Penn researchers uncover evolution of bitter gene

Broccoli

The taste of broccoli has long been the bane of picky eaters everywhere (and at least one American president). But appreciating the vegetable is not just a matter of having a cultured palate; some people can easily taste a bitter compound in broccoli that others have difficulty detecting. Now, a team of Penn researchers has helped uncover the evolutionary history of one of the genes responsible for this trait.

The international team interested in the gene known as TAS2R38 was led by Penn researchers Sarah Tishkoff, a Penn Integrates Knowledge Professor, and Michael C. Campbell, a postdoctoral fellow in the Perelman School of Medicine. It also included undergraduate and postdoctoral researchers from both the Genetics and Biology Departments.

Individuals with a certain version of this gene can taste a compound, phenylthiocarbamide (or PTC) which is chemically similar to naturally occurring bitter compounds present in many foods. Previous studies investigated variations in the PTC-sensitivity gene, but none had 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 [in those populations] you may not see elsewhere,” Tishkoff explains. “Our study gives us a clue about the evolutionary history of the TAS2R38 gene and how natural selection might be influencing the pattern of variation.”

Bitter

Tishkoff Lab

Researchers testing PTC sensitivity among the Yaaku people in Kenya.

By looking at the gene in 611 Africans from 57 diverse ethnic populations with distinct diets, as well as in 132 non-Africans, the researchers showed that the former group had more variation than the latter. By combining this information with an experiment that tested how well participants could actually taste the bitter compound, the study also revealed that Africans have a broader range of PTC taste sensitivity than typically seen outside of the continent.

Surprisingly, Campbell says, 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,” Campbell says. “This is suggestive that variation of this gene serves some other function beyond oral sensory perception.”    

This counter-intuitive discovery is in line with other recent studies, which found receptors similar to TAS2R38 in the lungs, upper airway and gut. If the variations of TAS2R38 have had an impact on breathing or digesting, alongside tasting, Tishkoff and Campbell say, the former traits might be the true focus of natural selection.

Originally published on December 8, 2011