Evolutionary mysteries of parasites

Kevin Bradley - Mon Mar 03 00:00:00 AST 2014

Up close with plasmodium malariae. (Creative Commons photo)

Investigating the evolutionary history of humans and other vertebrates can be difficult, but diving into the evolution of microbes is a challenge unto itself.  

“Most biodiversity is invisible to us,” explained Patrick Keeling to a room full of Dalhousie’s top life science researchers and students. “Animals, plants and fungi are important, but there is a vast variety of microbes as well.”

Dr. Keeling, who obtained his PhD in biochemistry at Dalhousie, working with Dal researcher and recent Herzberg Medal winner Ford Doolittle, is currently a faculty member at UBC. Last month, he visited campus as part of the Governor General Lecture Series hosted by the Royal Society of Canada, the country’s national academy of distinguished scholars in the arts, humanities and sciences.

Dr. Keeling’s lecture was titled “Here Be Dragons,” due to the unknown and mysterious nature of the microbial world. He talked about plasmodium malariae, a parasitic single-cell protozoan that is responsible for infecting humans with malaria disease.

An evolutionary mystery


“Plasmodium is particularly special, because it gets inside our cells,” he explained. “This organism lives in the darkness of our bodies, so it was a surprise to find that it has a chloroplast. In plants and algae, photosynthesis takes place in the chloroplast. Why would an intracellular parasite, like malaria, contain the structure that plants use for photosynthesis? Where did the chloroplast come from?”

Dr. Keeling explained that one of the reasons chloropasts are special is that they come from a process called endosymbiosis, where one cell eats another and retains it, the two fusing together and become one cell. This process is also responsible for the energy-producing mitochondria in our own cells.

In the case of the malaria-causing plasmodium, the answer to how the microbe obtained its chloroplast was found in an unlikely place. Researcher Bob Moore isolated a photosynthetic unicellular organism called Chromera at stony corals off the coast of Australia. By sequencing part of the genome of Chromera, Dr. Keeling and his team discovered that it was the most closely related relative to plasmodium. It was revealed that Chromera had secondary endosymbiosis with red algae, which finally answered how plasmodium got its chloroplast.

An emphasis on exploration


The fact that the evolutionary history of a major disease-causing organism could be revealed by a chance discovery among coral reefs serves as a case study for the importance of basic exploratory research, explained Dr. Keeling.

“We all sort of acknowledge that basic research is important, but there’s also a level of exploration that’s necessary to find the right questions. Sometimes we need to get in the boat and go look at things to see what we can find.”


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