Phenotypic variability and phylogenetic relationships of the genera Tolypothrix and Calothrix (Nostocales, Cyanobacteria) from running water

The taxonomy of heterocystous cyanobacteria belonging to the genera Calothrix and Tolypothrix has long been a matter of debate, but their phylogenetic relationships are still not well understood. Our aim was to compare the phylogeny and morphology of members of these genera, which exhibit basal-apical polarity. A phylogeny was reconstructed on the basis of 16S rRNA gene sequences and compared with the morphological characterization of new isolates and environmental samples. Strains isolated from several rivers and streams showed a high degree of tapering when they were cultured in a nutrient-rich medium. However, clear differences were apparent when they were transferred to a nutrient-poor medium. Some strains showed a low degree of tapering and other morphological features corresponding to the genus Tolypothrix, such as false branching, whereas others maintained the morphological characteristics of the genus Calothrix. Phylogenetic analysis was congruent with the phenotypic characterization, in which the strains and environmental samples of the Tolypothrix and Calothrix morphotypes could be clearly separated. Isolates with a low degree of tapering and natural samples of Tolypothrix distorta were grouped in the same cluster, but strains of the genus Calothrix fell into well separated clades. Results from this study showed that representatives of the genus Tolypothrix share most morphological and developmental properties and a high degree of 16S rRNA gene sequence similarity. However, although similar and sometimes overlapping morphologies may occur in isolates of the genus Calothrix, these morphotypes may be distinguished on the basis of their clear genetic divergence.