Female D. melanogaster, the fly used in the study, laying an egg. Image: Dow/Davies labs, Glasgow
Fruit flies are a common subject of studies concerning alcohol–they've been shown to drink harder when they're rejected, and to get better at learning while drunk–but why did fruit flies' affinity for drink evolve in the first place? Curiously enough, for fruit flies, alcohol can help combat parasites.
According to a study out of Emory University published in Science, female fruit flies will lay their eggs on alcohol-rich food sources (in the wild, that generally means decomposing fruit) if they see parasitic wasps nearby. Why's that? Endoparasitoid wasps are small, but they're huge killers of flies. They do so by injecting their eggs into fly larvae, along with a chemical that suppresses the fly larva's immune response; if the wasp larvae survives, it eats its way out. Gory, right?
It turns out that fruit flies are far more tolerant of alcohol than are wasps. That means that if an infected fly larva is hatched in an alcoholic environment, it can survive while the wasp larva dies. It's still more beneficial for a fly larva to grow up uninfected and without alcohol, but this study shows that if female flies see a female parasitic wasp–which spells doom for their offspring–they'll choose to medicate their larvae by laying them in an alcoholic environment.
Comparison of L heterotoma wasp male (top) and female.
Female fruit flies can distinguish between the two.
"The adult flies actually anticipate an infection risk to their children, and then they medicate them by depositing them in alcohol," Todd Schlenke, an evolutionary geneticist at Emory whose lab conducted the research, told Science Daily. "We found that this medicating behavior was shared by diverse fly species, adding to the evidence that using toxins in the environment to medicate offspring may be common across the animal kingdom."
The team's experiment was set up thusly: 300 flies were put in cages containing two food sources, one non-alcoholic and one containing a 6% alcohol solution, and allowed to do their mating thing. The control group, which never saw wasps, preferred to lay eggs in the non-alcoholic dish. But in the presence Leptopilina heterotoma females, a common fruit fly parasite, the flies laid a significantly greater portion of eggs on alcoholic dishes. Interestingly, they did not show that bias when in the presence of male wasps, which don't present a danger to fly larvae.
"Our data indicate that the flies can visually distinguish the relatively small morphological differences between male and female wasps, and between different species of wasps," Schlenke said.
The team then tested how strong the females like their kids' alcohol to be. Flies that saw no wasps actually preferred to lay eggs in a 3% solution, which the team notes aligns with previous research suggesting flies get a health benefit from consuming a small amount of ethanol. Females in the presence of wasps preferred to lay their eggs in dishes corresponding to the highest alcohol concentrations found in nature, around 12-15% (wine, basically).
But is there an actual evolutionary advantage, or are flies just drunks? In the first part of the study, the team measured the eclosion (egg-hatching) success of the flies, and found that when wasps weren't around, non-alcoholic eggs hatched at a higher rate than those that in the 6% solution, which suggests in normal conditions, alcohol is bad for fly fitness. But when wasps were allowed to prey on the eggs, those that were laid in alcoholic dishes survived at a higher rate than those that didn't.
In other words, it's better for flies to not be born in alcoholic conditions, but because wasps are better killers than ethanol is, it's basically better to have drunk babies than parasitized ones. Because of that fitness benefit, the team posits that fruit flies' decision-making over alcohol medication is indeed an evolved trait, which is supported by the fact that such behavior has been observed in a number of different fly species.