The parasite family tree: Tracing malaria’s evolution from a peaceful past

By Corey Buhay

SAN JOSE, Calif. — This is a story of betrayal.

Parasites have a reputation for their sinister side effects, and malaria is a clear example. There is a little explored skeleton in its closet, though — a link perhaps to a once-peaceful past.

Patrick Keeling of the University of British Columbia has taken a closer look at one of the malaria protozoan’s seemingly vestigial organelles: the plastid. Plastids are associated with photosynthesis, making their presence in blood-borne, darkness-dwelling pathogens suspicious. To better understand malaria, and the protozoa that cause it, Keeling and his lab are working to unravel the origin of this incongruous organelle. He presented their research at the 2015 American Association for the Advancement of Science annual meeting.

Malaria protozoa belong to a group of parasites called Apicomplexans — almost all of which have plastids. To figure out how Apicomplexans acquired those plastids, Keeling needed to trace their evolution back a step further. A breakthrough came when Dee Carter’s lab in Sydney, Australia, discovered photosynthetic algae related to Apicomplexans living in coral.

Keeling’s lab dove into gene sequencing. What they found was surprising: Not only had the plastids of those algae been previously misidentified, but what was once thought to be a novel phylum of bacteria turned out to be not bacteria at all, but plastids. What’s more, all the unknown, misidentified plastids were associated with coral reefs.

“This is not random, because coral reefs are poorly sampled. They’re a small fraction of the samples,” Keeling explained. The glaring correlation proved a definite link between coral and plastid development. It began to look as though coral reefs were the cradle of Apicomplexans’ rise to parasitism.

Apicomplexans are not unique in their residual plastids. It is actually common for photosynthetic species to lose that ability over evolutionary time.

“You can lose photosynthesis really easily even though it seems like something you really ought to keep,” Keeling said. Plastids, on the other hand, are hard to lose. The cell becomes dependent on metabolites the plastid produces, which makes it difficult for the structure to fade out of use, similar to the human appendix.

As Keeling and his lab compared Apicomplexans with similar organisms, they realized none of Apicomplexans’ body structures are actually unique. Plastids show up in plants and algae. Even the apparatus Apicomplexans use to parasitize humans appear in other protists. This structure is composed of “bags” that look like a bunch of deflated balloons.

“This basically allows an Apicomplexan to stick to a host … and shoot all sorts of stuff out of those bags,” Keeling said. One photosynthetic organism Carter’s lab came across, Chromera velia, may have used these bags to get inside the coral species where they maintain a symbiotic relationship.

“The whole delivery system predated the development of parasitism,” Keeling said. Those deflated balloons likely began as a tool for feeding, then became a tool for invading coral or other potential beneficiaries.

“So this organism had a way of getting inside these animal cells, and it probably honed this mechanism over time due to this beneficial symbiosis,” Keeling said. Eventually symbiotic relationships began to falter. Apicomplexans found themselves benefiting less from those partnerships, but they were already equipped with all the tools of invasion. The path of least resistance was a short evolutionary journey to parasitism.

The Apicomplexans betrayed their hosts and turned the tools of their trade to malicious ends. The betrayal was complete, but because of the plastid, evidence of the crime remained. It was a mystery that requires investigation to unravel, but Keeling and his lab are willing detectives.

Corey Buhay is a junior at the University of North Carolina at Chapel Hill where she studies environmental science. She is an avid hiker and backpacker, a writer, and a chemistry enthusiast. Follow her on Twitter at @coreylbuhay.

October 2, 2015

Drexel University Online