Molecular phylogeny of the heterocystous cyanobacteria (subsections IV and V) based on nifD

Department of Botany, Miami University, Oxford, OH 45056, USA Correspondence Susan R. Barnum barnumsr{at}muohio.edu The heterocystous cyanobacteria are currently placed in subsections IV and V, which are distinguished by cellular division in one plane (false branching) and in more than one plane (true branching), respectively. Published phylogenies of 16S rRNA gene sequence data support the monophyly of the heterocystous cyanobacteria, with members of subsection V embedded within subsection IV. It has been postulated that members of subsection V arose from within subsection IV. Therefore, phylogenetic analysis of nucleotide sequences of the nitrogen-fixation gene nifD from representatives of subsections IV and V was performed by using maximum-likelihood criteria. The heterocystous cyanobacteria are supported as being monophyletic, with the non-heterocystous cyanobacteria as their closest relative. However, neither subsection IV nor subsection V is monophyletic, with representatives of both subsections intermixed in two sister clades. Analysis of nifD does not support recognition of two distinct subsections. Published online ahead of print on 31 October 2003 as DOI 10.1099/ijs.0.02821-0. The GenBank/EMBL/DDBJ accession numbers for nifD sequences determined in this study are AY196951 – AY196958 . A table showing the current taxonomic placement of the heterocystous cyanobacteria and an alignment of nifD gene sequences are available as supplementary material in IJSEM Online.