Figure 5.

The equilibrium GC content under the mutation bias model, reproduced from Hildebrand et al.¹³³ Plot compares GC4 (current GC content at 4-fold degenerate sites) to GC4_pred (GC content at 4-fold degenerate sites predicted under mutational equilibrium, estimated from intraspecific analysis), across numerous bacterial genomes spanning eight phyla. Estimates of GC4_pred are based on the general approach described in Figure 4, but calculated at 4-fold generate sites only. The wide range of GC4_pred across bacterial genomes suggests variation in relative per-site mutational bias. Deviation between GC4_pred and GC4 indicates that mutation alone cannot explain current base composition and suggests that selection play an important role. original reference ¹³³ for a discussion of alternative explanations to selection, such as biased gene conversion (BGC), and evidence against these alternatives. While this figure suggests that AT-rich bacteria also show a deviation between GC4_pred and GC4, and in the opposite direction (less GC-rich than expected under mutation), this deviation was not significant among AT-rich genomes after adjusting for the infinite sites model.¹³³ The point corresponding to the aphid mutualist Buchnera is enclosed in a square, for reference. Other AT-rich bacteria shown include intracellular pathogens.