Toxoplasma gondii is a parasitic protozoan that has a widespread subclinical seropositivity and which can cause a severe — and often fatal — infection in immunocompromized individuals. Most infections are due to one of the three asexually propagated clonal strains, but the mechanisms that led to these lines of evolution has been unclear. In the January 17 Science, C. Su and colleagues at Washington University, St. Louis, USA, show that clonal expansion of T. gondii resulted from the concurrent acquisition of direct oral infectivity and bypass of the sexual phase (Science, 299:414-416, January 17, 2003).

C. Su et al. performed genetic polymorphism analyses and compared the present clonal lineages of T. gondii with ancient strains of around 1 million years. They observed that the present clonal lineages emerged within the last 10,000 years after a single genetic cross. In addition, they showed that the progeny were successful...

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