A US-led study of malaria parasites in wild apes in Africa suggests that the parasite that causes the most deadly form of the disease in humans, Plasmodium falciparum, did not come from chimpanzees as first thought but from gorillas.

You can read how lead investigator Dr Beatrice Hahn of the University of Alabama at Birmingham in the US, and colleagues from the US, the Republic of Congo, the Republic of Cameroon, France, and the UK, arrived at this conclusion in a paper published online in the journal Nature on 23 September.

Of the five species of human malaria-causing parasites carried by mosquitoes, P. falciparum is the most prevalent and the most deadly; it causes hundreds of millions of cases of malaria and more than one million deaths every year. However, its evolutionary roots are a much debated topic.

Until this study, it was thought that P. falciparum's closest relative was P. reichenowi, which infects chimpanzees, but this is based on small studies that only included a few apes, and even fewer in the wild.

For their study, Hahn and colleagues analysed the DNA of thousands of samples of droppings from wild-living African apes and discovered that the Plasmodium parasites most closely related to the human one are to be found in the western gorillas rather than chimpanzees or bonobos.

Their findings also suggest that all existing strains of the human form evolved from a single jump from one species to another.

Dr Daniel Jeffares, an evolutionary biologist at University College London, UK, who was not one of the authors of the paper, told Nature News that it would take more samples than the number used in this study to confirm that it was a one-off event that transmitted the parasite from gorillas to humans.

Nonetheless, he described the study findings as "striking", and that the paper was a "game changer" in terms of what we know about parasites.

Evolutionary biologist Paul Sharp of the University of Edinburgh, UK, a co-author of the paper, said they were now "wondering whether a cross-species jump like this could happen again in the future".

This could be a highly relevant question for today's anti-malaria campaigns, since this study also suggests there are potential Plasmodium reservoirs in apes living in the wild today.

In their investigation, Hahn, Sharp and colleagues identified and characterized Plasmodium species in nearly 3,000 fecal samples from wild apes living throughout central Africa. For their analysis they developed "a single-genome amplification strategy" which enabled them to search for specific DNA sequences of Plasmodium parasites.

They found evidence of Plasmodium infection in chimpanzees (Pan troglodytes) and western gorillas (Gorilla gorilla), but not in eastern gorillas (Gorilla beringei) or bonobos (Pan paniscus).

"Ape plasmodial infections were highly prevalent, widely distributed and almost always made up of mixed parasite species," they wrote.

The researchers analyzed more than 1,100 mitochondrial, apicoplast (a type of cellular organelle found in certain Plasmodium species) and nuclear DNA sequences from chimpanzees and gorillas and found that 99 per cent of them were "grouped within one of six host-specific lineages representing distinct Plasmodium species within the subgenus Laverania".

One of the Plasmodium species, from western gorillas, were nearly indentical to P. falciparum.

When they compared full-length mitchondrial sequences from human P. falciparum with the species from the gorilla specimens, they found they formed a "monophyletic lineage" indicating they have the same last ancestor (in other words, they are neighbouring twigs on the same branch of the evolutionary tree).

Hahn and colleagues concluded that:

"These findings indicate that P. falciparum is of gorilla origin and not of chimpanzee, bonobo or ancient human origin."

"Origin of the human malaria parasite Plasmodium falciparum in gorillas."
Weimin Liu, Yingying Li, Gerald H. Learn, Rebecca S. Rudicell, Joel D. Robertson, Brandon F. Keele, Jean-Bosco N. Ndjango, Crickette M. Sanz, David B. Morgan, Sabrina Locatelli, Mary K. Gonder, Philip J. Kranzusch, Peter D. Walsh, Eric Delaporte, Eitel Mpoudi-Ngole, Alexander V. Georgiev, Martin N. Muller, George M. Shaw, Martine Peeters, Paul M. Sharp, Julian C. Rayner & Beatrice H. Hahn.
Nature, Volume 467, Pages 420-425, Published online 23 September 2010.
DOI:10.1038/nature09442

Additional source: Nature News.

: Catharine Paddock, PhD

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