Phylogenetic evidence for recombination in SAG5 locus in Toxoplasma gondii.

The present study evaluated the hypothesis that genetic diversity in SAG5 genes was generated by recombination events. Three lines of evidence suggested that recombination occurred in SAG5 genes in T. gondii. The permutation test revealed strong signature of intragenic recombination, pairwise comparisons of nucleotide sequences of SAG5 genes revealed that SAG5A alleles have chimerical structures composed of segments derived through recombination events between different alleles, and phylogenetic trees reconstructed based on SAG5 sequences using neighbor-joining and maximum parsimony methods, showed statistically well-supported consensus clusters of T. gondii strains specific to each SAG5 gene. Topological discrepancies between trees based on the N-terminal variable domain and C-terminal conserved domain sequences, were observed, suggesting intragenic recombinetion between SAG5A and SAG5B/C genes. The results showed that recombination within SAG5 in T. gondii was a major evolutionary mechanism generating both allelic variation at SAG5 locus and contributing to genotypic diversity and to emergence of new T. gondii variants, allowing them to evade the host immune defence mechanism.