Molecular genetic divergence of the centric diatom Cyclotella and Discostella (Bacillariophyceae) revealed by nuclear ribosomal DNA comparisons

Abstract

The centric diatom Cyclotella, including the recently separated Discostella, is commonly present in freshwater and several species are important bio-indicators. Here, we describe molecular characteristics of the nuclear rDNA, spanning 18S to D1/D2 region of the 28S rDNA, of two genera Cyclotella and Discostella, particularly using Korean isolates of C. meneghiniana, Discostella sp. c.f. D. pseudostelligera. Molecular and morphological analyses showed that our isolates had nearly identical genotypes in rDNA and similar morphology as compared to presumably the same species from other geographical areas. Phylogenetic analyses of individual 18S and partial 28S rDNAs of Cyclotella sensu lato showed that all sequences were separated into two clades: one containing Cyclotella, the other Discostella including C. ocellata and C. bodanica. Statistical tests with pairwise genetic distance scores showed that the two genera were significantly different (one-factor ANOVA, p < 0.01). In addition, divergence in the partial 28S rDNA was significantly high (p < 0.01) as compared to 18S rDNA. This provides evidence that the two genera, Cyclotella and Discostella, belong to genetically well-separated groups. In addition, 28S rDNAs is a more suitable molecular marker for the discrimination of Cyclotella sensu lato.