Population Strucure and Virulence of T. gondii Figure 6. Evolutionary history of the upstream region in T. gondii compared to the out-group N. caninum. Pairwise sequence comparisons of genomic sequences between T. gondii strains and N. caninum showed the region surrounding ROP18 is highly syntenic with preservation of gene order and intergenic regions in N. caninum. The upstream region (UPS) of type III ROP18 (VEG) is found in N. caninum but is absent in type I (GT1) and type II (ME49), indicating that it was retained by type III* but lost in the ancestor to types I* and II*. The small region of dissimilarity between type I (GT1) and type II (ME49) corresponds to a slight difference in the rearrangements that presumably occurred in these lineages as shown in Figure 5A. X-axis is length in bp. Y-axis indicates the similarity values between two strains. doi:10.1371/journal.pgen.1000404.g006 PLoS Genetics | www.plosgenetics.org 9 March 2009 | Volume 5 | Issue 3 | e1000404