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      Diversity in the Globally Distributed Diatom Genus Chaetoceros (Bacillariophyceae): Three New Species from Warm-Temperate Waters

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          There is no author summary for this article yet. Authors can add summaries to their articles on ScienceOpen to make them more accessible to a non-specialist audience.

          Abstract

          Chaetoceros is one of the most species rich, widespread and abundant diatom genera in marine and brackish habitats worldwide. It therefore forms an excellent model for in-depth biodiversity studies, assessing morphological and genetic differentiation among groups of strains. The global Chaetoceros lorenzianus complex presently comprises three species known to science. However, our recent studies have shown that the group includes several previously unknown species. In this article, 50 strains, mainly from high latitudes and from warm-temperate waters, were examined morphologically and genetically and the results compared with those of field studies from elsewhere. The strains clustered into five groups, two of which are formed by C. decipiens Cleve and C. mitra (Bailey) Cleve, respectively. Their species descriptions are emended based on samples collected close to the type localities. The three other groups are formed by new species, C. elegans sp. nov., C. laevisporus sp. nov. and C. mannaii sp. nov. Characters used to distinguish each species are: orientation of setae, shape and size of the apertures, shape, size and density of the poroids on the setae and, at least in some species, characters of the resting spores. Our aim is to cover the global species diversity in this complex, as correct species delineation is the basis for exploring biodiversity, distribution of organisms, interactions in the food web and effects of environmental changes.

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          Most cited references 4

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          Simplicity-correlated size growth of the nuclear 28S ribosomal RNA D3 expansion segment in the crustacean order Isopoda.

          The expansion segments within the eukaryote nuclear 23S-like ribosomal RNA molecule are now well characterized in many diverse organisms. A different base compositional bias, a higher propensity for size variability, and an increased evolutionary rate distinguish these regions from the universally conserved "core" regions of the molecule. In addition, some expansion segments of higher eukaryotes exhibit significant sequence simplicity which is hypothesized to occur by slippage-mediated mutational processes. We describe the discovery of extreme size variation of the D3 expansion segment in the crustacean order Isopoda. Among 11 species D3 varies in size from 180 to 518 nucleotides but maintains a homologous secondary structure. The D3 size is significantly positively correlated to relative simplicity factor (RSF), indicating that growth is most likely by insertion of simple sequences. D3 size and RSF correlate approximately with a morphology-based phylogeny, and within oniscideans RSF increases as more recent divergences occur. The D3 of Armadillidium vulgare, with an RSF of 1.87, is the highest value recorded for any known expansion segment. Regions of high sequence simplicity in nuclear ribosomal RNA were previously only known from the higher vertebrate lineage. Here we demonstrate that this phenomenon occurs in a more extreme condition within a monophyletic invertebrate lineage. The extreme size changes identified could indicate that expansion segments are an extraneous element in the functioning ribosome.
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            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.
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              Bridging the Rubicon: phylogenetic analysis reveals repeated colonizations of marine and fresh waters by thalassiosiroid diatoms.

              Salinity imposes a significant barrier to the distribution of many organisms, including diatoms. Diatoms are ancestrally marine, and the number of times they have independently colonized fresh waters and the physiological adaptations that facilitated these transitions remain outstanding questions in diatom evolution. The colonization of fresh waters by diatoms has been compared to "crossing the Rubicon," implying that successful colonization events are rare, irreversible, and lead to substantial species diversification. To test these hypotheses, we reconstructed the phylogeny of Thalassiosirales, a diatom lineage with high diversity in both marine and fresh waters. We collected approximately 5.3kb of DNA sequence data from the nuclear (SSU and partial LSU rDNA) and chloroplast genomes (psbC and rbcL) and reconstructed the phylogeny using parsimony and Bayesian methods. Alternative topology tests strongly reject all previous colonization hypotheses, including monophyly of the predominantly freshwater Stephanodiscaceae. Results showed at least three independent colonizations of fresh waters, and whereas previous accounts of freshwater-to-marine transitions have been discounted, these results provide compelling evidence for as many as three independent re-colonizations of the marine habitat, two of which led to speciation events. This study adds valuable phylogenetic context to previous debate about the nature of the salinity barrier in diatoms and provides compelling evidence that, at least for Thalassiosirales, the salinity barrier might be less formidable than previously thought.
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                Author and article information

                Contributors
                Role: Editor
                Journal
                PLoS One
                PLoS ONE
                plos
                plosone
                PLoS ONE
                Public Library of Science (San Francisco, CA USA )
                1932-6203
                13 January 2017
                2017
                : 12
                : 1
                Affiliations
                [1 ]Guangzhou Key Laboratory of Subtropical Biodiversity and Biomonitoring, College of Life Science, South China Normal University, Guangzhou, P. R. China
                [2 ]Natural History Museum of Denmark, University of Copenhagen, Copenhagen, Denmark
                [3 ]Department of Biology, Faculty of Science, Ramkhamhaeng University, Bangkok, Thailand
                [4 ]Integrative Marine Ecology, Stazione Zoologica Anton Dohrn, Naples, Italy
                [5 ]Department of Oceanography and Center COPAS Sur-Austral, University of Concepción, Concepción, Chile
                [6 ]Instituto de Ciencias del Mar y Limnologia, Universidad Nacional Autónoma de México, Cd. Universitaria, Coyoacán, Cd. de México, México
                [7 ]Section of Marine Biology, Institute of Biology, University of Copenhagen, Copenhagen, Denmark
                University of Cambridge, UNITED KINGDOM
                Author notes

                Competing Interests: The authors have declared that no competing interests exist.

                • Conceptualization: YL AB ØM NL.

                • Data curation: YL AB CG ØM NL.

                • Formal analysis: YL AB CG WK CL DB ZC ØM NL.

                • Funding acquisition: NL WK AB YL CL.

                • Investigation: YL AB CG WK CL DB ZC ØM NL.

                • Methodology: YL AB ØM NL.

                • Project administration: NL YL.

                • Resources: YL AB CG WK CL DB ZC ØM NL.

                • Software: YL NL CG WK.

                • Supervision: NL YL ØM.

                • Validation: YL AB CG WK CL DB ZC ØM NL.

                • Visualization: YL AB ØM NL.

                • Writing – original draft: YL AB ØM NL.

                • Writing – review & editing: YL AB CG WK CL DB ZC ØM NL.

                Article
                PONE-D-16-32004
                10.1371/journal.pone.0168887
                5235366
                28085887
                © 2017 Li et al

                This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

                Page count
                Figures: 20, Tables: 2, Pages: 38
                Product
                Funding
                Funded by: funder-id http://dx.doi.org/10.13039/501100001809, National Natural Science Foundation of China;
                Award ID: 31370235
                Award Recipient :
                Funded by: funder-id http://dx.doi.org/10.13039/501100001809, National Natural Science Foundation of China;
                Award ID: 31570205
                Award Recipient :
                Funded by: Association of European Marine Biological Laboratories
                Award ID: 227799
                Award Recipient :
                Funded by: Association of European Marine Biological Laboratories
                Award ID: 1555
                Award Recipient :
                Funded by: Royal Thai Government
                Award Recipient :
                Funded by: Faculty of Science, University of Copenhagen
                Award Recipient :
                Funded by: Carlsbergfondet (DK)
                Award Recipient :
                Funded by: Dansk Center for Havforskning
                Award Recipient :
                Funded by: Stazione Zoologica Naples
                Award Recipient :
                Funded by: CNRS/LIA-MORFUN
                Award ID: Mission no 7569
                Award Recipient :
                Funded by: PASPA
                Award Recipient :
                Natural Science Foundation of China, Nos. 31370235 and 31570205. Association of European Marine Biological Laboratories, Grant Agreements 227799 and 1555. Royal Thai Government. Faculty of Science, University of Copenhagen. Carlsbergfondet (DK) no 2012_01_0556. Dansk Center for Havforskning. Stazione Zoologica Naples. CNRS/LIA-MORFUN Mission no 7569. PASPA.
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                All relevant data are within the paper and its Supporting Information files. The submitted sequences are available in the NCBI database, and the accession numbers are included in S1 and S2 Tables.

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