A polymorphism in a DNA polymerase alpha gene intron differentiates between murine virulent and avirulent strains of Toxoplasma gondii. 
The IC intron, found within the DNA polymerase alpha gene of Toxoplasma gondii, was used to evaluate the genetic relationship among 10 strains of T. gondii.  Sequence comparison detected polymorphisms within this 652 bp intron which correlated with murine virulence.  The results reported here suggest that T. gondii contains two lineages, corresponding with their virulence, evolving independently following their separation.  The extensive homology of the IC sequences within the virulent and avirulent groups affirms the close relationship of the strains within the group, as reflected by the identical nucleotide substitutions and dinucleotide insertions/deletions observed.  In addition, the presence of the Nde I restriction enzyme site within the IC intron of avirulent strains allows definition of a T. gondii strain as murine virulent or avirulent without needing to test it in vivo.