On the tongue of all mammals, including humans, there are receptors for identifying sour, bitter, sweet, and salty, and others specifically for detecting umami, or fifth taste, and it has now been discovered that there are a sixth taste, alkaline, which can be detected by an insect, the fruit fly or Drosophila melanogaster.
The study that has reached this conclusion has been carried out by researchers from the Universities of California and Pennsylvania, the Chinese Academy of Sciences and the Monell Center for Chemical Senses (United States) and has been published in Nature metabolism. The alkaline or basic taste characterizes substances with a high pH, such as caustic soda, and being able to detect it allows us to avoid eating food or staying in potentially dangerous environments, since the optimal physiological activities and enzymatic reactions of most organisms only they can occur in a narrow pH range, around 7.4, and too high a value could trigger “a life-threatening condition.”
Although in people the sense of taste serves to enjoy food, it helps animals to discover how nutritious a food is and constitutes an alert signal to detect harmful elements. Very acidic substances, such as hydrochloric acid –which has a pH of 0–, or very alkaline, such as sodium hydroxide –which has a pH of 14– are dangerous for living beings, but, although it is believed that some animals, such as cats, can detect basic taste, had only been demonstrated in one species of beetles.
Alkaline taste detection “drastically increases the evolutionary fitness” of these flies “by improving their survival, growth and reproduction”
The results of the new study show, however, that the fruit fly – which is frequently used as an animal model for the study of numerous human diseases and whose brain has recently been mapped – has receptors specially designed to identify the basic taste. Specifically, these researchers have discovered a gene that they have named alka because it contains the instructions that allow the insect to detect the alkalinity of a substance.
“Flies detect different flavors mainly using taste receptor neurons (RGNs), analogous to human taste receptor cells, present in the labellum, equivalent to our tongue,” explained Yali Zhang, biochemist at the Monell Center and lead author of the discovery. “In addition, they also use the NRG from the tarsi of their feet to detect taste substances.” This means that when they settle on a substance they already know if it is sweet, acid…, or alkaline, as has been observed now.
Detect taste as alert of a dangerous environment
The researchers have pointed out in their article that there are “many places where organisms can encounter high pH conditions in their ecosystem, such as in the food and water they consume”, and that life expectancy and survivability they are lower in flies fed moderately alkaline diets, and that continued exposure to such an environment “impairs development, shortens lifespan, and causes lethality.”
“Consequently -they add-, female flies avoid alkaline substrates when they choose a place to lay their eggs.” Taken together, the perception of this basic pH serves as an “essential self-protection strategy” that allows these insects and other animals to effectively avoid toxic environments during foraging and habitat selection. The authors therefore consider that the detection of this sixth taste “drastically increases the evolutionary fitness” of these flies “by improving their survival, growth and reproduction.”
To carry out the study, the researchers genetically modified fruit flies and gave these insects a choice between two foods and another group that had not been modified. One consisted of a neutral glucose solution (pH 7) and the other had sodium hydroxide (a base) added to it, causing its pH to rise. Most of the animals – both mutants and normals – chose the former, but some “showed significantly reduced aversion to alkaline foods” and even preferred them when the concentration was not very high.
The flies that chose the alkaline products were those in which the CG12344, or alka, gene had been altered. To confirm that the choice of these insects was due to their ability to perceive the alkaline taste and not to other factors, they created drosophila flies with a double mutation, adding to the one that made them prefer basic tastes another that prevented them from detecting the salty touch. typical of sodium hydroxide. The results were the same.
Although not all alkaline substances have to be toxic, most are. “pH is important for all living organisms, as they need their food to have specific pH ranges to live,” Zhang recalls. “In addition, it plays an essential role in the metabolism, physiology and nutrition of organisms, because many biological processes, such as enzymatic reactions, require precise pH levels (a pH of 7.4) to occur,” he indicates. . Bases, or alkaline substances, abound in ecosystems, and “strong alkalinity is physiologically damaging, causing alkalosis,” she concludes.
.
