LAUSR

ABSTRACT Thecadinium ovatum is one of the four species of Thecadinium sensu lato which has no apical pore, but has a ventral pore on the first apical plate, an unusual ‘multi-pimple-plate’ in the precingular series, and an antapical plate with several spines. Its original thecal plate formula was 3ʹ 6ʹʹ 6c 5s? 5ʹʹʹ 1ʹʹʹʹ. Morphological examination of cells from the Sea of Japan showed that T. ovatum has four apical plates and that the multi-pimple-plate is an anterior intercalary plate. The revised thecal plate formula of the species is 4ʹ1a 5ʹʹ6c 6s 5ʹʹʹ1ʹʹʹʹ. Molecular genetic data of T. ovatum were obtained for the first time. LSU rRNA gene sequences obtained by a single-cell PCR method from natural samples were used for taxonomic and phylogenetic studies. The sequence for T. ovatum did not show a close phylogenetic relationship to sequences of T. yashimaense, T. kofoidii and T. pseudokofoidii. The phylogenetic analysis of the D1/D2 and D8/D10 regions of the large subunit (LSU) rRNA from single cells indicated that Thecadinium ovatum formed the nearest sister branch to the Protoceratiaceae, including Pentaplacodinium usupianum, Pentaplacodinium saltonense, Protoceratium reticulatum, and Ceratocorys species. This clade was placed within the order Gonyaulacales. Consequently, T. ovatum does not belong to Thecadinium sensu stricto, nor does it show a phylogenetic relationship with other Thecadinium sensu lato species (T. yashimaense and Psammodinium inclinatum). Thus, T. ovatum should be transferred to an independent new genus that may be created after genetic information is available for the other morphologically close species.