Abstract
Molecular phylogenetic analyses of available small ribosomal subunit RNA sequences from cyanobacteria (Cyanophyta) and oxychlorobacteria (Prochlorophyta) were conducted to infer evolutionary relationships among oxygenic photoautotrophic bacteria and plastids. Matrices of pairwise evolutionary distances estimated under two models of molecular evolution provided a basis for phylogenetic analysis using two methods of tree inference. The sequences fell into ten discrete groups of taxa that were subsequently examined more thoroughly using maximum likelihood, maximum parsimony, and distance matrix methods. — Results confirmed those of previous studies, including the deep branching of Pseudanabaena strains and Gloeoebacter violaceus, and the monophyletic grouping of heterocystous cyanobacteria. Several phenotypic characters traditionally used to classify cyanobacteria and oxychlorobacteria do not correlate with the phylogenetic relationships inferred from sequence analysis. In particular, the cyanobacterial genera Synechococcus and Leptolyngbya are clearly polyphyletic. Numerous other genera are paraphyletic. An unexpected result is an association of the pleurocapsalean cyanobacterium Chroococcidiopsis thermalis as a sister taxon to the heterocyst-forming nostocalean and stigonematalean cyanobacteria, distinctly apart from other pleurocapsalean cyanobacteria. — The plastids of photosynthetic eukaryotes constitute a monophyletic subtree within the cyanobacterial line of descent with no apparent relationship to any oxychlorobacteria which are themselves polyphyletic. No strong candidate for the sister taxon to plastids is indicated. Other molecular phylogenetic analyses are reviewed in the light of these results.
Published Version
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