Morphological and phylogenetic analysis of Skeletonema costatum-like diatoms (Bacillariophyta) from the China Sea

Five Skeletonenia costatum-like diatoms isolated from different areas along the coast of the China Sea were studied with light microscopy and scanning electron microscopy. The five isolates differed in morphology, only one (SK-XM) being identified as Skeletonema costatum sensu stricto. The other four isolates did not correspond morphologically to any described Skeletonema species. In order to explore the relationships between our isolates and previously described Skeletonenia species, partial large subunit ribosomal DNA (D1-D2) sequences were obtained by PCR amplification, and included in a phylogenetic analysis with published sequences of other Skeletonenia spp. Skeletonenia costatum sensu stricto (SK-XM) grouped with S. subsalsium and another of our isolates (SK-DH). The latter had a short terminal rimportula and both 1:1 and 1:2 fultoportula linkages. A third isolate (SK-HH) contained several chloroplasts per cell, with intact fultoportulae linking sibling valves, and grouped in the same clade as S. tropicum. The other two isolates (SK-BH, SK-FQ) formed short chains and had peculiar fultoportulac that were oblique to the valve face at their bases and grouped with S. dohrnii. Sequence distance analysis indicated higher similarity between S. costatum (SK-XM) and SK-DH and between SK-HH, SK-FQ and SK-BH. Genetic similarity between the isolates paralleled their geographical location.