Toxoplasma gondii is a zoonotic parasite of many animals, including humans. Genetic comparison of parasites from a wide range of hosts from across the world has found that there are, in essence, just three clonal lines of this parasite. This is remarkable because T. gondii has a sexual cycle that occurs in its definitive cat host. Sexual reproduction in a population will tend to prevent the development of clonal lineages. Thus, the apparent paradox of clonal lineages in a sexual reproducing organism suggests that either sexual reproduction is happening very rarely or, if it is occurring, that there is a non-random mating pattern. Interestingly, the three clonal lineages of T. gondii have different infection phenotypes. Parasites from lineage I are highly virulent, indicated by their rapid killing of mice. Conversely, parasites from lineages II and III are relatively avirulent – they will not kill mice. Parasites that have established chronic infections in human and other hosts usually belong to lineages II and III.Grigg et al. have looked at these relationships further [1xSuccess and virulence in Toxoplasma as the result of sexual recombination between two distinct ancestries. Grigg, M.E. et al. Science. 2001; 294: 161–165Crossref | PubMed | Scopus (211)See all References[1]. From a survey of 15 loci, they found that there are, in essence, just two classes of alleles at these loci, although the variation between alleles varies substantially for different loci. For each locus, these two allelic classes are distributed randomly between parasites from the three lineages. Thus, for some loci, lineages I and II are of the same allelic class and lineage III is different; for other loci lineages I and III are of one allelic class and lineage II is different, and so on. Overall, this suggests that just two major, genetically distinct lines of parasites have led to the evolution of the three lineages now seen. The distribution of the allelic classes between parasites of each lineage suggests that the observed pattern is a result of allelic segregation rather than of genetic drift. Segregation is a consequence of sexual reproduction, hence the paradox (clonal lineages in a sexual reproducing organism) might be resolved; sexual reproduction between just two genetically distinct lines seems to have resulted in three clonal lineages of parasites. Parasite virulence is what matters to hosts – so what of the virulence in T. gondii? Grigg et al. have investigated the possibility that virulence arose as a result of allelic reassortment by crossing lines of parasites from lineages II and III (both avirulent), and testing the progeny for virulence. Out of 16 progeny parasite lines of the cross, 14 were avirulent. The remaining two progeny parasite lines were highly virulent – indeed, one was three orders of magnitude more virulent than either of its parents. It is probable that this virulent phenotype is a multi-locus trait, and therefore that its appearance in the progeny of a cross is simply a result of the reassortment of parental alleles at these loci.The virulence of parasites is a phenotype of interest to any potential host – including ourselves. This work shows that T. gondii virulent parasites can arise from avirulent parents, via the sexual phase of theT. gondii life cycle in the definitive cat host. This has the consequence that virulent parasite genotypes can be generated, which must be a cause for some concern. However, for crossing to occur, mixed infections would have to occur in the cat host and it is not clear how often this happens.