Summer 2008

A SPECIES OF freshwater zooplankton known as Daphnia dentifera suffers periodic epidemics of a virulent yeast parasite that kills most infected individuals within a few days. Despite the deadly nature of the parasite, however, overall populations of these “water fleas” in Michigan lakes seem little affected by the outbreaks.

photo by Meghan Duffy Image shows a group of Daphnia, two of which (first and third from the left) are infected with the yeast parasite.

Reporting in the April 2008 issue of the journal American Naturalist, researchers suggest that two factors work together to limit the parasite’s impact: rapid evolution of resistance to the yeast and highly selective consumption of infected Daphnia by fish.

This surprising combination of predation and evolution shows how seemingly unconnected factors can have a significant impact on parasitism, says Meghan Duffy, the article’s lead author and an assistant professor in the Georgia Tech School of Biology.

“There has been a belief that evolution and ecology occur on different time scales, but we are starting to realize that is wrong, especially for plankton and other small creatures that have very rapid life cycles,” she explains. “The combination of selective predation and rapid evolution protects that population from experiencing large effects.”

By contrast, a bacterial parasite that also attacks Daphnia did seem to have a significant effect on populations of the microscopic creatures, though the researchers aren’t sure whether the Daphnia develop resistance to that parasite.

Duffy and collaborator Spencer Hall of Indiana University studied outbreaks of both parasites for a period of several years in small lakes near the W.K. Kellogg Biological Station in Michigan. Not all lakes suffered outbreaks each year, and the yeast and bacterium outbreaks seemed to occur independently.

Healthy Daphnia are transparent, about one and one-half millimeters long when fully grown. They are important to the food chain, consuming algae and serving as food for smaller lake fish such as bluegills.

The yeast, Metschnikowia bicuspidata, gives Daphnia a milky color that helps fish see them more easily. As a result, fish eat infected Daphnia at a rate nine times greater than the rate at which they eat healthy ones. That serves to dramatically reduce the amount of the parasite in the overall water flea population.

photo courtesy Meghan Duffy Assistant professor Meghan Duffy collects samples at a Michigan lake.

“Just the effect of the fish eating them should make it more difficult for the parasite to stay in the population,” explains Duffy, who conducted the research as part of postdoctoral studies at the University of Wisconsin - Madison. “Conceptually, this is somewhat analogous to a quarantine that might be put into place during an epidemic. You selectively remove infected individuals from the general population.”

At the same time the fish are reducing the population of infected creatures, the Daphnia themselves are rapidly developing resistance to the yeast. Because they reproduce asexually and can produce new generations in as little as a week, evolution can take place quickly, Duffy notes.

“The population ultimately becomes so resistant that it passes the disease threshold, hitting a point at which the yeast parasite cannot persist in the population,” she says. “In asexual populations, the selection of resistant genes can be very efficient.”

After the epidemic’s threat passes, however, the Daphnia lose their resistance. That may be the result of genetic recombination that occurs during sexual reproduction – which takes place in the fall – or because evolution favors a different trait in the absence of the yeast threat.

“There often have to be tradeoffs because organisms can only do so many things,” says Duffy. “Perhaps they can deal with a parasite by becoming very resistant, but that may mean having fewer babies. Or they can remain susceptible and get infected a lot, but as long as they produce a lot of babies, the population may thrive.”

The impact of the bacterium Spirobacillus cienkowskii on the population appears to be greater than that of the yeast, Duffy says. Though the infection turns the Daphnia bright red, it doesn’t appear to affect consumption by fish as much as the yeast does. Because researchers can’t study the infection in the lab, they don’t know if the Daphnia are developing resistance to it.

For the future, Duffy is studying parasites in Georgia lakes and hopes to learn more about epidemics that may be occurring there. And she’d like to learn more about what sparks the outbreaks, why they have the effects she has observed and how diversity plays a role in rapid evolution of resistance to certain parasites.

“Diversity is important because more diverse populations should be able to evolve more rapidly,” she adds. “Our study suggests that losing diversity might make populations less able to evolve in response to threats such as parasites. That could potentially make epidemics more common and more severe when they do happen.”