A molecular test of cyanobacterial phylogeny: inferences from constraint analyses

Parsimony and neighbor-joining analyses of 16S rDNA nucleotide sequences of 68 species and strains of cyanobacteria and prochlorophytes supported a monophyletic Nostocales, a monophyletic Stigonematales, three independent lineages of prochlorophytes within the cyanobacteria, and a paraphyletic Chroococcales (p<0.0001) and Oscillatoriales (p = 0.0147). Within the Oscillatoriales, the genus Oscillatoria formed an unnatural taxon (p<0.0001) and needs major revisions. Using constraint analysis, the genus Microcystis was found to be monophyletic and the paraphyletic positions of Microcystis elabens and M. holsatica are probably due to long-branch attraction. Further, a separation of Chroococcales based on varying levels of polyunsaturated fatty acids is more consistent with nucleotide-based phylogenies than with existing morphological groupings. It is proposed that Chroococcales be redefined to exclude the genus Microcystis, and that a new order be erected for Microcystis. Finally, it is more parsimonious to assume a common cyanobacterial/prochlorophyte ancestor, than to evoke de novo synthesis of chlb in each prochlorophyte lineage plus in the lineage leading to green chloroplasts. This common ancestor is proposed to have contained both chlorophyll a and b plus phycobilins. Subsequent multiple losses of chlb in cyanobacteria and the loss of chla and phycobilins in prochlorophytes, led to the currently observed pigment distribution. It is therefore, recommended that Prochlorales be reclassified as cyanobacteria.