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April 18, 2013

Fruit Flies Need Medicine Too

Ever had childbirth pains or an eye infection?  Greek physician Hippocrates (still remembered by the Hippocratic oath of doctors to do no harm), in 400 BCE, would’ve prescribed you to chew on willow bark.  I don’t know about you, but when I have an eye infection (being unfamiliar with childbirth pains) one of the last things I contemplate doing is gnawing bark off trees.  But sure enough, in 1883 chemists working at the Bayer division of I.G. Farben in Germany synthesized a derivative of the active ingredient in willow bark called acetylsalicylic acid and called it aspirin, a very effective reliever of pain, inflammation and fever—as evidenced by the 80 billion tablets consumed each year in the United States alone! 

Whereever on planet Earth people have found themselves they’ve come up with remedies and rituals to cure diseases and other health problems.  Science has debunked many of these and few have proven effective against especially aggressive diseases such as small pox, but some have been upheld as legitimately effective treatments for various ailments.  The question that comes to my mind is how did the people know or come to figure out these medicines from the plants, animals, and minerals in the vastly diverse ecosystems of Earth that they found themselves in?  Was it a matter of trial-and-error/experimentation?  Was it an I’m-going-to-die-anyway moment and anything was worth trying?  Did they observe and mimic animals?  Was instinct/evolution a guide? Intuition or dreams?  Can you use your senses and determine what will help you by taste, touch, sight, smell, etc?  Is it a combination of these things or some other reason? 

I know of no answer to many of these questions, but an interesting clue comes from the non-human realm that points to an ancient, evolutionary origin of medication.  In essence proving that humans did not invent medicine, but rather inherited a tendency that we have vastly expanded and improved upon with our technologies. 

Evolutionarily speaking it seems that, to survive, many organisms, including humans, have had to supplement their innate immune systems with medications in order to survive the onslaught of pathogens and injuries that one inevitably encounters in a lifetime.  The fact that other organisms use medicine and that we use medicine implicates an interesting, interconnected history for this phenomena.  Now, what is the evidence that this link exists between us and the rest of the living world? 

There are many examples of medication in the biosphere: from chimpanzees who eat leaves not normally part of their diet to kill nematode parasites, to ants and bees who lacquer their homes with anti-microbial/anti-parasitic resins when the colony becomes infected to just name a few.  I am going to focus on one specific example to give you a feel for the complexity of the behaviors involved.

Drosophila melanogaster, Fruit Fly

The example concerns a famous mainstay of scientific study, the common fruit fly, Drosophila melanogaster, commonly found buzzing around a house near youThe fly larvae eat the fungi and bacteria that cause fruit to rot and ferment when overripe.  They have a certain resistance to the toxic effects of alcohol which is a good thing because levels can range from 5-15% in the rotting fruit where they are growing.  But there is another reason for this alcohol tolerance and it is that fruit fly larvae are parasitized by wasps who lay their eggs inside the fly larvae with an injection of venom to suppress their immune systems so that the wasp offspring can grow, eating up the fly larvae from the inside out, eventually, killing them.  Alcohol is a toxin to many of the wasps who parasitize the flies.  The unlucky wasps developing inside of the alcohol-consuming fly larvae die a horrible death where their internal organs liquify and get ejected out of their anuses. 

That fruit fly larvae live in an alcohol-rich environment that kills wasps is not proof of medicating behavior however.  What makes fruit flies an example of a species that medicates itself is that alcohol in the concentrations to kill developing wasps is toxic to the fly larvae as well after long enough durations and so must be done in response to being parasitized, not on a consistent basis. 

In fact, the flies can choose food for their offspring that minimizes the impacts of disease (in this case, being parasitized by the wasps) when the need arises.  Also, the fruit fly larvae can adjust their diet and environment to increase their blood alcohol levels when developing wasps are in their bodies. 

When adult fruit flies sense parasitic wasps in their environment they can anticipate the infection risk for their children They respond by medicating their offspring, putting them in an alcohol-rich environment.  Surprisingly, the flies can identify the wasps by sight alone and can distinguish between male and female wasps, as well as parasitizing wasps versus non-parasitizing wasps.  If the fruit fly parent sees a male wasp or a non-parasitizing species of wasp they will not seek out high alcohol environments for their offspring to grow in, only detection of nearby female wasps of parasitizing species consistently elicit the medicating behavior. 

Similarly, the larvae also know when they've been infected by the wasp eggs and will move to an environment with higher concentrations of alcohol and will begin eating food with higher concentrations of alcohol in an effort to rid themselves of the parasites in their bodies. 

Medicating behavior similar to the fruit flies' is being found more and more in the insect world.  Monarch butterflies infected by parasites, for example, will lay their eggs on plants with anti-parasitic chemicals which drastically reduce the levels of infection in their offspring.  Somehow they recognize that they are infected and they take action to save the next generation.
Monarch Butterfly

The researchers who amassed the information on fruit fly medication that I’ve been talking about pointed to the potential that alcohol might prove effective in treating parasitic diseases in humans, something not previously researched. 

We humans often fancy ourselves superior to all other living things and point to our sophisticated gadgets and life-lengthening medical technologies to prove our grander intelligence, but as researchers worldwide are beginning to discover......perhaps humans are not so many heads higher than the rest of the biosphere after all.  Many of the behaviors, including medication, that we've traditionally classified as uniquely human, may just be evolutionary tendencies that we've inherited alongside many other organisms. 

Perhaps another message to derive from the example of the fruit fly is one of respect, for it appears that even something as apparently lowly and simple as a fruit fly might have something to teach us about what it means to be human.  

Sources Cited:


Neil F. Milan, Balint Z. Kacsoh, Todd A. Schlenke. Alcohol Consumption as Self-Medication against Blood-Borne Parasites in the Fruit Fly. Current Biology, 2012; DOI: 10.1016/j.cub.2012.01.045     

B. Z. Kacsoh, Z. R. Lynch, N. T. Mortimer, T. A. Schlenke. Fruit Flies Medicate Offspring After Seeing Parasites. Science, 2013; 339 (6122): 947 DOI: 10.1126/science.1229625

-Seth Commichaux 

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