Flies confirm the existence of a sixth taste, alkaline

Flies taste more than humans.

The latter, and all mammals, have receptors on their tongues for sweet, salty, bitter, and sour.

A few decades ago the existence of other specific ones for umami was discovered.

Now, a group of scientists has shown that the fruit fly (

) detects what would be a sixth taste, alkaline.

Typical of substances that, such as strong water or salfumán, have a high pH, ​​this ability to detect it would give them an advantage when avoiding food and potentially dangerous environments.

Before humans gave it another cultural function (enjoyment), the sense of taste was a manager of scarcity.

Animals are programmed to go hungry, and taste gives them clues as to how nutritious a food is.

But this sense also serves as a sentinel: it warns that a compound may be in bad shape and be harmful.

The physiology and chemistry of all living beings develops in environments with a neutral pH (value of 7).

Extremely acidic substances (hydrochloric acid, with its pH 0, for example) are incompatible with life.

But the same thing happens with very alkaline ones (a base like sodium hydroxide has a pH of 14).

However, while other animals such as cats are suspected of detecting the basic taste as well, it had only been shown in one species of beetles.

To the short list we must add the fruit fly.

Scientists from the University of California, the Chinese Academy of Sciences and the Monell Center for Chemical Senses (United States) have discovered that it has receptors specifically dedicated to detecting basic taste.

This drosophila is one of the most common animals in laboratories.

Its genome is made up of just over 15,000 genes (humans have just over 20,000).

It serves as a model to study many human diseases and the first map of a larval brain was recently created.

But even if all its genes have been identified, the function of most of them is unknown.

What these researchers have published in the scientific journal

has been the discovery in this genetic haystack of a gene that they have baptized

, because it hides the instructions to detect the alkalinity of a substance.

"Flies detect different flavors using mainly taste receptor neurons (RGNs), analogous to human taste receptor cells, present in the labellum, equivalent to our tongue," says 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."

That is to say, when they land they know if that substance is sweet, acid... or alkaline, as you have seen now.

mutant flies

To find out, they used the CRISPR genetic modification technique, obtaining dozens of mutant flies that were missing a certain gene that they presumed to be related to the senses (one of the authors, Craig Montell, from the University of California, has spent years identifying fruit fly sensory receptors).

So they put two drops in front of the flies, one of glucose and the other with the same sugar, but to which they added sodium hydroxide, known as caustic soda, one of the most alkaline bases there is.

All the insects whose genes were not manipulated chose the sweet solution, avoiding the basic one.

The same thing happened with almost all the mutants, except for some that had blocked the expression of a gene of unknown function called CG12344.

These reduced the natural aversion to alkaline substances.

They even preferred droplets of sodium hydroxide if its concentration was not very high.

As there was the possibility that this attraction was due to the salt of the compound (sodium), they came to create flies with a double mutation, one so that they were not repulsed by hydroxide and another so that they did not perceive the salty taste.

Again, these drosophilas still did not avoid the alkaline solution.

In their eagerness to rule out alternative explanations, they observed how the different flies extended or retracted their proboscis, the kind of proboscis that ends in the labellum and that they use to feed.

The mutant flies did not pick it up when they detected the high pH substance.

To rule out that it was due to the compound itself, which is very alkaline, they repeated the tests with another base, sodium carbonate, with a lower pH.

They got the same results again.

In another series of already extreme experiments,

they manipulated the alkaline taste neurons of various insects with light using optogenetics in such a way that when they were brightly illuminated with the red part of the spectrum, they excited them.

This activation made the insects not only prefer the basic flavor, but also avoid the sweet one.

When we activate taste receptor neurons by exposing them to red light, flies perceive the taste of sucrose as alkaline, so they reject sucrose."

Yali Zhang, biochemist at the Monell Chemical Senses Center (United States)

“When we activate the NRGs by exposing them to red light, the flies perceive the taste of sucrose as alkaline, so they reject the sucrose that, under normal conditions, would be attractive to them.

This indicates that the alkaline taste can suppress the sweet taste,” explains Zhang.

“When you add something bitter, like caffeine, to a cup of chocolate milk, it will lessen its sweet taste and even make it bitter because the caffeine suppresses the sweet taste,” she compares.

Zhang and his colleagues had earned the right to rename the CG12344 gene

.

They had identified it, found its function, and shown how flies taste alkalinity.

Not all alkaline substances are necessarily toxic, but 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 adds. he.

Bases or alkaline substances are widely present in ecosystems and “strong alkalinity is physiologically harmful, causing alkalosis”, she concludes.

Humans have lost it, but in the rest of the animal world, if you don't detect a toxic compound, you die."

Juan Alcañiz, researcher at the Institute of Neurosciences of the Miguel Hernández University

Juan Alcañiz, who studies the senses of

at the Institute of Neurosciences of the Miguel Hernández University, highlights the sentinel value of these discovered receptors.

“It's not just about the food.

The females then have to lay their eggs and they have to avoid alkaline environments”, she recalls.

This may give an evolutionary advantage and most likely what is discovered in the fruit fly is also found in other insects and, why not, in mammals.

Alcañiz, who values ​​the discovery of a new receptor that is sufficient and specific for alkalinity, draws attention to the fact that taste is such an essential sense that there are hardly any pathologies of the taste system.

"Humans have lost it, but in the rest of the animal world, if you don't detect a toxic compound, you die."

The difficult thing is to know if what the fruit fly has is also present in humans.

The logic of the taste of some and others is similar: taste cells detect the same molecules by sending a series of signals to the brain.

But the receptors of insects and mammals have nothing to do with it.

Francisco Martín studies the molecular physiology of behavior at the Cajal Institute (CSIC).

“It is to be hoped that it will also happen in humans.

But the fly mechanism is not used by humans, ”he maintains.

The channel that is activated in flies resembles, but very distantly, a channel in human cells, which has nothing to do with taste.

"Vertebrates don't even have the fly gene," he adds.

Both Alcañiz and Martín agree that this new discovery opens the way to search for these basic receptors in humans.

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