Species detection and delineation in the marine planktonic diatoms Chaetoceros and Bacteriastrum through metabarcoding: making biological sense of haplotype diversity.

High-throughput sequencing (HTS) metabarcoding is commonly applied to assess phytoplankton diversity. Usually, haplotypes are grouped into operational taxonomic units (OTUs) through clustering, whereby the resulting number of OTUs depends on chosen similarity thresholds. We applied, instead, a phylogenetic approach to infer taxa among 18S rDNA V4-metabarcode haplotypes gathered from 48 time-series samples using the marine planktonic diatoms Chaetoceros and Bacteriastrum as test case. The 73 recovered taxa comprised both solitary haplotypes and polytomies, the latter composed each of a highly abundant, dominant haplotype and one to several minor, peripheral haplotypes. The solitary and dominant haplotypes usually matched reference sequences, enabling species assignation of taxa. We hypothesise that the super-abundance of reads in dominant haplotypes results from the homogenization effect of concerted evolution. Reads of populous peripheral haplotypes and dominant haplotypes show comparable distribution patterns over the sample dates, suggesting that they are part of the same population. Many taxa revealed marked seasonality, with closely related ones generally showing distinct periodicity, whereas others occur year-round. Phylogenies inferred from metabarcode haplotypes enable delineation of biologically meaningful taxa, whereas OTUs resulting from clustering algorithms often deviate markedly from such taxa.

A multigene phylogeny to infer the evolutionary history of Chaetocerotaceae (Bacillariophyta).

The diatom family Chaetocerotaceae (Bacillariophyta) is common in the marine plankton worldwide, especially in coastal areas and upwelling zones. Its defining character constitutes hollow processes, called setae, which emerge from the valves of the vegetative cells. The family comprises two extant genera: Bacteriastrum and Chaetoceros. Current systematics is based on morphological features of vegetative cells and resting spores and is summarised in a classification scheme subdividing Bacteriastrum in two sections, Isomorpha and Sagittata, and Chaetoceros in three subgenera: Hyalochaete, Chaetoceros (Phaeoceros) and Bacteriastroidea, and further into 22 sections. Phylogenies inferred from single molecular markers (18S and partial 28S rDNA) show only partial topological agreement and many poorly or unresolved basal ramifications. Since classification should not only satisfy practical needs but also reflect well-supported evolutionary relationships of the taxa under investigation, we inferred a multigene phylogeny of the family Chaetocerotaceae amplifying five genes of 100 strains encompassing six Bacteriastrum and 60 Chaetoceros species. We also compared the phylogenetic signal of nuclear, plastid and mitochondrial compartments to ascertain if the inferred tree topologies were congruent. Our results provided a robust multigene phylogeny of the family Chaetocerotaceae, offering a solid framework to test the validity of the traditional taxonomical classification. The genera Bacteriastrum and Chaetoceros were resolved as sister clades, whilst the subgenus Hyalochaete was found to be paraphyletic. Consequently, we rejected the subdivision in subgenera and only considered sections. Most of the already recognised sections were found to be monophyletic. We emended one section, rejected seven and erected three new ones. As a consequence of our proposed changes, all the sections investigated are supported by morphological and molecular characters alike. Thus, a natural classification is feasible for this important and very diverse marine planktonic family.

Two new species in the Chaetoceros socialis complex (Bacillariophyta): C. sporotruncatus and C. dichatoensis, and characterization of its relatives, C. radicans and C. cinctus.

The diatom genus Chaetoceros is one of the most abundant and diverse phytoplankton in marine and brackish waters worldwide. Within this genus, Chaetoceros socialis has been cited as one of the most common species. However, recent studies from different geographic areas have shown the presence of pseudo-cryptic diversity within the C. socialis complex. Members of this complex are characterized by curved chains (primary colonies) aggregating into globular clusters, where one of the four setae of each cell curves toward the center of the cluster and the other three orient outwards. New light and electron microscopy observations as well as molecular data on marine planktonic diatoms from the coastal waters off Chile revealed the presence of two new species, Chaetoceros sporotruncatus sp. nov. and C. dichatoensis. sp. nov. belonging to the C. socialis complex. The two new species are similar to other members of the complex (i.e., C. socialis and C. gelidus) in the primary and secondary structure of the colony, the orientation pattern of the setae, and the valve ultrastructure. The only morphological characters that can be used to differentiate the species of this complex are aspects related to resting spore morphology. The two newly described species are closely related to each other and form a sister clade to C. gelidus in molecular phylogenies. We also provide a phylogenetic status along with the morphological characterization of C. radicans and C. cintus, which are genetically related to the C. socialis complex.

Marine protist diversity in European coastal waters and sediments as revealed by high-throughput sequencing.

Although protists are critical components of marine ecosystems, they are still poorly characterized. Here we analysed the taxonomic diversity of planktonic and benthic protist communities collected in six distant European coastal sites. Environmental deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) from three size fractions (pico-, nano- and micro/mesoplankton), as well as from dissolved DNA and surface sediments were used as templates for tag pyrosequencing of the V4 region of the 18S ribosomal DNA. Beta-diversity analyses split the protist community structure into three main clusters: picoplankton-nanoplankton-dissolved DNA, micro/mesoplankton and sediments. Within each cluster, protist communities from the same site and time clustered together, while communities from the same site but different seasons were unrelated. Both DNA and RNA-based surveys provided similar relative abundances for most class-level taxonomic groups. Yet, particular groups were overrepresented in one of the two templates, such as marine alveolates (MALV)-I and MALV-II that were much more abundant in DNA surveys. Overall, the groups displaying the highest relative contribution were Dinophyceae, Diatomea, Ciliophora and Acantharia. Also, well represented were Mamiellophyceae, Cryptomonadales, marine alveolates and marine stramenopiles in the picoplankton, and Monadofilosa and basal Fungi in sediments. Our extensive and systematic sequencing of geographically separated sites provides the most comprehensive molecular description of coastal marine protist diversity to date.

The Protist Ribosomal Reference database (PR2): a catalog of unicellular eukaryote small sub-unit rRNA sequences with curated taxonomy.

The interrogation of genetic markers in environmental meta-barcoding studies is currently seriously hindered by the lack of taxonomically curated reference data sets for the targeted genes. The Protist Ribosomal Reference database (PR(2), http://ssu-rrna.org/) provides a unique access to eukaryotic small sub-unit (SSU) ribosomal RNA and DNA sequences, with curated taxonomy. The database mainly consists of nuclear-encoded protistan sequences. However, metazoans, land plants, macrosporic fungi and eukaryotic organelles (mitochondrion, plastid and others) are also included because they are useful for the analysis of high-troughput sequencing data sets. Introns and putative chimeric sequences have been also carefully checked. Taxonomic assignation of sequences consists of eight unique taxonomic fields. In total, 136 866 sequences are nuclear encoded, 45 708 (36 501 mitochondrial and 9657 chloroplastic) are from organelles, the remaining being putative chimeric sequences. The website allows the users to download sequences from the entire and partial databases (including representative sequences after clustering at a given level of similarity). Different web tools also allow searches by sequence similarity. The presence of both rRNA and rDNA sequences, taking into account introns (crucial for eukaryotic sequences), a normalized eight terms ranked-taxonomy and updates of new GenBank releases were made possible by a long-term collaboration between experts in taxonomy and computer scientists.

Diversity and toxicity of the planktonic diatom genus Pseudo-nitzschia from coastal and offshore waters of the Southeast Pacific, including Pseudo-nitzschia dampieri sp. nov.

To expand knowledge of Pseudo-nitzschia species in the Southeast Pacific, we isolated specimens from coastal waters of central Chile (36°S-30°S), the Gulf of Corcovado, and the oceanic Robinson Crusoe Island (700 km offshore) and grew them into monoclonal strains. A total of 123 Pseudo-nitzschia strains were identified to 11 species based on sequencing of the ITS region of the nuclear rDNA and on ultrastructural and morphometric analyses of the frustule in selected representatives of each clade: P. australis, P. bucculenta, P. cf. chiniana, P. cf. decipiens, P. fraudulenta, P. hasleana, P. multistriata, P. plurisecta, P. cf. sabit, the new species P. dampieri sp. nov., and one undescribed species. Partial 18S and 28S rDNA sequences, including the hypervariable V4 and D1-D3 regions used for barcoding, were gathered from representative strains of each species to facilitate future metabarcoding studies. Results showed different levels of genetic, and at times ultrastructural, diversity among the above-mentioned entities, suggesting morphological variants (P. bucculenta), rapidly radiating complexes with ill-defined species boundaries (P. cf. decipiens and P. cf. sabit), and the presence of new species (P. dampieri sp. nov., Pseudo-nitzschia sp. 1, and probably P. cf. chiniana). Domoic acid (DA) was detected in 18 out of 82 strains tested, including those of P. australis, P. plurisecta, and P. multistriata. Toxicity varied among species mostly corresponding to expectations from previous reports, with the prominent exception of P. fraudulenta; DA was not detected in any of its 10 strains tested. In conclusion, a high diversity of Pseudo-nitzschia exists in Chilean waters, particularly offshore.

Microsatellite primers in the planktonic diatom Pseudo-nitzschia multistriata (Bacillariophyceae).

PREMISE OF THE STUDY: Seven microsatellite loci were characterized for the toxic diatom Pseudo-nitzschia multistriata Takano (Takano) to investigate intraspecific variability and estimate population genetic structure over blooms, seasons, and sexual and vegetative reproduction. METHODS AND RESULTS: Selected microsatellites consisted of di- and trinucleotide repeats in the core region, and showed four to twelve alleles per locus in strains of P. multistriata collected in the Gulf of Naples (Italy). Primer pairs were species-specific since they positively amplified against conspecific strains from Portugal and Spain but failed to generate PCR products from the diatoms Pseudo-nitzschia pseudodelicatissima (Hasle) Hasle and Leptocylindrus minimum Gran. CONCLUSIONS: The seven selected microsatellite markers will be useful in studying population dynamics of Pseudo-nitzschia multistriata in space and time.

Resolving cryptic species complexes in marine protists: phylogenetic haplotype networks meet global DNA metabarcoding datasets.

Marine protists have traditionally been assumed to be lowly diverse and cosmopolitan. Yet, several recent studies have shown that many protist species actually consist of cryptic complexes of species whose members are often restricted to particular biogeographic regions. Nonetheless, detection of cryptic species is usually hampered by sampling coverage and application of methods (e.g. phylogenetic trees) that are not well suited to identify relatively recent divergence and ongoing gene flow. In this paper, we show how these issues can be overcome by inferring phylogenetic haplotype networks from global metabarcoding datasets. We use the Chaetoceros curvisetus (Bacillariophyta) species complex as study case. Using two complementary metabarcoding datasets (Ocean Sampling Day and Tara Oceans), we equally resolve the cryptic complex in terms of number of inferred species. We detect new hypothetical species in both datasets. Gene flow between most of species is absent, but no barcoding gap exists. Some species have restricted distribution patterns whereas others are widely distributed. Closely related taxa occupy contrasting biogeographic regions, suggesting that geographic and ecological differentiation drive speciation. In conclusion, we show the potential of the analysis of metabarcoding data with evolutionary approaches for systematic and phylogeographic studies of marine protists.

Empirical evidence for concerted evolution in the 18S rDNA region of the planktonic diatom genus Chaetoceros.

Concerted evolution is a process of homogenisation of repetitive sequences within a genome through unequal crossing over and gene conversion. This homogenisation is never fully achieved because mutations always create new variants. Classically, concerted evolution has been detected as "noise" in electropherograms and these variants have been characterised through cloning and sequencing of subsamples of amplified products. However, this approach limits the number of detectable variants and provides no information about the abundance of each variant. In this study, we investigated concerted evolution by using environmental time-series metabarcoding data, single strain high-throughput sequencing (HTS) and a collection of Sanger reference barcode sequences. We used six species of the marine planktonic diatom genus Chaetoceros as study system. Abundance plots obtained from environmental metabarcoding and single strain HTS showed the presence of a haplotype far more abundant than all the others (the "dominant" haplotype) and identical to the reference sequences of that species obtained with Sanger sequencing. This distribution fitted best with Zipf's law among the rank abundance/ dominance models tested. Furthermore, in each strain 99% of reads showed a similarity of 99% with the dominant haplotype, confirming the efficiency of the homogenisation mechanism of concerted evolution. We also demonstrated that minor haplotypes found in the environmental samples are not only technical artefacts, but mostly intragenomic variation generated by incomplete homogenisation. Finally, we showed that concerted evolution can be visualised inferring phylogenetic networks from environmental data. In conclusion, our study provides an important contribution to the understanding of concerted evolution and to the interpretation of DNA barcoding and metabarcoding data based on multigene family markers.

Diatom Genus Hyalosira (Rhabdonematales emend.) and Resolution of its Polyphyly in Grammatophoraceae and Rhabdonemataceae with a New Genus, Placosira, and Five New Hyalosira Species.

Hyalosira gene sequences are divided into two clades within different families. We examined authentic material of Hyalosira (isotype material of H. obtusangula, synonymous with H. delicatula) and voucher specimens of published sequences, and pooled our observations of Hyalosira-like taxa from benthic and epizoic habitats in several parts of the globe. The two molecular clades corresponded to two morphological groups, with Hyalosira obtusangula associated with Grammatophoraceae. We emend the description and provide lectotypification for Hyalosira and propose Placosira to encompass the taxa in the other clade, associated with Rhabdonemataceae. We propose that Hyalosira has uniseriate to triseriate striae, sometimes different on valve face and mantle. Copulae in most species had shallow septa, though in one they were moderately deep. All species had girdle bands bearing two rows of areolae separated by a midrib. We name five new species of Hyalosira. Morphologies of taxa in the Placosira clade were superficially similar to Hyalosira but differed in having areolae with ricae, a single row of areolae on the girdle bands, and tubular rimoportulae on the valve-face-mantle junction. Hyalosira hustedtiana Patrick should revert to its original position in Striatella until the appropriate genus can be determined. We emend Rhabdonematales to encompass Rhabdonemataceae, Grammatophoraceae and Tabellariaceae.

Internal transcribed spacer polymorphism in Pseudo-nitzschia multistriata (Bacillariophyceae) in the Gulf of Naples: recent divergence or intraspecific hybridization?

The planktonic diatom Pseudo-nitzschia multistriata is a potentially toxic species recorded during late summer-autumn in the Gulf of Naples (Tyrrhenian Sea, Italy). We analysed the genetic structure by amplifying the internal transcribed spacer (ITS-1-5.8S-ITS-2) region of the ribosomal DNA of 44 strains isolated along 2 years. Polymorphism in the ITS region was detected by direct sequencing and the PCR-products from selected strains were thus cloned to assess intra-strain ITS variability. Strains clustered into three main types: type A and B - differing by 0.6% sequence divergence - and type A/B, showing both A and B variants within the same genome. The three types showed no differences in the large subunit sequences (LSU) of the rDNA, ultrastructure of the valve, secondary structure of ITS-1 and ITS-2, ploidy level and they were sexually compatible. Based on the results of these multiple approaches, we can state that the three ITS-types belong to the same reproductive unit (or "species" sensu Mayr [(1942). Systematics and the Origin of Species. Columbia University Press, New York]). We suggest that ITS polymorphism in P. multistriata may be related to the contemporary occurrence of different but still inbreeding populations which either diverged recently or originated in different geographic areas and became sympatric in the studied area.

High genetic diversity and connectivity in the polyploid invasive seaweed Asparagopsis taxiformis (Bonnemaisoniales) in the Mediterranean, explored with microsatellite alleles and multilocus genotypes.

The red alga, Asparagopsis taxiformis, has recently expanded its distribution range into the Western Mediterranean Sea, and populations have now even been found on the Portuguese South coast. All Western Mediterranean populations belong to a single mitochondrial cryptic lineage (referred to as lineage 2 in earlier studies) and probably result from a recent invasion of Indo-Pacific origin. Here we investigate fine-scale population genetic diversity and structure within and among Mediterranean populations of lineage 2 using eight microsatellite loci and compare the obtained patterns with those observed in a Californian population of the same lineage. To generate an appropriate analytical method suitable to the polyploid status of this species, we score microsatellite loci as in a dominant marker system. Thereafter, we produce robust descriptors inferred from frequencies of both microsatellite alleles and multilocus genotypes. Populations from California and the Mediterranean Sea differ considerably in their levels of genetic diversity. In the Mediterranean, populations reproduce predominantly sexually and exhibit high levels of genotypic variation, suggestive either of multiple introductions or of a single introduction by a genetically diverse and large group of individuals. Bayesian clustering revealed one or possibly two weakly supported panmictic subpopulations, indicative of extensive admixture. The expansion of this lineage is rapid, possibly due to the absence of eco-physiological barriers to gene flow throughout its invasive trajectory.

Global distribution and diversity of Chaetoceros (Bacillariophyta, Mediophyceae): integration of classical and novel strategies.

Information on taxa distribution is a prerequisite for many research fields, and biological records are a major source of data contributing to biogeographic studies. The Global Biodiversity Information Facility (GBIF) and the Ocean Biogeographic Information System (OBIS) are important infrastructures facilitating free and open access to classical biological data from several sources in both temporal and spatial scales. Over the last ten years, high throughput sequencing (HTS) metabarcoding data have become available, which constitute a great source of detailed occurrence data. Among the global sampling projects that have contributed to such data are Tara Oceans and the Ocean Sampling Day (OSD). Integration of classical and metabarcoding data may aid a more comprehensive assessment of the geographic range of species, especially of microscopic ones such as protists. Rare, small and cryptic species are often ignored in surveys or mis-assigned with the classical approaches. Here we show how integration of data from various sources can contribute to insight in the biogeography and diversity at the genus- and species-level using Chaetoceros as study system, one of the most diverse and abundant genera among marine planktonic diatoms. Chaetoceros records were extracted from GBIF and OBIS and literature data were collected by means of a Google Scholar search. Chaetoceros references barcodes where mapped against the metabarcode datasets of Tara Oceans (210 sites) and OSD (144 sites). We compared the resolution of different data sources in determining the global distribution of the genus and provided examples, at the species level, of detection of cryptic species, endemism and cosmopolitan or restricted distributions. Our results highlighted at genus level a comparable picture from the different sources but a more complete assessment when data were integrated. Both the importance of the integration but also the challenges related to it were illustrated. Chaetoceros data collected in this study are organised and available in the form of tables and maps, providing a powerful tool and a baseline for further research in e.g., ecology, conservation and evolutionary biology.

Plastid inheritance in the planktonic raphid pennate diatom Pseudo-nitzschia delicatissima (Bacillariophyceae).

Plastid inheritance was followed during sexual reproduction in the raphid pennate diatom Pseudo-nitzschia delicatissima, using rbcL haplotypes as plastid identification tools. Pseudo-nitzschia species are dioecious and show functional anisogamy with 'male' mating type+(PNd(+)) cells and 'female' PNd(-) cells. Vegetative cells possess two plastids. In P. delicatissima, meiosis results in two gametes that both contribute two plastids to the zygote. The latter initially contains four plastids, but during auxospore development two of these four seem to disappear, and the initial cell emerging from the auxospore appears to contain only two. Here we assessed if the plastids are inherited strictly unipaternally, strictly biparentally, or randomly. We traced the source of the plastids in the F(1) generation by using PNd(+) and PNd(-) parental strains with different rbcL genotypes, here denoted AA (homoplastidial, with two plastids of rbcL haplotype A) and BB (homoplastidial; two plastids of haplotype B). Results showed that 16 out of 96 strains raised each from single F(1) cells had retained two paternal (PNd(+)) plastids, 20 had two maternal (PNd(-)) plastids and the remaining 60 had one maternal and one paternal plastid. This pattern is in accordance with the hypothesis that either two of the four plastids are eliminated during auxospore formation, or that all plastids are retained in the auxospore and segregate in pairs joining at random during the first mitotic division of the initial cell. Heteroplastidic F(1)-strains retained the AB genotype throughout the vegetative phase of their life cycle. The finding that 60 out of 96 F(1) strains were heteroplastidial contrasts with an absence of such genotypes in our strains raised from single cells sampled in the Gulf of Naples.

Global diversity and biogeography of Skeletonema species (bacillariophyta).

Recent studies have shown that the cosmopolitan diatom Skeletonema costatum sensu lato is composed of several morphologically and genetically distinct species. To assess whether the separate species have a cosmopolitan distribution, we analysed 184 strains from marine and estuarine sites worldwide. We identified the strains using light and electron microscopy, and we sequenced the hyper-variable region of nuclear LSU rDNA. All recently described species were genetically distinct, and all but two were morphologically distinct. Variability was found for the only ultrastructural character used to distinguish Skeletonema dohrnii and S. marinoi, which cannot be identified based on morphology alone. Furthermore, multiple genetically distinct taxa, which may represent cryptic species, were found within the S. menzelii and S. tropicum clades. We found that all currently recognized species of Skeletonema are widespread, however, gaps seem to occur in their geographical ranges. For example, some species are found in both the northern and southern temperate latitudes whereas other species appear to have only subtropical to tropical ranges. Skeletonema pseudocostatum and S. grethae seem to have more restricted geographical ranges because the former was not found along American coasts and the latter was encountered only in US waters. A taxonomic update is provided for Skeletonema strains currently available in several culture collections, which could aid reinterpretation of results obtained in comparative studies using these strains.

dinoref: A curated dinoflagellate (Dinophyceae) reference database for the 18S rRNA gene.

Dinoflagellates are a heterogeneous group of protists present in all aquatic ecosystems where they occupy various ecological niches. They play a major role as primary producers, but many species are mixotrophic or heterotrophic. Environmental metabarcoding based on high-throughput sequencing is increasingly applied to assess diversity and abundance of planktonic organisms, and reference databases are definitely needed to taxonomically assign the huge number of sequences. We provide an updated 18S rRNA reference database of dinoflagellates: dinoref. Sequences were downloaded from genbank and filtered based on stringent quality criteria. All sequences were taxonomically curated, classified taking into account classical morphotaxonomic studies and molecular phylogenies, and linked to a series of metadata. dinoref includes 1,671 sequences representing 149 genera and 422 species. The taxonomic assignation of 468 sequences was revised. The largest number of sequences belongs to Gonyaulacales and Suessiales that include toxic and symbiotic species. dinoref provides an opportunity to test the level of taxonomic resolution of different 18S barcode markers based on a large number of sequences and species. As an example, when only the V4 region is considered, 374 of the 422 species included in dinoref can still be unambiguously identified. Clustering the V4 sequences at 98% similarity, a threshold that is commonly applied in metabarcoding studies, resulted in a considerable underestimation of species diversity.

Annotated 18S and 28S rDNA reference sequences of taxa in the planktonic diatom family Chaetocerotaceae.

The species-rich diatom family Chaetocerotaceae is common in the coastal marine phytoplankton worldwide where it is responsible for a substantial part of the primary production. Despite its relevance for the global cycling of carbon and silica, many species are still described only morphologically, and numerous specimens do not fit any described taxa. Nowadays, studies to assess plankton biodiversity deploy high throughput sequencing metabarcoding of the 18S rDNA V4 region, but to translate the gathered metabarcodes into biologically meaningful taxa, there is a need for reference barcodes. However, 18S reference barcodes for this important family are still relatively scarce. We provide 18S rDNA and partial 28S rDNA reference sequences of 443 morphologically characterized chaetocerotacean strains. We gathered 164 of the 216 18S sequences and 244 of the 413 28S sequences of strains from the Gulf of Naples, Atlantic France, and Chile. Inferred phylogenies showed 84 terminal taxa in seven principal clades. Two of these clades included terminal taxa whose rDNA sequences contained spliceosomal and Group IC1 introns. Regarding the commonly used metabarcode markers in planktonic diversity studies, all terminal taxa can be discriminated with the 18S V4 hypervariable region; its primers fit their targets in all but two species, and the V4-tree topology is similar to that of the 18S. Hence V4-metabarcodes of unknown Chaetocerotaceae are assignable to the family. Regarding the V9 hypervariable region, most terminal taxa can be discriminated, but several contain introns in their primer targets. Moreover, poor phylogenetic resolution of the V9 region affects placement of metabarcodes of putative but unknown chaetocerotacean taxa, and hence, uncertainty in taxonomic assignment, even of higher taxa.

Diatom diversity through HTS-metabarcoding in coastal European seas.

Diatoms constitute a diverse lineage of unicellular organisms abundant and ecologically important in aquatic ecosystems. Compared to other protists, their biology and taxonomy are well-studied, offering the opportunity to combine traditional approaches and new technologies. We examined a dataset of diatom 18S rRNA- and rDNA- (V4 region) reads from different plankton size-fractions and sediments from six European coastal marine sites, with the aim of identifying peculiarities and commonalities with respect to the whole protistan community. Almost all metabarcodes (99.6%) were assigned to known genera (121) and species (236), the most abundant of which were those already known from classic studies and coincided with those seen in light microscopy. rDNA and rRNA showed comparable patterns for the dominant taxa, but rRNA revealed a much higher diversity particularly in the sediment communities. Peculiar to diatoms is a tight bentho-pelagic coupling, with many benthic or planktonic species colonizing both water column and sediments and the dominance of planktonic species in both habitats. Overall metabarcoding results reflected the marked specificity of diatoms compared to other protistan groups in terms of morphological and ecological characteristics, at the same time confirming their great potential in the description of protist communities.

Microsatellite markers in an invasive strain of Asparagopsis taxiformis (Bonnemaisoniales, Rhodophyta): insights in ploidy level and sexual reproduction.

Eight polymorphic nuclear microsatellite loci were identified from the invasive Indo-Pacific Mediterranean strain of Asparagopsis taxiformis. Microsatellite markers were tested against a panel of specimens collected along the Italian (Elba, Naples) and Californian (Catalina Island) coasts, all belonging to the same mitochondrial lineage. In addition, we used Hawaiian specimens, belonging to a closely related mitochondrial lineage. The markers amplified in all of the specimens but failed consistently in thalli of two more distantly related mitochondrial lineages of A. taxiformis as well as in specimens belonging to the sister species Asparagopsis armata. Since haploid female individuals among the Mediterranean specimens contained cystocarps, genotyping was performed on supposedly haploid female specimens and supposedly diploid cystocarps separately. As expected, external allelic contribution was detected in the cystocarps. However, even after removal of these reproductive structures, gametophyte thalli exhibited patterns consisting of up to three alleles in all of the tested populations indicating polyploidy. An elevated number of distinct genotypes (up to 85%) were found per population, suggesting high intra-population variation. Results showed high genetic similarity between the two Mediterranean populations screened and lower similarity between these two and the Californian one within the same mitochondrial lineage. Lowest similarity was found between these three and the Hawaiian population belonging to the other related mitochondrial lineage 1.

Reproductive isolation among sympatric cryptic species in marine diatoms.

Pseudo-nitzschia is a marine cosmopolitan genus of chain-forming planktonic diatoms. As for the vast majority of phytoplankton organisms, species identification within this genus mostly relies upon morphological features. Taxa were initially identified based on cell shape and gross morphology of their composite silica cell wall, called the frustule. Yet, observations of the frustule in electron microscopy showed many additional characters for species identification and results of molecular studies have demonstrated that genetically distinct groups might exist within morpho-species. However, these studies have not addressed the biological meaning of these genetic differences. Here, we bridge that gap by comparing ultrastructural features and sequence data (three ribosomal and one plastid marker) of 95 strains with results of mating experiments among these strains. Experiments were performed on two morphologically distinct entities: P. delicatissima and P. pseudodelicatissima. Each of the two entities consisted of multiple genetically distinct and reproductively isolated taxa, all occurring in sympatry: P. delicatissima was composed of three phylogenetic and reproductively distinct groups, whereas P. pseudodelicatissima consisted of up to five. Once these taxa had been defined both genetically and biologically, subtle ultrastructural differences could be detected as well. Our findings not only show that cryptic genetic variants abound in sympatry, but also that they are reproductively isolated and, therefore, biologically distinct units.