Codon richness index (CRI) for rhizobial genomes. All gene classifications were based on comparisons against S. meliloti, unless explicitly stated otherwise. Panels: (a), S. meliloti (compared with A. tumefaciens). (b), A. tumefaciens. (c), M. loti. (d), B. melitensis. CRIs were calculated according to the method described by Medrano-Soto et al. . Symbols: red circles, syntenic genes. Blue plus signs, non-syntenic. Green squares, homologs in plasmids. Gray crosses, species-specific genes. Horizontal lines denote the species-specific thresholds for low and high CRI.