-appressed during interphase and mitosis. After cytokinesis, the chloroplasts moved onto the parental valve and remained there during the formation of the new hypovalve and until separation and re-arrangement of the sibling cells within the cell chain had been completed. Clones were almost always heterothallic and cultures of opposite mating type isolated from different localities were compatible. Meiosis I was cytokinetic and accompanied by chloroplast division. Meiosis II involved karyokinesis but not cytokinesis and preceded the rearrangement and contraction of the two gametes. Sexual reproduction involved physiological anisogamy. With one exception, gamete behaviour was clone-specific, gametes being active in clones of one mating type but passive in clones of the other mating type. Auxospore development was accompanied by deposition of a transverse and then a longitudinal perizonium. Infrequently, triploid auxospores and presumably haploid auxospores were produced. The four chloroplasts of diploid auxospores did not divide, and behaved synchronously during the two acytokinetic mitotic cycles accompanying the deposition of the initial thecae. Just before the first division of the initial cell, the chloroplasts shifted onto the valves (two per valve). The division of the initial cell was not accompanied by chloroplast division and so the two daughter cells received two chloroplasts each. Two modes of abrupt cell size reduction were detected. One occurred during initial cell formation when part of the expanded auxospore aborted. The other pattern was more gradual and was observed in growing cultures; during successive cell divisions a frustule constriction appeared and intensified, one chloroplast split into two, and part of the protoplast aborted. A simple naming system is proposed for mating types in pennate diatoms.

In search of new tractable diatoms for experimental biology

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.

TAXONOMY, LIFE CYCLE, AND AUXOSPORULATION OF NITZSCHIA FONTICOLA (BACILLARIOPHYTA)1

Nitzschia fonticola (Grunow) Grunow is a member of Nitzschia sect. Lanceolatae, a group of taxonomically intractable but ecologically important and widespread diatoms. We investigated the morphology and life cycle in three clones of N. fonticola and all exhibited reduced sexuality, with pedogamous production of auxospores in unpaired gametangia. The auxospores of all clones contained tangles of striplike elements that lay outside the perizonium and were distinct from it in structure and ontogeny. We introduce a new term, incunabula, to refer to such components of the auxospore wall. Semicryptic variation was detected: one clone differed from the other two in valve size and shape, stria density, and fibula density, as well as its nuclear large subunit ribosomal DNA (LSU rDNA) sequence. The implications of reduced sexuality for the taxonomy of sect. Lanceolatae are discussed. A lectotype is designated for N. fonticola from among original material of Grunow, and the application of the name is clarified further by designating illustrations and the LSU sequence AM182191 from one of our clones as epitypes.