Evidence for the co-evolution of BldB and Hpb pairs. The proportion of variable sites in sets of orthologous BldB amino acid sequences is plotted against the Hpb sequences from the same locus. For example, there are 12 variable sites in the 76 amino-acid alignment of BldB sequences from group 1 in Figure 5, while the corresponding 274 amino-acid alignment for Hpb group 1 has 71 variable sites. The presence of amino acid variability in BldB is positively correlated with variability in Hpb at the same locus (Spearman’s ρ = 0.609, p-value = 0.01605). Points for the locus that corresponds to the internally duplicated bldB genes are shown in blue.