Submitted to: Proceedings of the National Academy of Sciences
Publication Type: Peer reviewed journal
Publication Acceptance Date: 8/1/2007
Publication Date: 10/1/2007
Citation: Khan, A., Fux, B., Su, C., Dubey, J.P., Darde, M.L., Ajioka, J.W., Rosenthal, B.M., Sibley, L.D. 2007. Recent global swee[p of Toxoplasma gondii driven by a single monomorphic chromosome. Proceedings of the National Academy of Sciences. 104:14872-14877. Interpretive Summary: Toxoplasma gondii is a prevalent parasite of humans and other vertebrates, and certain strains are particularly dangerous to human health. We have characterized genetic variability among a broad sampling of Toxoplasma gondii isolates, revealing strains endemic to North and South America, respectively. In each region, certain strains have propegated as asexual clones, although sexual recombination has played a greater role in diversifying some South American lineages. In striking contrast to these patterns, a particular version of one chromosome has become established in distinct genomic backgrounds, both north and south of the Central American Isthumus. We do not yet understand why this chromosomal variant has experienced such disproportionate success, but its gene content points towards possible explanations that may unlock keys to the widespread prevalence of this important zoonotic parasite.
Technical Abstract: Toxoplasma gondii is a highly prevalent protozoan parasite that infects a wide range of animals and threatens human health by contaminating food and water. A markedly limited number of clonal parasite lineages have been recognized as predominating in North American and European populations, whereas strains from South America are comparatively diverse. Here, we show that strains from North America and Europe share distinct genetic polymorphisms and these are nonoverlapping with polymorphisms seen in strains from the South. A striking exception to this geographic segregation is a monomorphic version of one chromosome (Chr1a) that characterizes virtually all northern and many southern isolates. Using a combination of molecular phylogenetic and phenotypic analyses, we conclude that northern and southern parasite populations diverged in isolation over a period of ~106 yrs, but that the monomorphic Chr1a has swept each population within the past 10,000 years. Like its definitive feline hosts, T. gondii may have entered South America and diversified there after re-establishment of the Panamanian land bridge. Since then, recombination has been an infrequent but important force in generating new T. gondii genotypes. Genes unique to a monomorphic version of a single parasite chromosome may have facilitated a recent global sweep of a limited umber of highly successful T. gondii lineages.