Success and virulence in toxoplasma as the result of sexual recombination between two distinct ancestries

Toxoplasma gondii is a common human pathogen causing serious, even fatal, d isease in the developing fetus and in immunocompromised patients. Despite i ts ability to reproduce sexually and its broad geographic and host range, T oxoplasma has a clonal population structure comprised principally of three lines. We have analyzed 15 polymorphic loci in the archetypal type I, II, a nd III strains and found that polymorphism was limited to, at most, two rat her than three allelic classes and no polymorphism was detected between all eles in strains of a given type. Multilocus analysis of 10 nonarchetypal is olates likewise clustered the vast majority of alleles into the same two di stinct ancestries. These data strongly suggest that the currently predomina nt genotypes exist as a pandemic outbreak from a genetic mixing of two disc rete ancestral lines. To determine if such mixing could lead to the extreme virulence observed for some strains, we examined the F-1 progeny of a cros s between a type II and III strain, both of which are relatively avirulent in mice. Among the progeny were recombinants that were at least 3 logs more virulent than either parent. Thus, sexual recombination, by combining poly morphisms in two distinct and competing clonal lines, can be a powerful for ce driving the natural evolution of virulence in this highly successful pat hogen.