Distribution and molecular phylogeny of the dinoflagellate genus Ostreopsis in Thailand

• We found two clades and two subclades of Ostreopsis from Thailand.
• Among them, Ostreopsis sp. 7 clade and O. cf. ovata Thailand subclade were novel.
• Ostreopsis sp. 7 strains were distributed in the Andaman Sea.
• O. cf. ovata and Ostreopsis sp. 6 strains were distributed in the Gulf of Thailand.
• The strains of Ostreopsis sp. 6 and Ostreopsis sp. 7 showed toxicity in mice.

The toxic benthic dinoflagellate genus Ostreopsis has been connected to the production of palytoxin and its analogs in many tropical and temperate areas. Although the type species, O. siamensis, was originally described from the Gulf of Thailand in 1901, little is known about the species composition and distribution of the genus Ostreopsis in Thailand. In this study, a total of 64 Ostreopsis strains isolated from the Andaman Sea as well as the Gulf of Thailand were investigated by analyzing the nucleotide sequences of the LSU rDNA D1/D2, D8/D10 and ITS-5.8S rDNA regions. Phylogenetic analyses (BI and ML) resulted in some of the strains being assigned to previously described clades, O. cf. ovata and Ostreopsis sp. 6, and revealed the existence of a novel clade named Ostreopsis sp. 7, which exhibited large genetic distances from the other clades. Among O. cf. ovata, several strains from Thailand were formed into a new subclade, the Thailand subclade, whereas a few strains belonged to the South China Sea subclade. Morphometric characteristics such as the cell sizes of the two O. cf. ovata subclades and those of Ostreopsis sp. 7 were not significantly different from each other (p > 0.05). Their characteristics were similar but slightly different from those of O. ovata and were significantly different from those of Ostreopsis sp. 6 (p < 0.05). Toxicities of Ostreopsis from Thailand were evaluated using mouse bioassay. Strains of Ostreopsis sp. 6 and Ostreopsis sp. 7 tested were highly toxic, while the two subclades of O. cf. ovata strains seemed to be nontoxic. This study suggests that toxic Ostreopsis sp. 7 is distributed in the Andaman Sea, whereas the two subclades of O. cf. ovata and toxic Ostreopsis sp. 6 are distributed in the Gulf of Thailand.