How microbes evolve from critter to killer

A group of small orange fish swim around a mini forest of yellow ocean plants and fungi. The water around them is brilliantly blue.

How microbes evolve from critter to killer

The University of British Columbia’s Patrick Keeling traces the evolution of parasites — including the one that causes malaria — to figure out how they went from benign microbes to agents of sometimes deadly disease
February 11, 2015
A plasmodium, a single-celled organism that causes malaria in humans, pictured through a microscope.

Plasmodium is a single-celled organism that causes
malaria in humans. By comparing Plasmodium with its
non-parasitic relatives, researchers in Patrick Keeling’s
lab at the University of British Columbia are re-
constructing the story of how it evolved from a free-
swimming organism into a deadly parasite.
Credit: Thomas Bresson/Computer Hotline, via
WikiMedia Commons

Pick up a litre of seawater and you’re holding tens of millions of unseen cells, yet most of these microbes are unknown to science. As humans, we tend to pay attention to only the small subset of microorganisms that directly affect us, usually by causing disease. Understanding why — and how — organisms evolve from harmless, unnamed lifeforms into deadly pathogens is the mission of Patrick Keeling and his team at the University of British Columbia.

Keeling’s lab studies protists, a group of single-celled organisms that are generally larger and more complex than bacteria. One well-known protist is Plasmodium, the parasite that causes malaria. But there are many more like it, surviving in everything from bats to shellfish, most of them still a mystery. “It’s a fair bet that there’s at least as many species of them as there are animals,” says Keeling. And that’s just the parasitic ones: other protists live independently in ponds, lakes, oceans or soil. In fact, based on its internal structures, we now know that Plasmodium evolved from a non-parasitic ancestor, one that produced energy from sunlight via photosynthesis, as plants do.

Funding from the Canada Foundation for Innovation has provided Keeling’s lab with advanced microscopy tools and DNA sequencing equipment, which they use in concert to help figure out the relationships between protists’ genes, appearance and behaviour. “You can watch these things move, even film them if you want, then pick up a single cell and sequence its genome,” says Keeling. Among other things, this allows the team to make connections between species that are genetically close, but that look and behave very differently.

A few years ago, Australian scientists found a group of photosynthetic algae living in coral reefs that were nevertheless closely related to Plasmodium. By studying these cousins of the parasites, Keeling and his team have come up with a theory about the common ancestor they once shared. It too was likely free-living and photosynthetic, but it also had a piercing structure that it used to attack and eat other cells. That same piercing structure allowed it to burrow into the cells of corals, where it could stay protected from predators. At first this relationship suited both parties; in return for protection, the photosynthetic protists produced food for their coral hosts, just as many algae that live within the cells of modern corals do today. But at some point, the protist lost its ability to make its own food from sunlight. At that point, it became a parasite, and over millions of years it evolved to infect other animals besides coral. Eventually, one of those hosts evolved into us.

Keeling and his colleagues still need to track down and study more members of Plasmodium’s family tree to confirm their theory. The insights could help in the fight against parasitic infections, for example, by showing drug-makers new biochemical pathways to target. But as Keeling says, “almost every significant scientific discovery has been made by accident.” We can’t predict exactly what we’ll learn from a better understanding of protist evolution, which is all the more reason to study it.

WATCH: Find out more about the Canada Foundation for Innovation.