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Malaria co-opted its symbiotic tools to become a parasite

by Lindsay Jolivet
Mar 2 / 15

The type of parasite that causes malaria was once algae that lived in symbiosis with other organisms. Now researchers have found that they had all of the genetic tools needed to evolve from symbiont to parasite — from friend to foe — all along.

Apicomplexans, the group of parasites that brought us malaria and toxoplasmosis, infect the cells of their animal hosts, latching on using spores from what look like small bags, and reproducing once inside. That may seem like quintessential parasite behaviour, but it isn’t, says CIFAR Senior Fellow Patrick Keeling (University of British Columbia).

“Basically you have a delivery system with bags of stuff, and you change what’s in the bags,” Keeling said during a presentation at the American Association for the Advancement of Science annual meeting. “The whole system existed long before parasitism. It just got co-opted into being used for parasitism.”

Keeling and his team sequenced parts of the genomes of several apicomplexan relatives, which are not parasites, but algae that live in coral or predators that eat other microbes. They found that all of the genes linked with parasitism exist in their friendlier relatives too.

“There wasn’t a new kit bag of ‘parasitology genes’ that were invented. It was more subtle than that,” said Keeling, who is part of CIFAR’s program in Integrated Microbial Biodiversity. This contradicts existing textbook theories about the origin of parasitism. The new results are published in Proceedings of the National Academy of Sciences.

The study also resolved a mystery about an organelle apicomplexans have been carrying around all this time, called a plastid. Plants and algae use plastids to help with photosynthesis, but apicomplexans lost photosynthesis a very long time ago; in fact, it was probably lost many times. But they never shed the plastid. “Malaria lives in red blood cells in our bodies. It’s in the dark. It doesn’t do photosynthesis —why does it have a plastid?” Keeling asked.

The plastid’s existence pointed the way toward the parasites’ photosynthetic past, and the sequencing revealed why the plastid is not a spare part today. An ancient apicomplexan ancestor lost a biochemical pathway that produced essential metabolites. The plastid also produces those metabolites, therefore almost all apicomplexans depend on them to survive.

So why did they become parasitic? The story remains incomplete, but existing clues suggest an ancient ancestor of malaria could have lived symbiotically in coral when it lost photosynthesis. “That left it with its beautiful invasion mechanism, which led quite naturally to becoming a parasite,” Keeling said.

CIFAR is supporting a field expedition for Keeling and other scientists to visit a coral reef research station and seek samples of other relatives of the malaria line to learn more about this strange family tree.