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How did a Toxoplasma chromosome spread globally?

A parasite of animals (most notably cats) and humans alike, Toxoplasma gondii can boast a global distribution. But it’s not exactly the same organism the world over: one clonal type predominates in Europe and North America and another leads the pack in South America. One thing they do have in common is a chromosome called Chrla, which appears to interact with a major T. gondii virulence factor. Chrla is present in T. gondii isolates from all these regions. So, even though T. gondii from North America and Europe has many marked differences from South American isolates, you can still find Chrla in all those locales.

How did this come about? Understanding the population structure of T. gondii helps us understand how it acquires and spreads the various elements that enable it to cause disease, so the authors of a paper in mBio this week looked closely at the genome of T. gondii isolates to try and figure it out.

They found that Chrla is very similar in North America/Europe and in South America, which suggests that the chromosome has been transferred recently between the diverse T. gondii populations in South America and into the T. gondii that subsequently expanded clonally in North America and Europe. The fact that Chrla is very similar in both South America, where T. gondii propagates sexually, and in North America/Europe, where it propagates asexually, suggests that it confers a selective advantage no matter how the parasite propagates.

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