Trypanosoma brucei, a protozoan that causes African sleeping sickness, is a bizarre product of symbiosis: An ancient ancestor of the parasitic organism swallowed a microscopic alga and evolved into a deadly plant-animal hybrid. Molecular parasitologist Fred Opperdoes of the Catholic University of Louvain in Belgium reached this conclusion after scrutinizing T. brucei's genome.
The microbe swims freely through the blood and feeds off its host, behaving like the single-celled animal it is, but it produces enzymes and lipids thought to be exclusive to plants. Opperdoes and his colleagues found that the parasite's DNA harbors at least 16 plant genes, including one that codes for an enzyme that plants use to produce sugar via photosynthesis. In T. brucei, the enzyme works instead to break down sugar sucked from the victim's blood. "A billion years ago, a primitive, free-living, amoebalike protozoan ate an alga, and the alga stayed inside," Opperdoes says. "This was a tremendous advantage, because an organism that previously had to swim around to find its food was now capable of photosynthesis. Later it turned into a parasite and the alga was lost, but some of its genes moved into the nucleus." This discovery could lead to novel ways to fight T. brucei. Herbicides that attack the plant-based part of the genome could finally spell the hardy parasite's downfall.