Yali Zhang, a neurobiologist at the Monell Center for Chemical Senses in Philadelphia, and his colleagues, to test the flies’ preference for foods with different pH levels, presented them with a dish containing a sweet gel that was half neutral in pH and half mixed with sodium hydroxide to make it alkaline. . Each half was also mixed with red or blue. After eating the favorite gel, the flies’ transparent abdomens turned red, blue, or purple (if they ate both).
The researchers found that the higher the pH of the alkaline food, the more the flies would skip it and go for the more neutral food. But a group of flies did not perform well in distinguishing between two meals. The screening of these flies showed that they have a mutation in a gene called Alkaliphil (or Alka for short). It was found that this gene is active in the taste neurons in the fleshy tip of their snout (equivalent to the tongue of mammals) and also in the cells at the end of their legs and tentacles.
By conducting 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 Baze, the receptor protein opens a channel in the cell membrane to allow negatively charged chloride ions to exit the neuron, immediately sending a message to the fly’s brain to avoid the food. Lyman says most sensory receptors contain channels that control the movement of positively charged ions into the cell rather than the exit of negatively charged ions out of the neuron.
Artificially activating alka gene-expressing cells using an optogenetic technique caused the insects to withdraw their proboscis, apparently assuming that the sugar solution was too alkaline.
Sweet, sour, alkaline
This particular finding probably isn’t directly applicable to vertebrates because they don’t have the alca gene, Liman says. Although humans and flies can sense similar tastes, such as sweet, bitter and sour, they use different receptors to do so, he says. However, the study could stimulate further research into whether vertebrates have similar types of chloride receptors and whether they detect distinct alkaline tastes.
Lyman says the new discovery in flies could provide new insights into how these insects respond to environmental cues, for example in making decisions about where to lay their eggs, as well as in pest control.
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