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Researchers: Fruit flies force their young to drink alcohol to kill off parasites

Researchers from Emory University have discovered that fruit flies force their young to drink alcohol. However, there is a perfectly reasonable explanation behind this seemingly unconscionable behavior. When fruit flies detect parasitic wasps in their environment, they place their eggs in an alcohol-soaked piece of fermenting fruit, basically forcing their young to drink alcohol to medicate against infection. The study’s findings were published in the journal Science.

“Hosts have numerous defenses against parasites, of which behavioral immune responses are an important but underappreciated component. Here we describe a behavioral immune response that Drosophila melanogaster uses against endoparasitoid wasps. We found that when flies see wasps, they switch to laying eggs in alcohol-laden food sources that protect hatched larvae from infection,” write the authors in the study’s abstract.

According to Todd Schlenke, an evolutionary geneticist at Emory University, the adult fruit flies identify the wasps by sight.

The larvae of the common fruit fly have evolved some resistance to the toxic effects of the alcohol levels in overripe, fermenting fruit. Alcohol levels in their natural habitat can range up to 15 percent.

Parasitic wasps are big-time killers of fruit flies. The wasps place their eggs inside the fruit fly larvae, along with venom that tries to suppress their hosts’ cellular immune response. If the flies do not succeed at killing the wasp egg, a wasp larva will consume the fruit fly larva when it hatches.

Previously, Schlenke and his team found that fruit fly larvae infected with wasps favor foods high in alcohol. This is because the flies have evolved a high tolerance of the toxic effects of the alcohol, but the deadly wasps have not.

Schlenke notes that the fruit fly larvae increase their blood alcohol levels, “so that the wasps living in their blood will suffer.” He adds that “behavior can also be a big part of an organism’s immune defense.”

The researchers wanted to see whether fruit fly parents could determine when their young were at risk for infection, and whether they then looked for sources of alcohol.

Female fruit flies were placed in one cage with parasitic wasps and another cage with no wasps. Both cages contained two petri dishes with yeast. The yeast in one of the petri dishes was combined with 6 percent alcohol, while the yeast in the other petri dish did not contain alcohol. After one day, the petri dishes were taken out of the cages and the researchers counted the eggs that the fruit flies had laid.

The researchers found that in the cage with parasitic wasps, 90 percent of the eggs were located in the dish containing alcohol. In the cage with no wasps, only 40 percent of the eggs were discovered in the alcohol dish.

“The fruit flies clearly change their reproductive behavior when the wasps are present,” Schlenke notes.

The researchers also learned that fruit flies prefer to lay their eggs in alcohol when female wasps are present.

The researchers theorized that the flies were making their decisions based on pheromones. They recreated the experiment using two groups of mutated flies. One group did not have the ability to smell, and the second group could not see. The flies that were unable to smell, however, still chose the petri dish with alcohol over the dish without alcohol when female wasps were present. Blind flies, on the other hand, chose the dish without alcohol, even with female wasps in the cage.

“This result was a surprise to me,” Schlenke says. “I thought the flies were probably using olfaction to sense the female wasps.”

There are some small visual differences between female and male wasps, but Schlenke notes that “the small, compound eyes of flies are believed to be more geared to detecting motion than high-resolution images.”

The researchers also learned that fruit flies can differentiate between different species of wasps, and will only select the petri dish with alcohol when wasp species that infect larvae, not fly pupae, are present.

The researchers also linked the exposure to female parasitic wasps to alterations in a fruit fly neuropeptide. In the past, the reduced level of neuropeptide F has been linked to alcohol-seeking behavior in fruit flies.

“We found that when a fruit fly is exposed to female parasitic wasps, this exposure reduces the level of NPF in the fly brain, causing the fly to seek out alcoholic sites for oviposition,” Schlenke says. “Furthermore, the alcohol-seeking behavior appears to remain for the duration of the fly’s life, even when the parasitic wasps are no longer present, an example of long-term memory.”

Schlenke and his colleagues believe that a number of fly species medicate their young to protect against parasitic wasps.