No abstract

Analyses of 115-2 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 115-2. 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.

The hydrodictyacean green algal lineage has been the focus of much research due to the fossil record of at least some members, their ornamented cell walls, and their distinctive reproductive strategies. The phylogeny of the family was, until recently, exclusively morphology based. This investigation examines hydrodictyacean isolates from several culture collections, focusing on sequences from ribosomal data: 18S rDNA, 26S rDNA (partial), and internal transcribed spacer (ITS)-2 data. Results from phylogenetic analyses of independent and combined data matrices support the Hydrodictyaceae as a monophyletic lineage that includes isolates of Chlorotetraedron, Hydrodictyon, Pediastrum, Sorastrum, and Tetraedron. Phylogenetic analyses of rDNA data indicate that the three-dimensional coenobium of Hydrodictyon is evolutionarily distinct from the three-dimensional coenobium of Sorastrum. The more robust aspects of the ITS-2 data corroborate the 18S þ 26S rDNA topology and provide a structural autapomorphy for the Hydrodictyaceae and Neochloridaceae, that is, an abridgment of helix IV in the secondary structure. The rDNA data do not support monophyly of Pediastrum but rather suggest the existence of four additional hydrodictyacean genera: Monactinus, Parapediastrum, Pseudopediastrum, and Stauridium.

A comparative study of the phylogeny of Scenedesmaceae based on rRNA gene sequences (ITS1/5.8S/ITS2), cell morphology and cell wall ultrastructure resulted in the acceptance of the genus Acutodesmus and the description of the new genera Verrucodesmus and Chodatodesmus. A new species Pectinodesmus holtmannii and 11 new combinations were erected: Chodatodesmus mucronulatus, Verrucodesmus verrucosus, V. parvus, Pectinodesmus pectinatus f. tortuosus, Acutodesmus bajacalifornicus, A. bernardii, A. deserticola, A. dissociatus, A. distendus, A. nygaardii, A. obliquus var. dactylococcoides. It was shown that the new genera Verrucodesmus and two of the Chodatodesmus strains have enlarged ITS2 helices (helix I in Verrucodesmus and helix III in Chodatodesmus). The occurrence of zoospores of Scenedesmaceae in nature was discussed.

No abstract