THE MORPHOLOGICAL AND PHYLOGENETIC STUDIES ON TWO NEWLY RECORDED THALASSIOSIRA SPECIES FROM CHINA

To investigate the species diversity of Thalassiosira in China and provide molecular data for further phylogenetic studies, monoclonal strains have been isolated and established from some representative sites in Chinese coast. The morphology was studied using light and electron microscopy, and the hypervariable D1‒D3 region of the nuclear large subunit ribosomal RNA-encoding gene region (LSU rRNA gene) was amplified and sequenced. Two newly recorded Thalassiosira species (T. anguste-lineata (Schmidt) Fryxell & Hasle and T. minicosmica Lee & Park) for China were reported based on the combination of morphology and molecular data. Their detailed morphology were described and compared with allied taxa. The results of molecular phylogenetic tree based on sequences data of D1—D3 region of LSU rRNA indicated that neither T. anguste-lineata nor T. minicosmica were clustered with T. nordenskioeldii, the type species of the genus Thalassiosira, suggesting great genetic difference is present among Thalassiosira taxa, and that the present morphology-based genus Thalassiosira is not a natural group because it is separated by some other allied genus, such as Skeletonema, Cyclotella, Planktoniella. These results revealed that the present reported Thalassiosira species should be paraphyletic group and will be gradually revised into multiple independent genera in future phylogenetic study. However, it is difficult to carry out specific taxonomic revisions because of limited molecular data that is an important constraint for the advancement of related researches.