Amino acid substitution rates in relation to bilaterian ancestor sequences. In each graph, the x-axes indicate mean numbers of substituted aa in comparison to ancestor sequences in each taxa, with standard deviations (error bars). A, Pax6 PD+HD; B, Zic ZF; C, actin; D, ATPS; E, Aldolase. Bracketed numbers left of the taxa names indicate the numbers of sequences subjected to the analysis. aa, length of aa sequences subjected to the analysis; Pinv, proportion of invariant aa among all tested sequences. The animal species are listed with their sequence accession numbers [see Additional file 2].