Tipsy Fruit Flies on a Mission
Fruit flies may seem as if they lead an uneventful life. They look for old fruit to lay their eggs. The maggots then hatch and graze on the yeast and bacteria that make the fruit rot.
In reality, however, these flies have to do battle with horrifying enemies. Tiny wasps seek out the maggots and lay eggs inside them. The wasps develop inside the still living flies, feeding on their tissues. When the wasps reach adult size, they crawl out of the dying bodies of their hosts.
The flies are not helpless victims, however. In the journal Current Biology, Todd Schlenke, an Emory University biologist, and his colleagues report a remarkable defense the insects use: To kill their parasites, the flies get drunk.
Dr. Schlenke discovered this tactic while studying the common fruit fly species Drosophila melanogaster. As they eat yeast, they also eat the alcohol that the yeast produce while breaking down sugar. Their fermentation can leave a rotting banana with an alcohol concentration higher than that of a bottle of beer.
This boozy environment can be toxic to animals. The only reason Drosophila melanogaster thrives on rotting fruit is that it has evolved special enzymes that quickly detoxify alcohol.
Dr. Schlenke was well aware that many insects gain defenses from their food. Monarch butterflies, for example, are protected from birds by the toxic compounds they get from the milkweed plants they eat. To see how alcohol influences the enemies of the flies, Dr. Schlenke unleashed a parasitic wasp, Leptopilina heterotoma.
Dr. Schlenke allowed the wasps to attack two kinds of fly larvae: one kind reared on alcohol-free food, and another that ate food spiked with 6 percent alcohol. In the presence of alcohol, the wasps laid 60 percent fewer eggs, possibly because of the fumes wafting from the food. “Presumably the wasps felt really ill,” Dr. Schlenke said.
It turned out that alcohol was even worse for their eggs. Wasps growing in flies that ate alcohol-free food always grew normally. But inside boozing flies, 65 percent of the wasps died.
Dr. Schlenke discovered they suffered a hideous death: Each wasp’s internal organs had shot out of its anus. “All their guts are outside the wasps,” he said. “I don’t know how to explain that.”
This deadly effect occurred only if the flies consumed alcohol after the wasps laid eggs in them. Taking in alcohol beforehand, by contrast, had little effect. This discovery led Dr. Schlenke to wonder if the flies might seek out alcohol to kill the wasps, using it like a medical drug. “I wondered if they were smart enough to know that,” he said.
To find out, he and his colleagues filled petri dishes with alcohol-rich food on one side and alcohol-free food on the other. They then placed flies that did not have wasps inside them on the alcohol-free side. A day later, they found that 30 percent of the flies had crawled over to the side with alcohol. When they repeated the experiment with wasp-infested flies, 80 percent of the flies headed for the spirits. “There’s a big difference there,” Dr. Schlenke said.
Likewise, when the flies started out on the alcohol side of the dish, 40 percent of the healthy flies crawled to the other side after 24 hours. Many infected larvae started moving to the other side as well, but then returned to the alcohol. Dr. Schlenke speculates that they were exploring for even higher alcohol concentrations that would be even more toxic to their parasites.
“They know the wasps are infecting them, and they seek out the alcohol,” Dr. Schlenke said. “The flies self-medicate by getting schnockered.”
Some wasps appear to have evolved ways around this tipsy defense. Dr. Schlenke repeated these experiments on another species, L. boulardi, which unlike the other wasp can lay its eggs only in D. melanogaster. Dr. Schlenke found that the specialist wasp L. boulardi suffered far less when its host consumed alcohol. Only 10 percent of its larvae died, compared with 65 percent for L. heterotoma. Dr. Schlenke suspects that its specialization allowed L. boulardi to overcome the alcohol. “The wasps are tracking their hosts over evolutionary time,” he said.
“This article is exciting in several ways,” said Michael Singer, a biologist at Wesleyan University who was not involved in the study. Over the years, scientists have gathered a few examples of animals medicating themselves. Chimpanzees eat plants with antiparasitic compounds when they get intestinal worms, for example. Dr. Singer and his colleagues have shown that woolly bear caterpillars go out of their way to feed on toxic plant leaves when parasitic flies lay eggs in them. But Dr. Schlenke’s research is the first to show that an animal uses alcohol as medicine.
Alcohol is common in nature, and Dr. Schlenke speculates that other species may seek it out to self-medicate. When it comes to humans, however, Dr. Schlenke has no idea whether a bout of heavy drinking has any effect on a parasite.
“As far as I can tell, no one’s ever tested whether we humans can make life hard for our bloodborne pathogens by getting our blood alcohol levels up,” he said.
By CARL ZIMMER
Published: February 16, 2012