Seeking the true Oscillatoria: a quest for a reliable phylogenetic and taxonomic reference point

Reliable taxonomy of any group of organisms cannot be performed without phylogenetic reference points. In the historical “morphological era”, a designated type specimen was considered fully sufficient but nowadays this principle can prove to be problematic and challenging especially when studying microscopic organisms. However, within the last decades there has been tremendous advancement in microscopy imaging and molecular biology offering additional data to systematic studies in ways that are revolutionizing cyanobacterial taxonomy. Unfortunately, most of the existing herbarium specimens or even iconotypes of old established taxa often cannot be subjects of modern analytic methods. Such is the case for the widely known cyanobacterial genus Oscillatoria which was introduced by Vaucher in 1803. In this study, we establish an epitype of the type species Oscillatoria princeps and provide an emended generic description based on data obtained from light as well as electron microscopy. We also present phylogenetic relationships of the genus Oscillatoria sensu stricto, a well-supported monophyletic clade, to other related cyanobacterial taxa using DNA sequence data of the 16S rRNA gene region. Our epitypification of Oscillatoria and the establishment of a phylogenetic DNA benchmark will aid in improving the understanding of cyanobacterial diversity and deeper level taxonomy.