Variable selective pressure along the Sca1 sequences. The x axis represents the alignments of the Sca1 sequences. The predicted passenger and autotransporter domains expand from positions 1 to 800, and from positions 801 to the end, respectively. The N-terminal signal peptide is not included in the alignment because the sequence was not determined for some of the Rickettsia species under study. The y axis represents the percent of invariable sites per window of 50 sites attributed to each amino acid position in the Sca1 protein alignment. Predicted positively selected sites and their mean of ϖ (calculated as the average of ϖ over the 11 site classes of model M8, weighted by the posterior probabilities; right axis) appear as stars. The curve represents the averages of means of ϖ calculated for sliding windows of 50 codon sites (right axis).