New genetic research from Penn has found that Africans can detect bitter tastes better and more easily than Europeans or Asians—a trait that may have made it easier for African peoples to detect potentially poisonous plants in nature.

According to an ongoing study from the lab of Sarah Tishkoff, the David and Lyn Silfen University Associate Professor and a Penn Integrates Knowledge Professor with joint appointments in the schools of Medicine and Arts and Sciences, there is a striking amount of diversity in the so-called “bitter gene” among African populations—far more diversity, in fact, than in other studied populations around the world.

And with diversity, she says, comes a more sensitive palate.

While previous studies have analyzed the gene TAS2R38, which is known to be associated with variants in bitter taste response, this is the first study of its kind to investigate this pattern of genetic diversity in a wide range of distinct African populations. Common foods with a bitter taste include dandelion greens, coffee, unsweetened chocolate and the aptly named bitter melon.

This research is part of a wide-ranging genetic diversity project that began when Tishkoff went to Africa to collect phenotype data for a number of variable traits, including bitter taste perception. In this study, people in Cameroon and Kenya were administered vials containing various strengths of the synthetic bitter substance PTC. They were then scored according to the point at which they could first detect the bitter substance.

Back in the lab, Michael Campbell, a postdoctoral researcher in Tishkoff’s lab, sequenced the TAS2R38 gene looking for patterns of diversity.
“We also looked for signatures for selection, and then I correlated the genetic information with the phenotype data to see what kind of result we would get,” says Campbell.

What they found was a high number of haplotypes—or combinations of different forms of genes located on the same chromosome—associated with bitter taste perception in Kenyans and Cameroonians. Typically, in non-Africans, two major haplotypes known as PAV and ADI combine in a predictable variety of ways, resulting in a range of ability to detect bitterness. But in Africans, Campbell says, the results were surprising. Specifically, they saw greater variety of haplotypes in the Africans than in other groups.

“Not only are we seeing PAV and ADI in Africans, but we’re also seeing a number of other haplotypes that are associated with bitter taste detection at some levels,” Campbell says. “They’re at a much higher frequency in Africans than we see in non-Africans.”

What this means, Campbell says, is that Africans have a much greater sensitivity to bitterness than other people.

The question now is why this variation exists.

“There are evolutionary forces at play here, but we’re just not quite sure which ones—yet,” Campbell says. “It’s been speculated that perhaps it’s due to the importance of being able to detect bitter substances in nature, because bitter substances in plants are likely indicative of toxins that the plant may contain. So, there’s basically a survival advantage to being able to detect these substances.”

Campbell and others will continue to pursue bitterness even further in an attempt to understand how individuals or populations are able to detect bitter substances and to identify patterns of variation within populations.
This project is just the beginning, however. In Tishkoff’s lab, she and her team of five postdocs and three graduate students are studying lactase persistence in African populations and malaria susceptibility—a disease that affects a huge swath of the African population.

The overall goal is to establish a database of genetic diversity among diverse African populations, which will be important for the identification of the genetic basis of diseases prevalent in African and African-American populations, including diabetes and prostate cancer.

“This is just the tip of the iceberg,” says Campbell.