The vinegar fly Drosophila melanogaster is one of the most well-studied organisms on the planet, having been introduced to the world of genetics a century ago. But most people know them as unwelcome guests in the kitchen, especially if bananas are left out one day too long. Despite their widespread use in biological research, the mystery has remained as to how they made the switch from the wild to becoming our very unwanted house guests.
A collaboration between researchers at Lund University in Sweden and the University of Wisconsin have uncovered the origins of D. melanogaster in a new paper titled, “Wild African Drosophila melanogaster Are Seasonal Specialists on Marula Fruit” published in Current Biology. For decades, researchers have searched for their origins and now a Swedish-American research team has succeeded.
“The ancestors of the flies in our fruit bowls lived in southern Africa. About 10,000 years ago they moved in with their neighbors: humans. Their offspring then colonized the world. It’s actually quite awesome,” said Marcus Stensmyr, Ph.D., senior lecturer at Lund University.
D. melanogaster displays preference toward certain fruit and strongly favors citrus for egg laying. The presence of a distinct host partiality is intriguing and implies that D. melanogaster likely has had a close association with a specific fruit, or group of fruit, with characteristics akin to citrus. The Miombo and Mopane forests carry an impressive diversity of fruit-bearing plants. The researchers targeted the marula fruit as one that stood out to have multiple characteristics that are known preferences of D. melanogaster.
The researchers sought to ask if D. melanogaster from native habitats use marula. To answer this, they traveled to Southern Africa in search of forest-dwelling D. melanogaster and marula. Specifically, they searched the Matopos national park in Southwestern Zimbabwe. With the help of traps, Dr. Stensmyr and colleagues captured fruit flies and found that traps close to marula fruit were quickly filled with flies, while traps at other locations remained empty or only attracted a few flies.
The researchers already knew that overripe, rotten, citrus fruits are the fruit flies’ favorite in the fruit bowl. They, therefore, tested what the flies preferred in the wild: marula or citrus. The answer was marula.
“At home in the kitchen the flies feast on whatever is starting to rot in the fruit bowl, even though they like citrus fruits best. In the wild they are far pickier, they prefer marula fruit,” said Dr. Stensmyr. They are drawn to particular aromatic substances from marula that activate receptors on the antennae. When these are activated, it’s a sign that it’s a good place to lay eggs.”
More specifically, the researchers took the study one step further to identify the chemicals released by the marula that activate the odorant receptors that mediate species-specific host choice (Or22a) and oviposition site selection (Or19a.)
The authors wrote that “the Or22a-expressing neurons—ab3A—respond strongly to the marula ester ethyl isovalerate, a volatile rarely encountered in high amounts in other fruit.” They also show that Or22a differs among African fly populations sampled from a wide range of habitats and that flies from Southern Africa, most of which carry a distinct allele at the Or22a/Or22b locus, have ab3A neurons that are more sensitive to ethyl isovalerate than other flies.
However, fruit flies are not the only animal that has preferred marula throughout history. The authors note that archaeological finds have shown that the San people, one of the indigenous tribes in the investigated area, have had a special relationship with marula fruit throughout history. In one cave, archaeologists found more than 24 million walnut-sized marula pips. The researchers’ conclusion is that the San people’s love of marula fruit explains why the fruit flies moved in with people long ago. Over time the flies adjusted to living inside and became increasingly tolerant of ethanol in rotten fruit. “The fly has developed into a generalist that eats and breeds in all sorts of fruit. But originally it was a real specialist that only lived where there was marula fruit,” concluded Dr. Stensmyr.
So, the next time that you find yourself trying to rid your kitchen of those pesky fruit flies, you have the marula fruit to thank.