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BIOLOGICAL SCIENCES / POPULATION BIOLOGY
Recent transcontinental sweep of Toxoplasma gondii driven by a single monomorphic chromosome

A. Khan, B. Fux, C. Su, J. P. Dubey, M. L. Darde^¶, J. W. Ajioka^||, B. M. Rosenthal^,, and L. D. Sibley^,

Department of Molecular Microbiology, Washington University School of Medicine, 660 South Euclid Avenue, St. Louis, MO 63130; Department of Microbiology, University of Tennessee, Knoxville, TN 37996; Animal Parasitic Disease Laboratory, Animal and Natural Resources Institute, Agricultural Research Service, U.S. Department of Agriculture, Beltsville, MD 20705; ^¶Faculté de Médecine, EA3174, Biological Resource Center for Toxoplasma, 87042 Limoges, France; and ^||Department of Pathology, University of Cambridge, Tennis Court Road, Cambridge CB2 1QP, United Kingdom

Edited by John J. Mekalanos, Harvard Medical School, Boston, MA, and approved August 1, 2007 (received for review March 13, 2007)

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 that are mutually exclusive from polymorphisms 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 from a common ancestor in isolation over a period of 10⁶ yr, and 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 reestablishment 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 population sweep of a limited number of highly successful T. gondii lineages.

biogeography | evolution | pathogen | transmission | virulence

Author contributions: A.K., B.F., C.S., and L.D.S. designed research; A.K., B.F., and C.S. performed research; A.K., C.S., J.P.D., M.L.D., J.W.A., and B.M.R. contributed new reagents/analytic tools; A.K., J.W.A., B.M.R., and L.D.S. analyzed data; and B.M.R. and L.D.S. wrote the paper.

The authors declare no conflict of interest.

This article is a PNAS Direct Submission.

This article contains supporting information online at www.pnas.org/cgi/content/full/0702356104/DC1.

To whom correspondence may be addressed. E-mail: benjamin.rosenthal{at}ars.usda.gov or sibley{at}wustl.edu

© 2007 by The National Academy of Sciences of the USA

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PNAS 2007 104: 14545-14546. [Full Text]