Fruit Flies Medicate Young Against Parasites

Many students remember being dragged to the doctor’s office for vaccinations as kids. As it turns out, fruit flies suffer similar childhood plights.

A new study conducted by Emory College biology professor Todd Schlenke confirms the innate ability of the Drosophila melanogaster, or common fruit fly, to medicate its young with alcohol injections as a defense against invading parasites.

These flies, which feed on bacteria living on the surfaces of rotting fruit, inject distilled alcohol into their larvae to kill off parasites – specifically small, endoparasitoid wasps that lay eggs inside of their offspring. Once a wasp egg hatches inside fly larvae, the juvenile wasp ultimately eats its way out of its unfortunate host, emerging as an adult.

Schlenke’s 2012 study proved the fly larvae’s tendency to self-medicate, but these recent findings show that the adult flies can actually see the wasps and anticipate the larvae-killing infections.

“They’ve been naturally selected to behave this way,” Schlenke said. He sees immune systems in all organisms as “a much more complex set of behaviors.”

“It’s much more common than we originally thought,” he added, citing a 2010 study by Emory Ecologist Jaap de Roode in which monarch butterflies revealed an innate preference for milkweed, a substance toxic to their parasites and predators.

Zachary Lynch, a second-year Emory graduate and Population, Biology, Ecology and Evolution (PBEE) program student, works in Schlenke’s lab. Co-advised by both Schlenke and de Roode, Lynch is currently investigating medication behaviors in other fly species.

In Drosophila melanogaster and other Drosophila species, according to Lynch, such behaviors even coevolved with alcohol tolerance.

In the experiment, flies were given the chance to lay eggs in food containing zero or six percent ethanol. Most flies chose the ethanol, and 60 percent successfully killed the invading wasps.

According to Nathan Mortimer, a postdoctoral fellow in Schlenke’s lab who focuses on psychological immunity, a chemical alteration occurred within the brains of flies that had seen wasps.

“This suggests that signaling in the flies’ brains changed when they encountered their parasites, and that this led to a change in their behavior that helped their offspring resist parasitic infection,” Mortimer said.

Balint Kacsoh (’12C), who will study molecular and cellular biology at Dartmouth this fall, designed and conducted the research.

Kascoh is in the midst of performing two follow-up experiments on other Drosophila melanogaster flies that specialize in other harmful food sources, not including alcohol.

“We wondered whether these flies are also able to utilize unique, albeit toxic, food sources in order to protect their offspring,” Kacsoh said. “We are finding that this is the case with the two additional toxins and multiple fly species we are texting.”

Schlenke, who has been conducting this research for the last couple of years, sees his team’s findings as another example of each organism’s tendency to adapt to and use its environment.

“We looked at a bunch of fly species that live among fruits,” Schlenke said. “They all do the same thing.” Flies that don’t live among fruit, he added, reveal no preference for ethanol medication. However, this behavior is seen among mammals such as chimpanzees who eat certain plants to treat deficiencies, Schlenke added.

“The broader idea,” he said, “is that organisms – not just humans – have behaviors that prevent infection.”

– By Lydia O’Neal