THE SECONDARY STRUCTURE OF THE ITS2 TRANSCRIPT IN CYCLOTELLA AND STEPHANODISCUS (THALASSIOSIRACEAE, BACILLARIOPHYTA)

Abstract

Sequence variation of the ribosomal DNA internal transcribed spacer II (ITS2) among species of the diatom genus Stephanodiscus were investigated by Zechman et al. (1994). In addition, Zechman et al. (1994) sequenced a repeat from Cyclotella meneghiniana to determine the taxonomic range over which the ITS2 region could be used for diatom systematics. It was mentioned that, for Cyclotella, much of the ITS2 region was not alignable with that for Stephanodiscus species, and therefore, a generic-level comparison within the Thalassiosiraceae may not be possible, i.e. only interspecific relationships could be assessed with ITS2 sequences. In this study secondary structures of the ITS2 transcript inCyclotella and Stephanodiscus were derived. The secondary structure seemed to be more conservative than the primary sequence. The higher-order structure shares conservation patterns, e.g., with green algae and flowering plants. However, Cyclotella could be used as an outgroup for rooting a phylogenetic tree concerning sister group relations within Stephanodiscus. Therefore, it is shown that Stephanodiscus neoastraea is closely related to S. hantzschii and not, as previously proposed, to the Stephanodiscus-niagarae-complex.

Key words:

• Cyclotella
• Bacillariophyta
• diatoms
• green algae
• internal transcribed spacer (ITS)
• molecular systematics
• phylogeny
• rRNA
• secondary structure
• Stephanodiscus
• yeast