Figure 3

Signatures of natural selection and genetic drift in endosymbiotic bacteria of insects. The strength of selection was determined as the ratio between non-synonymous nucleotide substitutions per non-synonymous site (d_N) and synonymous nucleotide substitutions per synonymous site (d_S) (ω=d_N/d_S) for each genome pair. (a) To determine the relative strength of selection on endosymbiotic genomes, we divided the ω of each symbiotic gene (ω_e) by that of its ortholog in its free-living bacterial relatives (ω_f) and compared this ratio (R) with 1. Values of R>1 imply that ω_e>ω_f, hence endosymbiotic genes evolved under relaxed selective constrains or under increased genetic drift. Conversely, R<1 implies stronger constrains on endosymbiotic genes than on their free-living bacterial orthologs. (b) The relative efficiency of natural selection, or genetic drift, for each of the endosymbiotic genomes of this study was compared. Differences were tested using Wilcoxon’s rank test with significant values being indicated with *P<0.05, **P<0.01 and ***P<10⁻⁶.