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Sharks Possess “Surprising” Bitter Taste Perception

A shark.
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Many organisms secrete toxic compounds to deter predators and ensure survival. A type of receptor that enables humans to perceive tastes as bitter – taste 2 receptors (T2Rs) – also helps some organisms recognize these compounds so that they can avoid consuming them.

A team, led by Professor Sigrun Korsching from the Institute of Genetics at the University of Cologne, discovered that certain cartilaginous fish species, including shark and sawfish, possess the T2R gene.

This finding came as a surprise to the researchers, as it was believed that these receptors only occurred in bony vertebrates.

Their findings were published in the Proceedings of the National Academy of Sciences.

11 bitter substances activated sharks’ receptors

Advances in next-generation sequencing enabled Korsching and colleagues to scour the DNA of 17 cartilaginous fish, a feat that would have once proved challenging given the large size of these organisms’ genomes.

Twelve of the fish carried one copy of the T2R gene, which the researchers named T2R1. The discovery of a single T2R gene implies that it is the original form of the bitter taste receptor, Korsching and colleagues say, as it has not been affected by gene duplication or specialization of the encoded receptors.

To explore which ligands activate the T2R1­-encoded receptor, the researchers introduced the T2R1 gene from the whitespotted bamboo shark, Chiloscyllium plagiosum (C. plagiosum) and the small-spotted catshark, Scyliorhinus canicula (S. canicula), into immortalized cell lines.

Collectively, they screened 94 human bitter substances in the cell lines, discovering 11 substances that activated both sharks’ T2R1 receptors, including colchicine and bile acid. “The ligand repertoire of bamboo shark represents a subset of that of the catshark, with roughly similar thresholds,” Korsching and colleagues write.

In a previous study, the researchers had shown that some of the ligands tested here also activated the taste receptor of the West Indian Ocean coelacanth, Latimeria chalumnae, a species of ancient bony fish. “The extent to which this function has been conserved is astonishing, i.e., through the entire evolution of vertebrates,” says Korsching.

The original taste receptor

Both sharks’ receptors also responded to the application of xenobiotic compounds and steroids. In early- and late-derived bony vertebrates, such as zebrafish and humans, there are separate receptors that respond to these compounds. This, the researchers say, is consistent with the notion that the shark T2R reflects “the original ligand repertoire of the ancestral bitter taste receptor at the evolutionary origin of this family.”

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“These findings give us new insights into the evolution of these receptors,” says Korsching. “We can look back almost 500 million years on the molecular and functional origin of an entire family of bitter taste receptors. Because that is how old the last common ancestor of cartilage and bony fish is.”

Reference: Behrens M, Lang T, Korsching SI. A singular shark bitter taste receptor provides insights into the evolution of bitter taste perception. PNAS. 2023;120(48):e2310347120. doi: 10.1073/pnas.2310347120

This article is a rework of a press release issued by the University of Cologne. Material has been edited for length and content.