A new perspective on and re-assessment of SAG2 locus as the tool for genetic analysis of Toxoplasma gondii isolates.

SAG2 locus, the coding gene of the P22 protein, has been widely used for the molecular epidemiology of Toxoplasma gondii and characterization of the parasite isolates with two separate polymerase chain reaction (PCR)-restriction fragment length polymorphism (RFLP) processes. To re-assess the resolution power and suitability of this genetic marker for molecular characterization of the parasite isolates, a number of 27 Toxoplasma strains from different zymodeme patterns were used in the present study. Both codon and non-codon regions of the SAG2 locus of all 27 strains were amplified and subjected to sequencing and nucleotide alignment. Nucleotide variations clustered the three major genotypes (I, II and III). Some minor genotypes, unidentifiable by SAG2-RFLP, could be identified by sequence comparison. However, there were other genotypes that could not be differentiated from the major types due to having identical sequences. This suggests that a remarkable number of field isolates representing several minor types will be miss-clustered with the major types by using the traditional SAG2-PCR-RFLP method. It was concluded that this technique seems not to be suitable for Toxoplasma population study. Thus, the utilization of more variable markers and other discriminatory methods are also recommended.