Investigating deep phylogenetic relationships among cyanobacteria and plastids by small submit rRNA sequence analysis

Small subunit rRNA sequence data were generated for 27 strains of cyanobact eria and incorporated into a phylogenetic analysis of 1,377 aligned sequenc e positions from a diverse sampling of 53 cyanobacteria and 10 photosynthet ic plastids. Tree inference was carried out using a maximum likelihood meth od with correction for site-to-site variation in evolutionary rate. Confide nce in the inferred phylogenetic relationships was determined by constructi on of a majority-rule consensus tree based on alternative topologies not co nsidered to be statistically significantly different from the optimal tree. The results are in agreement with earlier studies in the assignment of ind ividual taxa to specific sequence groups. Several relationships not previou sly noted among sequence groups are indicated, whereas other relationships previously supported are contradicted. All plastids cluster as a strongly s upported monophyletic group arising near the root of the cyanobacterial lin e of descent.