But what happens when new research suggests that long-standing relationships between some species aren’t what we originally thought? A new study led by Museum scientists presents the most comprehensive tree of life for malaria parasites to date—and reveals that microorganisms in the diverse genus Plasmodium, which includes hundreds of species, contains groups that are only distantly related. But because Plasmodium is such a well-known genus, regrouping and renaming based on the newly mapped relationships isn’t imminent.
“If we worked on a group of fishes or beetles, we would just split them up and put them into new genera,” said Susan Perkins, curator in the Museum’s Division of Invertebrate Zoology and co-author of the new study, out today in the journal Royal Society Open Science. “But to do this based on our tree would involve changing the name of the most deadly protozoan parasite in the world, and there’s too much inertia working against us. We have to be bad taxonomists in this case and let it continue to be called Plasmodium.”
Malaria parasites can infect a range of animals, including mammals, birds, and reptiles. Of the 500 species currently described there are five known to infect humans, which happens when female mosquitos in the genus Anopheles bite and transmit the parasite through the salivary gland into the host’s blood. But since many species of this parasite are only deadly to some animals and not others, tracking the relationships between them can be challenging for researchers, and extra care must be taken in analysis to account for the fact that the chemical base of the parasites’ genome is often biased toward A (adenine) and T (thymine) couples.
To build the new malaria tree, Perkins and colleagues sampled 58 Plasmodium species from eight of the currently recognized genera of malaria parasites, and utilized DNA sequence data from more than 20 genes. To account for the genome imbalance—which can affect up to 80 percent of the parasite’s genome—Spencer Galen, a comparative biology Ph.D.-degree student in the Museum’s Richard Gilder Graduate School who led the Museum’s research team, and Janus Borner, a research fellow from the German Academic Exchange Service's PRIME fellowship, used new genetic markers developed by Borner to include malaria species from evolutionary lineages, such as those that infect deer, turtles, bats, and many bird species.
“Malaria parasites are far more diverse than most people realize, and the picture we’re painting shows yet another level of complexity,” said Galen.
The researchers’ new tree yielded surprising results. Among them was the finding that that groups within the genus Plasmodium, which include species like Plasmodium falciparum (the deadliest to humans), are not close relatives—some aren’t even carried by mosquitos nor can they replicate in red blood cells. And that’s not all: malaria parasites from a turkey vulture, as well as those previously identified by Perkins in white-tailed deer and other hoofed mammals, may also qualify for new names.
Findings from the study also address debate over the parasites’ origins in vertebrate groups, confirming previous work that shows malaria parasites were likely first found in some dinosaurs and birds before making a single jump via mosquitos to mammals. And as for why there is such diversity among the parasites’ evolutionary lineages? Bats were likely a major factor, acting as reservoir hosts for the parasites.
Based on the new findings, researchers say that name changes may be needed to underscore newly uncovered relationships, or lack of relationships, among malaria parasites. But for now, there’s no rush to revise existing taxonomy since Plasmodium is synonymous with malaria transmission. Instead, the researchers suggest referring to malaria parasites by their subspecies names to avoid confusion.