Sweeping Through Toxoplasma

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Science  21 Sep 2007:
Vol. 317, Issue 5845, pp. 1651-1653
DOI: 10.1126/science.317.5845.1651d

Though particularly notorious for its association with cats, the protozoan parasite Toxoplasma gondii is ubiquitous among vertebrates, to the extent that a quarter of the human population is infected. Unusually, it propagates both sexually and asexually, but tends to have a clonal population structure with three geographically distinct lineages. Unexpectedly, all lineages share a nearly monomorphic version of one chromosome, which has become fixed in natural populations. Khan et al. have traced the population structure of T. gondii by sequencing introns and find that South American strains show variation not seen in other lineages. The date of this divergence correlates with the reappearance of the Panamanian land bridge roughly 1 million years ago and the southerly migration and diversification of the Felidae. The lineages appear to have evolved in two phases, first by the generation of northern and southern haplogroups emerging by rare recombination events, and second by a global sweep spreading the monomorphic chromosome into the South American haplogroups. The selective advantage of the monomorphic chromosome has yet to be revealed, although it is probably involved in virulence and transmission. — CA

Proc. Natl. Acad. Sci. U.S.A. 104, 14872 (2007).

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