Field of Science


To Kill Parasites, Flies Self-Medicate with Booze

Everyone negotiates hazards in their lives. Your food is poisonous, say. Everything wants to eat you. Parasitic wasps are laying eggs in your body that will eventually hatch and chew their way out. To balance the difficulties of invertebrate existence, fruit flies have developed a grim strategy. Baby flies that are infected with parasites turn to alcohol, aiming to ingest just enough to kill their invaders without also offing themselves.

Drosophila melanogaster is the fruit fly species familiar to biologists and people who don't empty their fruit bowls fast enough. It prefers to eat--and lay its eggs inside--rotting produce. Fermenting fruit holds ethanol, which is toxic. But fruit flies have evolved to be relatively resistant to ethanol, allowing them to survive where other insects wouldn't be able to.

For example, parasitic wasps. Though these insects don't have the fruit fly's resistance to alcohol, some parasitic wasps are attracted to the smell of rotting fruit because it tells them where to find new victims. The wasps attack fly larvae, laying their eggs inside the hapless maggots. The wasp parent also injects venom that prevents the young fly's immune system from attacking their eggs. Inside the larva, the wasp eggs hatch into young that proceed to eat the fly from the inside out.

Researchers at Emory University wanted to know how young flies' consumption of toxic, ethanol-containing food affects their parasitic squatters. Is alcohol a weapon in the fight between fly and wasp?

They first confirmed that ethanol is more toxic to parasitic wasps than to fruit flies. This tolerance testing took place in petri dishes holding fly food with various concentrations of ethanol. Adult flies and wasps were placed in the dishes to breathe in the alcohol fumes. Then the insects were observed to see how long it took before they "could no longer stay upright on their feet," says researcher Todd Schlenke.

So living on more-alcoholic food discourages parasitic wasps from hanging around and laying eggs inside you. But what about fly larvae that are already infected--can they use alcoholic food to their advantage?

The researchers allowed wasps to attack their fly larvae. Then they put the larvae, now carrying a cargo of wasp eggs, on alcoholic or nonalcoholic food. Three days later, they checked up on the flies and wasps. More wasp larvae had died inside the flies eating alcoholic food. Additionally, the still-living wasp larvae inside those flies were often grossly malformed.

The crucial question, though, is whether the fly larvae choose to eat alcoholic, relatively toxic food when they have parasites inside them. Do flies self-medicate?

To test this, the researchers put fly larvae in petri dishes divided in half: one side held nonalcoholic food, and the other alcoholic. A day later, larvae that started on the dry side were more likely to have hit the liquor if they were infected with parasites. Infected larvae on the ethanol-containing side tended to initially crawl away--this food is toxic, after all--but returned later. Overall, flies that held baby wasps inside them preferred the alcoholic side of the dish. They also were more likely to survive to adulthood than their infected-but-sober peers.

It may seem incredible enough that tiny maggots can recognize and respond to a parasitic infection by carefully dosing themselves with toxic food. But the story has a final twist.

All along, the researchers were actually studying two species of parasitic wasp. One is a "specialist"--it only infects D. melanogaster and its close relatives. The other is a "generalist," laying its eggs in whatever species of fruit fly is nearest. The specialist parasite also has some resistance to ethanol, though not as much as the fruit flies. Presumably, it's evolved to tolerate the environment where its prey live. But the generalist has lower ethanol tolerance.

The specialist also has greater tolerance of fly hosts that eat alcoholic food. While this behavior killed off a large number of generalist parasites, it was less effective on the specialists. As if they knew this, fly larvae infected with these hardier parasites were less likely to opt for alcoholic food.

It's possible, Todd Schlenke says, that the fly larvae can tell which type of wasp is living inside them and adjust their behavior accordingly. They might identify a unique antigen from the wasp's eggs or venom. There's a second, more sinister possibility: The specialist wasps might be influencing the flies' behavior, making them less interested in consuming alcohol.

This kind of mind control is very possible for parasites. There are fungi that make their ant hosts carry them to a high place, then shoot spores out of the ants' heads. There's a parasitic worm that gets grasshoppers to drown themselves. A certain fluke convinces fish to flag down passing birds, thrashing and displaying their bellies in the water, because the parasite needs to be swallowed by a bird before it can lay its eggs.

Not all fruit flies living on decaying fruit. Some prefer poisonous mushrooms, or toxic rotting cactuses. It's possible that these flies self-medicate against parasites in a similar way. And who knows what other species might be using alcohol, or different toxins in their environment, to fight off invaders? Even humans could have evolved such a trick without knowing it. If scientists can't prove it, at least it'll be an excuse for your midweek drinking that your friends haven't heard before.

Neil F. Milan, Balint Z. Kacsoh, & Todd A. Schlenke (2012). Alcohol Consumption As Self-Medication Against Blood-Borne Parasites In The Fruitfly Current Biology

Photo: Peter Clark/Flickr

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