Molecular phylogeny of Staurastrum meyen ex Ralfs and related genera (Zygnematophyceae, Streptophyta) based on coding and noncoding rDNA sequence comparisons

Nuclear-encoded small subunit rDNA, 1506 group I intron, and internal transcribed spacer sequences were obtained from 39 strains representing five core desmid genera, Staurastrum, Staurodesmus Teil., Cosmarium Corda ex Ralfs, Xanthidium Ehr. ex Ralfs, and Euastrum Ehr. ex Ralfs (Desmidiaceae, Zygnematophyceae), and used individually and concatenated to assess phylogenetic relationships between putatively allied members of the family. To identify positional homology between divergent noncoding sequences, secondary structure models were generated and their reliability assessed by screening the alignment for compensating base changes. The phylogeny based on coding and noncoding sequence comparisons confidently resolved a monophyletic core of the genus Staurastrum but also revealed the artificial nature of the traditional genus. Twenty distinct species representing a wide range of morphotypes of Staurastrum formed a strongly supported generic clade that was further split into three well-resolved lineages. The phylogenetic relationships revealed within Staurastrum were in conflict with all previous formal or informal classifications of the genus. The genera Staurodesmus and Cosmarium were shown to be highly polyphyletic, and some morphologically similar taxa displayed high sequence divergence that exceeded generic boundaries. Apparently, the taxonomic significance of some morphological characters in Staurastrum and other desmid genera has been greatly overestimated.