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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+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. [ABSTRACT FROM AUTHOR]

The phylogenetic affinities of Lobocharacium coloradoense were investigated by analysis of combined 18S and 26S rDNA data. Results from both parsimony and likelihood methods supported a close alliance among Lobocharacium , Characiosiphon , and Characiochloris. These three taxa formed a clade near the base of the “Dunaliella ” group within the chlamydomonad lineage. Protosiphon , which exhibits a siphonous habit similar to Characiosiphon and Lobocharacium , was not resolved as a close ally of the latter two taxa. The Lobocharacium alliance was characterized by the presence of an attachment pad associated with the nonmotile vegetative stage and pyrenoids that possess cytoplasmic invaginations. The pyrenoid feature is an ultrastructural trait that has now been observed in five different chlorophycean lineages. The Lobocharacium–Characiosiphon–Characiochloris clade is not predicted by any classifications of green algae. Additional taxon and data sampling need to be completed to resolve inconsistencies between the molecular phylogenetic evidence and at least some of the current family-level taxa. [ABSTRACT FROM AUTHOR]

Ultrastructural analyses of the flagellar apparatus suggested that Sphaeroplea, Atractomorpha, the Hydrodictyaceae, and the Neochloridaceae, all of which produce biflagellate motile cells with directly opposed (DO) basal bodies, are allied in an order Sphaeropleales. Recent studies of 18S rDNA sequence data supported an alliance of the DO group, but no data from Sphaeroplea and its allies were included. This investigation presented a test of the phylogenetic hypothesis suggested by the flagellar apparatus evidence using sequence data from the nuclear-encoded small-subunit rDNA (18S) and large subunit rDNA (26S) genes, combined with additional taxon sampling. Results from phylogenetic analyses weakly supported monophyly of biflagellate DO taxa and indicated that pyrenoids with cytoplasmic invaginations are present in numerous distinct lineages. Analysis of both molecular data sets supported a class Chlorophyceae comprised of at least six major groups that generally correspond to currently recognized orders or families: Chaetophorales, Chae- topeltidales, Chlamydomonadales, Sphaeropleales, Sphaeropleaceae, and Oedogoniales. In addition, Cylindrocapsa, Elakatothrix, Treubaria, and Trochiscia formed a seventh chlorophycean clade that is new to science. This investigation demonstrated that the 26S rDNA gene provides more phylogenetic signal, per unit sequence, than the 18S rDNA gene and that combined analysis yields topologies with more robust support than independent analysis of either data set. [ABSTRACT FROM AUTHOR]

Ettlia is a green algal genus comprised of several economically important species that have contributed to a taxonomic nightmare for phycologists - the recognition that coccoid green algae are polyphyletic on a grand scale. This taxonomic confusion is slowly being resolved with the aid of molecular evidence and, in some cases, new structural data. The most recent investigation using 18S ribosomal (r)DNA data corroborated an extraordinarily close alliance between Haematococcus pluvialis, a predominantly flagellate unicell, and the coccoid alga, Ettlia carotinosa. This putative evolutionary relationship between the green algal species H. pluvialis and E. carotinosa has, once again, placed coccoid algae at the center of systematic debate. The only apparent similarity between these two species is the ability to produce astaxanthin. Because the 18S rDNA results were not particularly robust, additional data from the internal transcribed spacer 2 rRNA and 26S rRNA genes were collected. Results from analyses of these data corroborated a close alliance between E. carotinosa and H. pluvialis. In addition, the 18S rDNA data set was expanded using new, published data from other species of Ettlia. Phylogenetic analyses of this updated matrix confirmed the existence of at least four distinct Ettlia lineages. These observations indicated that (1) additional taxonomic revision of the genus Ettlia is needed, (2) E. carotinosa need not be merged into the genus Haematococcus, (3) the case of H. pluvialis and E. carotinosa is an example of ciyptic similarity, and (4) the extraordinarily close alliance between E. carotinosa and H. pluvialis offers an ideal system to study patterns of gene expression and morphogenesis in unicellular eukaryotes. [ABSTRACT FROM AUTHOR]