Molecular phylogeny of baculiform desmid taxa (Zygnematophyceae)

The baculiform, rod-like morphotypes belong to several phylogenetic lineages within Desmidiaceae (Zygnematophyceae, Viridiplantae). Some, for example the genus Pleurotaenium, form independent lineages, but reductive evolution of complicated desmid cells toward baculiform morphology also occurred in individual lineages, for example Micrasterias. In this genus, the rod like Triploceras forms evolved from more complex ancestors. In this study, we tested for an independent position of the subtropical and tropical genus Triplastrum, previously separated from Triploceras on the basis of morphological data. In addition, monophyly of Pleurotaenium was also investigated with multiple isolates corresponding to seven species of this genus, including the morphologically dissimilar P. nodosum and P. ovatum. Finally, two isolates of Docidium baculum were also investigated. Molecular phylogenetic analysis of concatenated rbcL?+?coxIII sequence data implied that the baculiform taxa investigated were in three distantly related positions within Desmidiaceae. The genus Triplastrum proved to be unrelated to Triploceras, because it clustered in the “omniradiate” lineage of Desmidiaceae among morphologically dissimilar taxa. The genus Pleurotaenium was monophyletic, but P. ovatum was recovered in a weakly supported sister position to all the other members of the genus. The trnG^ucc phylogeny of Pleurotaenium taxa concurred with the rbcL?+?coxIII phylogram, and generally revealed the poor morphological concepts of some species in this genus. The most common taxa P. ehrenbergii and P. trabecula were resolved as polyphyletic because their strains were distributed among several strongly supported clades. However, strains of P. nodosum and P. archeri formed separate, well supported lineages within the genus.