Phylogenetic Relationships of Scenedesmus and Scenedesmus-like Coccoid Green Algae as Inferred from ITS-2 rDNA Sequence Comparisons

Abstract

Analyses of ITS-2 ribosomal DNA from Scenedesmus and Scenedesmus-like strains showed that this molecule is well suited for resolving evolutionary relationships at various taxonomic levels. The analyses clearly suggested the separation of the investigated strains into two groups which corresponded to the “subgenera” Desmodesmus and Scenedesmus/Acutodesmus. No clear distinction between Acutodesmus and Scenedesmus was seen. It is proposed to regard both groups as distinct genera, Desmodesmus gen. nov. and Scenedesmus sensu strictu, and to merge Acutodesmus into Scenedesmus. Monophyly of both genera, Scenedesmus and Desmodesmus, is further supported by certain secondary structure features of ITS-2. The relationships resolved by the ITS-2 analyses are congruent with findings from cell wall ultrastructure, but not with those from other morphological features such as cell wall surface structures or cell shape. Didymocystis inermis and Dicloster acuatus, though somewhat similar in vegetative cell morphology to species of Scenedesmus and Desmodesmus, were seen as members of independent genera because of substantial differences in primary structure and length variations in ITS-2 when compared with each other and the latter two genera.

Key words

Scenedesmus - Acutodesmus - Desmodesmus - Dicloster - Didymocystis - ITS rDNA phylogeny - ITS-2 rDNA secondary structure