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Letter

Common inheritance of chromosome Ia associated with clonal expansion of Toxoplasma gondii

¹Department of Molecular Microbiology, Washington University School of Medicine, St. Louis, Missouri 63110, USA; ²The Wellcome Trust Sanger Institute, The Wellcome Trust Genome Campus, Hinxton, CB10 1SA, United Kingdom; ³Department of Pathology, University of Cambridge, Cambridge, CB2 1QP, United Kingdom; ⁴MRC Biostatistics Unit, Institute of Public Health, Cambridge, CB2 2SR, United Kingdom; ⁵National Research Center for Protozoan Diseases, Obihiro University of Agriculture and Veterinary Medicine, Inada-cho, Obihiro, Hokkaido, 080-8555, Japan; ⁶Department of Parasitology, Institute of Medical Science, The University of Tokyo, 4-6-1, Shirokanedai, Minato-ku, Tokyo, 108-8639, Japan; ⁷Department of Medical Genomics Sciences, Graduate School of Frontier Sciences, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8562, Japan; ⁸The Institute for Genomic Research, Rockville, Maryland 20850, USA; ⁹Department of Biology, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA

Toxoplasma gondii is a globally distributed protozoan parasite that can infect virtually all warm-blooded animals and humans. Despite the existence of a sexual phase in the life cycle, T. gondii has an unusual population structure dominated by three clonal lineages that predominate in North America and Europe, (Types I, II, and III). These lineages were founded by common ancestors 10,000 yr ago. The recent origin and widespread distribution of the clonal lineages is attributed to the circumvention of the sexual cycle by a new mode of transmission—asexual transmission between intermediate hosts. Asexual transmission appears to be multigenic and although the specific genes mediating this trait are unknown, it is predicted that all members of the clonal lineages should share the same alleles. Genetic mapping studies suggested that chromosome Ia was unusually monomorphic compared with the rest of the genome. To investigate this further, we sequenced chromosome Ia and chromosome Ib in the Type I strain, RH, and the Type II strain, ME49. Comparative genome analyses of the two chromosomal sequences revealed that the same copy of chromosome Ia was inherited in each lineage, whereas chromosome Ib maintained the same high frequency of between-strain polymorphism as the rest of the genome. Sampling of chromosome Ia sequence in seven additional representative strains from the three clonal lineages supports a monomorphic inheritance, which is unique within the genome. Taken together, our observations implicate a specific combination of alleles on chromosome Ia in the recent origin and widespread success of the clonal lineages of T. gondii.

¹¹ Corresponding author.

E-mail ja131{at}cam.ac.uk; fax 44-1223-333346.

Article published online before print. Article and publication date are at http://www.genome.org/cgi/doi/10.1101/gr.5318106

Supplemental material is available online at www.genome.org. The sequence data from this study have been submitted to EMBL under accession nos. AM055942 [GenBank] and AM055943 [GenBank] . Accession numbers for full-length cDNAs associated with chromosomes Ia and Ib are AK223637 [GenBank] –AK223774 [GenBank] .

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