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Patrick Guilfoile: Drink to their health, How fruit flies use alcohol to protect their young

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Fruit flies — the little insects that buzz around our bananas in the kitchen during the summer — have to worry about more than how to find enough to eat in our homes. These small insects face attacks from a variety of pathogens and parasites, and they have developed strategies to protect themselves from these assaults. In particular, there are wasps that lay eggs in fruit fly larvae. 

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These wasps ultimately hatch, then eat and kill the parasitized fruit flies. In a recent report, though, scientists from Emory University showed fruit flies aren’t just helpless victims of the wasps. These fruit flies use alcohol to ensure the best chance of survival for their offspring. (Ripe or rotten fruit, the preferred food for fruit flies, often contains some level of alcohol as a result of naturally-occurring yeast converting some of the sugar in fruit to ethanol.)

The researchers conducted experiments by putting pregnant female fruit flies in cages with or without wasps. In the absence of wasps, fruit flies laid their eggs in food with low levels of alcohol. In the presence of female wasps, though, the fruit flies mainly laid their eggs in food containing the highest levels of ethanol (12-15 percent). This proved to be an effective strategy. In the absence of wasps, flies reared on a low-ethanol diet had a higher survival rate. However, in the presence of wasps, flies reared on a high ethanol diet had a higher survival rate, as the wasps are more easily killed by high ethanol levels, as compared with fruit flies.

The scientists conducted additional tests to understand exactly what was going on. For example, they used mutant flies that lacked the ability to smell, but these flies still laid their eggs in food with high amounts of alcohol, if wasps were present. In contrast, mutant flies lacking the ability to see tended to lay eggs in foods with lower ethanol content, whether or not wasps were present. This indicated the flies used visual clues to detect the presence of wasps. Using other mutant fruit flies, the researchers were even able to track the changes in brain chemistry resulting from the visual detection of wasps that led to the flies laying eggs in high-alcohol foods.

These experiments document the ability of fruit flies to “prescribe” a medication for their offspring — in this case — alcohol. In the presence of a parasitic wasp, the flies ensure that their eggs are laid in high-alcohol foods, thereby increasing the likelihood that their young will survive. It appears that fruit flies, at least under certain conditions, adhere to the toast “drink to your health.”

More information in “Fruit flies medicate offspring after seeing parasites” by Balint Kacsoh and others “Science” 339:947-949 February 22, 2013

— Patrick Guilfoile has a doctorate in bacteriology and is the associate vice president for academic affairs at Bemidji State University.

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