PHILADELPHIA—Researchers at the University of Pennsylvania have uncovered the evolutionary history of one of the genes responsible for a person’s ability to taste bitter compounds, according to a new study published in the journal Molecular Biology and Evolution.
The researchers investigated the gene TAS2R38, which codes for a bitter taste receptor protein with the same name. People with a certain version of the gene can taste a compound, phenylthiocarbamide, or PTC, which is chemically similar to naturally occurring bitter compounds, called glucosinolates, present in many foods, including cruciferous vegetables like broccoli and Brussels sprouts. These “tasters" find such foods to have a bitter taste that people with a different version can’t detect. As a result, “nontasters" have been shown to consume fewer cruciferous vegetables.
Modern humans originated in Africa, and populations from that region have the highest levels of genetic diversity globally. Previous studies had looked at variations in the PTC-sensitivity gene, but none had ever studied a large sample of diverse African populations with different cultures, ethnicities or diets.
“Because there is more genetic variation in African populations, you’re likely to see unique variants you may not see elsewhere," the researchers said. “Our study of variation at the TAS2R38 gene in Africa and correlations with taste perception and diet gives us a clue about the evolutionary history of the gene and how natural selection might be influencing the pattern of variation."
Genes that influence perception are of particular interest to geneticists because those genes are under strong evolutionary pressure; organisms with senses that are well adapted to their environment have better chances to survive and reproduce. PTC-sensitivity’s potential impact on nutrition, or the ability to detect bitter-tasting toxins, would therefore make it an obvious target for natural selection.
By looking at the TAS2R38 gene in 611 Africans from 57 diverse ethnic populations with distinct diets (for example, Pygmy hunter-gatherers and Maasai pastoralists), as well as in 132 non-Africans, the researchers showed that Africans had more variation than non-Africans, including several never-before-seen rare mutations.
The researchers also tested the correlation between genetic variation at this gene and levels of PTC tasting ability in 463 Africans. In an experiment that was challenging to carry out across a wide swath of the African continent, participants sampled successively concentrated solutions of PTC and water until they were able to detect the bitter taste. When correlated with the participants’ genetic data, the study revealed that Africans have a broader range of PTC taste sensitivity than typically seen outside of Africa, and that relatively new rare mutations also decrease an individual’s ability to taste PTC.
Comparing different African populations confirmed that the PTC-sensitivity gene is millions of years old, meaning it predates the evolution of modern humans and likely existed in the common ancestor of modern humans and Neanderthals.
The study also revealed local diet did not have an effect on the evolution of any of the PTC-sensitivity gene variants. “Although we typically see a lot of genetic variation among diverse African populations, the frequency of TAS2R38 variants is fairly similar across different ethnicities, cultures and diets," they said. “This is suggestive that variation at this gene serves some other function beyond oral sensory perception."
The counterintuitive discovery is in line with other recent studies that found receptors similar to TAS2R38 in the lungs, upper airways and gut. If the variations of the TAS2R38 gene have had an undiscovered impact on breathing or digesting, alongside tasting, the former traits might be the true focus of natural selection.
“We now believe the chemical senses play key sentinel roles at points of entry to the body like the mouth, airways and gastrointestinal tract," they said. “It is possible that, in addition to detecting bitter-tasting thyroid toxins, products of this gene help to defend against ubiquitous pan-African threats, such as inhaling injurious compounds or growing undesirable bacteria in airway mucus or intestines."
