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      Excessive proliferation of new dinoflagellate generic names: the case of Torquentidium (Ceratoperidiniaceae, Dinophyceae)

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      Cochlodinium, Dinoflagellata, dinoflagellate, Dinophyceae, Gymnodinium, Gyrodinium, Pseliodinium, Torquentidium, molecular phylogenetics, taxonomy

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          Summary

          New genera of unarmoured dinoflagellates represented by a single or very few species have largely increased in the last two decades. An example is the new genus name Torquentidium H.H. Shin, Z. Li, K.W. Lee & Matsuoka 2019 (Shin et al., Eur. J. Phycol. 54: 249-262) recently proposed for species that were transferred into Pseliodinium in 2018. The SSU- and LSU rRNA gene sequences of the type species of Pseliodinium and Torquentidium only differed by 0.3% and 2%, respectively, and they clustered together as a monophyletic group with high support. Based on these tree topologies and/or the sequence divergences, each dinoflagellate species would be classified in its own genus. The morphological character proposed to distinguish Pseliodinium and Torquentidium is that the cingulum encircled the cell 1 and 1.5 times, respectively. This kind of arbitrary numeric morphometric character, proposed one century ago, is currently considered invalid for the generic split of unarmoured dinoflagellates. For example, Polykrikos geminatum (with ~1.5 cingular turns) clusters between species of Polykrikos (1 turn). Within the Ceratoperidiniaceae, the arrangement of the amphiesmal vesicles in apical structure complex was used for distinction at the genus level (i.e., Kirithra), while this character is identical for Pseliodinium and Torquentidium. For the first time, an unarmoured dinoflagellate genus contains species with smooth cell covering (T. convolutum, T. helix) and species with longitudinal striae (T. pirum). However, the original description as Gymnodinium pirum in 1895 erroneously illustrated two distinct species, and since then nobody has ever reported surface markings in T. pirum. The morphological and molecular criteria used for the split of Torquentidium from Pseliodinium are unsupported, contributing to the excessive proliferation of monotypic genera. Torquentidium pirum and T. convolutum are synonyms, and Torquentidium is a superfluous generic name and junior synonym of Pseliodinium.

          Abstract

          New genera of unarmoured dinoflagellates represented by a single or very few species have largely increased in the last two decades. An example is the new genus name Torquentidium H.H. Shin, Z. Li, K.W. Lee & Matsuoka 2019 (Shin et al., Eur. J. Phycol. 54: 249-262) recently proposed for species that were transferred into Pseliodinium in 2018. The SSU- and LSU rRNA gene sequences of the type species of Pseliodinium and Torquentidium only differed by 0.3% and 2%, respectively, and they clustered together as a monophyletic group with high support. Based on these tree topologies and/or the sequence divergences, each dinoflagellate species would be classified in its own genus. The morphological character proposed to distinguish Pseliodinium and Torquentidium is that the cingulum encircled the cell 1 and 1.5 times, respectively. This kind of arbitrary numeric morphometric character, proposed one century ago, is currently considered invalid for the generic split of unarmoured dinoflagellates. For example, Polykrikos geminatum (with ~1.5 cingular turns) clusters between species of Polykrikos (1 turn). Within the Ceratoperidiniaceae, the arrangement of the amphiesmal vesicles in apical structure complex was used for distinction at the genus level (i.e., Kirithra), while this character is identical for Pseliodinium and Torquentidium. For the first time, an unarmoured dinoflagellate genus contains species with smooth cell covering ( T. convolutum, T. helix) and species with longitudinal striae ( T. pirum). However, the original description as Gymnodinium pirum in 1895 erroneously illustrated two distinct species, and since then nobody has ever reported surface markings in T. pirum. The morphological and molecular criteria used for the split of Torquentidium from Pseliodinium are unsupported, contributing to the excessive proliferation of monotypic genera. Torquentidium pirum and T. convolutum are synonyms, and Torquentidium is a superfluous generic name and junior synonym of Pseliodinium.

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          Most cited references 4

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          Apical Groove Type and Molecular Phylogeny Suggests Reclassification of Cochlodinium geminatum as Polykrikos geminatum

          Traditionally Cocholodinium and Gymnodinium sensu lato clade are distinguished based on the cingulum turn number, which has been increasingly recognized to be inadequate for Gymnodiniales genus classification. This has been improved by the combination of the apical groove characteristics and molecular phylogeny, which has led to the erection of several new genera (Takayama, Akashiwo, Karenia, and Karlodinium). Taking the apical groove characteristics and molecular phylogeny combined approach, we reexamined the historically taxonomically uncertain species Cochlodinium geminatum that formed massive blooms in Pearl River Estuary, China, in recent years. Samples were collected from a bloom in 2011 for morphological, characteristic pigment, and molecular analyses. We found that the cingulum in this species wraps around the cell body about 1.2 turns on average but can appear under the light microscopy to be >1.5 turns after the cells have been preserved. The shape of its apical groove, however, was stably an open-ended anticlockwise loop of kidney bean shape, similar to that of Polykrikos. Furthermore, the molecular phylogenetic analysis using 18S rRNA-ITS-28S rRNA gene cistron we obtained in this study also consistently placed this species closest to Polykrikos within the Gymnodinium sensu stricto clade and set it far separated from the clade of Cochlodinium. These results suggest that this species should be transferred to Polykrikos as Polykrikos geminatum. Our results reiterate the need to use the combination of apical groove morphology and molecular phylogeny for the classification of species within the genus of Cochlodinium and other Gymnodiniales lineages.
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            A New Clade, Based on Partial LSU rDNA Sequences, of Unarmoured Dinoflagellates

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              Molecular phylogeny and morphology of Torquentidium gen. et comb. nov. for Cochlodinium convolutum and allied species (Ceratoperidiniaceae, Dinophyceae)

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                Author and article information

                Journal
                ScienceOpen Preprints
                ScienceOpen
                10 November 2019
                Affiliations
                [1 ] Carmen Campos Panisse 3, E-11500 Puerto de Santa María, Spain
                Article
                10.14293/S2199-1006.1.SOR-.PPWN5AK.v1

                This work has been published open access under Creative Commons Attribution License CC BY 4.0 , which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Conditions, terms of use and publishing policy can be found at www.scienceopen.com .

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