A fruit flyCredit: Shutterstock
Researchers have discovered an entirely new kind of taste receptor that allows fruit flies (Drosophila melanogaster) to detect alkaline substances — those that have a high pH — and avoid toxic meals and surfaces.
Finding a new taste receptor in such a well-studied animal is unexpected, says Emily Liman, a neurobiologist at the University of Southern California in Los Angeles. “I found it quite surprising that there is this whole new type of taste,” says Liman. “It is really beautiful work.”
The discovery, published in Nature Metabolism this week1, could “potentially inform future research into the physiological mechanisms underlying alkaline taste in other organisms”, says Lai Ren, a biochemist at the Kunming Institute of Zoology at the Chinese Academy of Sciences.
Taste sensation
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Most animals function in a narrow pH range, which makes sensing acids and bases important for their survival. Studies in the past few decades have detailed the receptors, cells and neural circuits involved in detecting sour and acidic tastes2, which occur at low pH, but sensing alkaline substances is not well understood, says Lai. Some early work in people3 and cats4 suggests — but does not prove — that the alkaline sensation could be a type of taste, he adds.
The researchers who led the latest study chose to investigate whether flies have a receptor specific to high pH, because previous work has shown that the insects can sense a broad spectrum of tastes.
To test the flies’ preferences for foods with different pH levels, Yali Zhang, a neurobiologist at the Monell Chemical Senses Center in Philadelphia, Pennsylvania, and his colleagues presented them with a Petri dish of sweet gel — one half was maintained at a neutral pH of 7, and the other half was mixed with sodium hydroxide to make it alkaline. Each half was also coloured either red or blue. After munching on their preferred gel, the flies’ translucent bellies turned red, blue or purple (if they ate from both halves).
The researchers found that the higher the pH of the alkaline food, the more the flies rejected it for the more neutral cuisine. But one group of flies was not as good at distinguishing between the two meals. Screening of these flies revealed that they had a mutation in a gene that the researchers named alkaliphile (or alka for short). The gene was found to be active in taste neurons at the fleshy tip of their elongated proboscis — the equivalent of the mammalian tongue — as well as in cells at the ends of their feet and antennae.
In cell studies, the researchers found that the alka gene expresses a receptor protein, whose normal activity is stimulated by alkaline solutions. In the presence of a base, the receptor protein opens a channel in the cell membrane to let negatively charged chloride ions escape the neuron, immediately relaying a message to the fly’s brain to avoid the food.
Most sensory receptors involve channels that control the movement of positively charged ions into the cell, not negatively charged ions out of it, says Liman.
Artificially activating cells expressing the alka gene using a technique called optogenetics caused the insects to recoil their probosces from a sugary solution, apparently under the impression that it was too alkaline to consume.
Sweet, sour, alkaline
Liman says the specific finding will probably not be directly applicable to vertebrates, because they do not have the alka gene. Although flies and people can experience many similar sensations, such as sweet, bitter and sour, the receptors they use are different, she says. Still, the study could spur research into whether vertebrates have similar types of chloride receptor, and whether they experience a distinct alkaline taste, she says.
The discovery in flies could bring new insights into how insects more generally respond to environmental cues, for example in decisions on where to lay their eggs, as well as for pest control, she says.