The tangled taxonomic history of Dictyococcus, Bracteacoccus and Pseudomuriella (Chlorophyceae, Chlorophyta) and their distinction based on a phylogenetic perspective

Phylogenetically diverse algae, especially unicellular forms from similar habitats, can have nearly indistinguishable morphology when viewed with light microscopy. Consequently, historical taxonomy of many green coccoids based on light microscopy is not only inaccurate but also highly tangled because over time taxonomists have emphasized different traits. In the case of the three morphologically similar genera, Bracteacoccus, Dictyococcus and Pseudomuriella, we present an example of molecular data helping to resolve phylogenetic relationships of coccoids and facilitating the interpretation of morphology. The three genera are shown to be distinct lineages by our phylogenetic analysis of 18S and rbcL. sequence data. Our photomicroscopic observations suggest that Bracteacoccus and Pseudomuriella are morphologically cryptic genera, whereas Dictyococcus can be clearly distinguished by its chloroplast morphology. Additionally, we show that Bracteacoccus engadinensis and Dictyococcus schumacherensis are more closely related to Pseudomuriella aurantiaca than to other examined Bracteacoccus and Dictyococcus representatives, respectively. We therefore propose the transfer of these species to the genus Pseudomuriella.