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Fruit flies consume alcohol to combat wasps

By BRYAN KOHRS | March 7, 2013

While humans continue to bicker about the legal drinking age, fruit flies have developed their own unique policy: alcohol before birth. A new study out of Emory University has shown that fruit flies lay their eggs in an alcoholic environment, essentially feeding their unhatched young booze, to protect them from parasitic wasps. This new spin on parenting is a creative defense tactic that can actually save the lives of fruit fly larvae.

Tiny endoparasitic wasps, those that live inside other organisms, commonly prey on fruit fly larvae. These wasps will inject their own eggs into the larvae of fruit flies, hatching inside them and eating the fruit fly larvae from the inside out. The fruit fly needs to kill the wasp eggs before they hatch to avoid this essential death sentence. However, while the fruit fly’s immune system works to kill these parasites, the wasps release venom that suppresses the organism’s immune response.

So what else can a fruit fly do to kill these parasitic wasps? They can drink.

“[Fruit flies] like alcohol and naturally consume a good amount of alcohol,” Todd Schlenke, a professor at Emory University and co-author of a new study on this topic, wrote in an email to The News-Letter.

Fruit ferments as it rots, which enables up to 15 percent of the fruit to be alcohol by volume. Since fruit flies live on and around rotting fruit, alcohol is a natural part of their environment and has become part of their diet. The wasps on the other hand have no such tolerance for alcohol and Schlenke’s research shows that the fruit flies utilize this evolutionary advantage to fight the parasites.

Mother fruit flies protect their young from wasps by dipping their eggs in alcohol, causing an influx that raises the blood alcohol levels of the larvae and the wasps. This alcohol kills the parasites, who are not accustomed to dealing with alcohol, and saves the majority of the fruit flies.

Schlenke and his team set up a controlled experiment to test this immune behavior. Adult female fruit flies were released into two separate cages, one that contained wasps and the other holding a control group without wasps. Two separate petri dishes were also set in the cages. One dish contained yeast, the laboratory nourishment used for fruit fly larvae, and one contained yeast mixed with six percent alcohol, roughly the alcohol percentage of a beer and more than a fruit fly enjoys.

“Fruit flies prefer alcoholic food, but only up to about three percent alcohol by volume,” Schlenke wrote. “We use six percent in our experiments because it’s at the higher range of what might be found in naturally rotting fruits, and it’s not a level fruit flies would normally like.”

After 24 hours, the researchers removed the petri dishes from both cages and counted the number of eggs in each dish. They found that in the cage containing the parasitic wasps, 90 percent of the eggs laid were in the alcoholic mixture, while only 40 percent of the eggs laid in the cage without wasps were in the alcoholic mixture.

Despite the protective benefits, laying eggs in alcohol can have adverse effects.

“There is increased mortality for flies laid as eggs on six percent alcohol food,” Schlenke wrote. “Only about 75 percent make it to the adult stage whereas about 95 percent do when laid on non-alcohol food. This is likely why flies don’t normally lay eggs in food with this high of an alcohol concentration. However, the cost is worth it when there are wasps around.”

The researchers hypothesized that fruit flies recognize the wasps by the smell of their pheromones, and thus were able to base their behaviors off of the presence or absence of wasps. To test this, they performed the same experiment with two separate mutated fruit flies. The first mutated group could not see, while the other could not smell. The group that could not smell preferred to lay their eggs in alcohol, while the group that could not see had no preference.

This showed that fruit flies actually recognize the wasps by sight, a somewhat shocking discovery. In fact, the scientists believe this is an innate behavior for fruit flies as the strains used by Schlenke and his team were grown in a laboratory and were many generations removed from contact with wasps in nature.

Apart from protection against wasps, fruit flies may garner other benefits from alcohol at the right concentration.

“Flies grown on three percent alcohol food are larger and live longer than flies not given any alcohol,” Schlenke wrote.

The fruit fly species certainly enjoys alcohol and knows how to use it well in everyday life. But does this mean that they, like humans, can become alcoholics?

“I don’t know if you could say they suffer from an addiction to it,” Schlenke wrote. “They do get drunk if you force them to consume excess alcohol, but they don’t drink that much when they’re given a choice. On the other hand, there was a study in Science a few months ago showing that male flies rejected by females consume more alcohol...”

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