Patterns and role of diversifying selection in the evolution of Toxoplasma gondii SAG5 locus

The higher intergenotypic polymorphism of the surface antigen genes 5 (SAG5)A, SAG5C, and SAG5E in Toxoplasma gondii was proposed to be the outcome of positive selection pressure favoring variation within these loci. However, the exact nature and magnitude of this selection is not completely known. To address this issue, the amino acids on which natural selection may operate were identified by comparing the ratios of nonsynonymous and synonymous substitutions (pN/pS) of homologous DNA sequences in strains belonging to the three major genotypes of T. gondii. Both positive and negative selections were detected and are likely to have contributed to shaping the patterns of nucleotide substitution and polymorphism in SAG5 genes. Several sites identified in SAG5 loci as likely to be under positive selection suggesting that diversifying selection may have promoted divergence in these genes. Also, it was noted that the SAG5 genetic loci contain many areas that exhibit signs of purifying selection; some of these areas might be the attractive candidates for drug targets. Phylogenetic analysis using the neighbor-joining and maximum parsimony methods grouped the SAG5 sequences of T. gondii strains into three distinct statistically well-supported evolutionary lineages. These findings carry important implications for human and veterinary toxoplasmosis epidemiology and may provide important insights into the pathways through which virulence has evolved in T. gondii.