[Reading Science] How Do Fruit Flies Avoid Insecticides?

US Research Team Identifies Alkaline Taste Receptor in Drosophila
First Discovery Among Animals, Previously Only Sour Taste Mechanism Known
Focus on Whether Humans and Other Vertebrates Can Also Perceive Alkaline Taste

Research on how humans and other animals detect and distinguish tastes to safely consume food is deepening. A U.S. research team has confirmed for the first time among animals that the fruit fly (Drosophila melanogaster) possesses a receptor that detects alkaline taste. This reveals the secret of how fruit flies sense strongly alkaline substances to avoid toxic food and insecticides.

The research team at the Monell Chemical Senses Center in Philadelphia, USA, published a paper containing these findings on the 20th in the international journal Nature Metabolism.

Fruit fly. Stock photo.

Most animals can survive only within a narrow pH (hydrogen ion concentration) range, making the ability to detect alkalinity and acidity essential for survival. pH refers to the hydrogen ion concentration and is divided into 14 levels. The lower the value, the more acidic; the higher, the more alkaline (basic); and the middle (pH 7) is neutral.

Recent studies have identified receptors, cells, and neural circuits involved in detecting sour taste, i.e., acidic substances, in animals. However, how alkaline taste is detected has remained unknown. Some studies on humans and cats have suggested that alkaline sensation might be a type of taste, but this has not been proven.

Based on previous research showing that fruit flies detect a wide range of tastes, the team investigated whether they possess receptors capable of sensing high pH levels, i.e., alkaline substances.

The researchers applied neutral red jelly on one half of a dish and blue jelly alkalized with sodium hydroxide on the other half, then fed these to fruit flies. This setup was designed to investigate which pH level of food the flies preferred. The results showed that fruit flies ate less as alkalinity increased and preferred the neutral jelly. Interestingly, some fruit flies could not distinguish between the two jellies. Genetic analysis comparing these flies with normal ones revealed that the flies unable to differentiate neutral and alkaline substances had genetic mutations. These mutations affected genes involved in sensory neurons located at the elongated proboscis, legs, and antenna tips of the fruit fly, which correspond to the tongue in mammals, rendering them nonfunctional.

At the cellular level, the team discovered that this gene produces a receptor protein that detects alkaline substances. When this receptor protein senses alkaline substances, it opens channels in the cell membrane, allowing negatively charged chloride ions (-) to exit the nerve cells, transmitting information to the brain and sending a message to avoid the food. Using optogenetics technology, the researchers artificially activated this gene and observed that fruit flies retracted their proboscis as if detecting alkalinity even while drinking a sugar solution. This demonstrated that the gene plays a key role in fruit flies’ detection of alkaline substances.

However, these findings do not directly apply to vertebrates such as humans, as vertebrates do not possess genes directly involved in detecting alkaline substances in this way.

Emily Liman, a professor at the University of Southern California, said, "Fruit flies and humans experience similar tastes such as sweet, bitter, and sour, but their receptors differ." She added, "This research could trigger studies on whether vertebrates have similar types of base receptors as fruit flies and whether they can detect strong alkaline tastes." She also noted, "It provides new insights into how insects decide where to lay eggs or respond to environmental factors such as insecticide application for pest control."