Redefinition of the genus Thecadinium (Dinophyceae) using morphological and molecular data, and description of Thecadinium pseudokofoidii sp. nov.

ABSTRACT

The type of the polyphyletic genus Thecadinium, T. kofoidii, differs from other species of the genus in the position and number of thecal plates. We sequenced partial nuclear small subunit (SSU) and large subunit (LSU) rDNA and ITS-5.8S of two T. kofoidii ecotypes collected in winter and summer and compared molecular phylogenetic inferences with morphological observations. Molecular phylogenies based on SSU, LSU, and ITS-5.8S sequences, including sequences of summer and winter strains and those from GenBank, revealed two distinct Thecadinium lineages. The first contained two phylogroups: (1) winter strains together with Atlantic T. kofoidii from GenBank that showed obvious differentiation from (2) summer strains. The second lineage contained sequences of T. yashimaense and showed that species of the second lineage should be placed in a separate genus. Genetic differences between the winter and summer strains suggested that they are two different species: T. kofoidii and T. pseudokofoidii sp. nov. Morphologically, T. pseudokofoidii differed from T. kofoidii in number, size, and position of pores on the large lateral hypothecal plates. Thecadinium pseudokofoidii had about 28 to 48 (37 ± 4.2) pores (diameter: 0.31–0.76, 0.51 ± 0.08 µm) located mainly along the margins of large lateral plates; whereas T. kofoidii had about 24 to 72 (48 ± 11) pores (diameter: 0.22–0.49, 0.36 ± 0.06 µm) scattered over the entire surface of these plates. Moreover, cells of T. pseudokofoidii were 23–37 µm (31.2 ± 2.2) long and 19–32 µm (26.2 ± 2.3) deep and had a stronger ornamented theca, in particular cingular and third postcingular plates, compared to T. kofoidii cells that were 27–34 µm (30.7 ± 1.7) long and 21–27 µm (24.0 ± 1.5) deep and less ornamented. Both species had a unique feature: the right anterior sulcal plate contacted the P_o plate. According to morphological and molecular data, Thecadinium was separated into two groups: Thecadinium sensu stricto including T. kofoidii and T. pseudokofoidii sp. nov., and Thecadinium sensu lato including the rest of the genus.

KEYWORDS:

• Morphology
• Phylogeny
• rDNA
• Sand-dwelling dinoflagellates
• Sea of Japan
• Taxonomy

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Funding

This study was financially supported by research grants of the Russian Science Foundation (project no. 14-50-00034) and the FEB RAS program ‘Dalnii Vostok’ (project no. 18-4-050).