Wednesday, 20 July 2011

Where Did Toxoplasma gondii Come From?

Toxoplasma gondii, one assumes, evolved as a parasite of cats. Is this a safe assumption? All the texts tell us that the cat is the only animal in which T. gondii completes the sexual phase of its life cycle, which is strong evidence for the cat being the original host. It is possible that there are other hosts in which T. gondii can produce gametocytes and reproduce sexually – maybe we just haven’t found them yet. Maybe we haven’t looked exhaustively either. Nonetheless, in the absence of evidence to the contrary, I’ll stick with the assumption that T. gondii evolved in cats.

T. gondii was first discovered in Tunisia in 1908. By coincidence, Northern Africa is also one of the places where the cat is thought to have been domesticated (there or present day Iraq) and this explains my somewhat illogical but long held assumption that T. gondii probably evolved in Africa. There is actually no reason why the parasite couldn’t have evolved in some other feline species and spread to domestic cats later.

One theory has it that T. gondii evolved in South America.

[caption id="attachment_309" align="alignleft" width="162" caption="Did prehistoric jaguars have T. gondii?"]Prehistoric cat statue from Guatemala, Simon Burchell[/caption]

The paper “Globalization and the Population Structure of Toxoplasma gondii,” reports an odd distribution of genotypes: one is found worldwide, one is found everywhere but South America, and several more are found only in South America. The authors' (Lehmann et al, PNAS July 25, 2006) interpretation is that there was an early split, originating in South America and leaving two populations to evolve in isolation from each other. One evolved in the “old world” and today is found everywhere but South America. The other is found everywhere, but apparently only spread very recently from its origins. Several more never left South America.

So maybe Toxoplasma gondii came from a prehistoric cat species in South America. It's a place to start.

(Image by Simon Burchell. Creative Commons)

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