Phylogenetic relationships of Scenedesmus and Scenedesmus-like coccoid green algae as inferred from ITS-2 rDNA sequence comparisons

Analyses of ITS-2 ribosomal DNA from Scenedesmus and Scenedesmus-like strai ns showed that this molecule is well suited for resolving evolutionary rela tionships at various taxonomic levels. The analyses clearly suggested the s eparation of the investigated strains into two groups which corresponded to the "subgenera" Desmodesmus and Scenedesmus/Acutodesmus. No clear distinct ion 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 gene ra, Scenedesmus and Desmodesmus, is further supported by certain secondary structure features of ITS-2. The relationships resolved by the ITS-2 analys es are congruent with findings from cell wall ultrastructure, but not with those from other morphological features such as cell wall surface structure s or cell shape. Didymocystis inermis and Dicloster acuatus, though somewha t similar in vegetative cell morphology to species of Scenedesmus and Desmo desmus, were seen as members of independent genera because of substantial d ifferences in primary structure and length variations in ITS-2 when compare d with each other and the latter two genera.