Phylogenetic tree of 16S rRNA genes from the four marine picocyanobacteria. Neighbor-joining tree with Kimura 2-parameter correction. The bootstrap value (1,000 replications) is shown in boldface. Lengths of the branches dA, dB, dC and dX (see text) are given below the branches. N1, node 1, branchpoint between MED4 and SS120; N2, node 2, branchpoint between MIT9313 and Node 1.