Systematics of primary osmotrophic euglenids: a molecular approach to the phylogeny of Distigma and Astasia...

Nuclear-encoded SSU rRNA genes from nine strains of Distigma and three strains of Astasia were sequenced and analysed phylogenetically with maximum-likelihood and maximum-parsimony methods. It could be demonstrated that the genus Distigma is paraphyletic, consisting of two distinct clades: one comprises four strains of the type species, Distigma proteus, and the other includes four strains of Distigma curvatum, Distigma gracile, Distigma sennii and Distigma elegans. These findings are well corroborated by morphological characteristics. The investigated species of Astasia are closely related to members of the Rhabdomonadida, thus rendering the genus Astasia polyphyletic, with Astasia longa branching within the phototrophs. All of the species investigated cluster in a well-supported group of primary osmotrophic euglenids that are not derived from photosynthetic ancestors. The recovered clades are characterized by their sequence diversity. After different evolutionary rates among lineages had been determined, a modified slow-fast approach was used to differentiate phylogenetic signal from noise. Finally, a revised systematic scheme based on phylogenetic relationships is suggested to render euglenid taxonomy more transparent: primary osmotrophic euglenids are classified as Aphagea, and members of the D. curvatum group are transferred into the new subgenus Parvonema.