AN EARLY ORIGIN OF PLASTIDS WITHIN THE CYANOBACTERIAL DIVERGENCE IS SUGGESTED BY EVOLUTIONARY TREES BASED ON COMPLETE 16S RIBOSOMAL-RNA SEQUENCES

It is generally accepted that the plastids arose from a cyanobacterial ancestor, but the exact phylogenetic relationships between cyanobacte ria and plastids are still controversial. Most studies based on partia l 16S rRNA sequences suggested a relatively late origin of plastids wi thin the cyanobacterial divergence. In order to clarify the exact rela tionship and divergence order of cyanobacteria and plastids, we studie d their phylogeny on the basis of nearly complete 16S rRNA gene sequen ces. The data set comprised 15 strains of cyanobacteria from different morphological groups, 1 prochlorophyte, and plastids belonging to 8 s pecies of plants and 12 species of diverse algae. This set included th ree cyanobacterial sequences determined in this study. This is the mos t comprehensive set of complete cyanobacterial and plastidial 16S rRNA sequences used so far. Phylogenetic trees were constructed using neig hbor joining and maximum parsimony, and the reliability of the tree to pologies was tested by different methods. Our results suggest an early origin of plastids within the cyanobacterial divergence, preceded onl y by the divergence of two cyanobacterial genera, Gloeobacter and Pseu danabaena.