Nuclear ITS and LSU sequence analysis of Skeletonema costatum (Bacillariophyta)-like species from the China Sea

Five strains of Skeletonema costatum-like species were isolated from different coastal regions of the China Sea, where red tides occurred. The internal transcribed spacer regions (ITS-1, 5.8S rDNA and ITS-2) and the partial large subunit rDNA (D1-D2 hyper-variable domain) regions of these strains were sequenced and aligned with sequences obtained from other Skeletonema strains. The five strains showed considerable sequence variation and also length variation among their ITS regions, whereas their partial LSU regions exhibited less variation. Their 5.8S rDNA were identical. The LSU and ITS trees provided different topologies and phylogenetic inferences suggested that the five isolates belonged to at least three different species. The strain from Yellow Sea might be S. tropicum; the strains from East Sea and Xiamen were grouped in the same clade in both trees; the other two strains clustered in the LSU tree, but belonged to different clades in the ITS tree. Results reveal considerable diversity in the genus Skeletonema from the Chinese coast, especially strains causing red tides.