The new research provides evidence underlining the significance of bitter taste perception, and suggests that a genetic mutation which makes certain people sensitive to the taste of a bitter compound appears to have been advantageous for certain human populations in Africa.
And the reason why the trait was selected may not be immediately to do with just taste, but rather a molecular receptor that may also play important roles in immune response or metabolism, suggest the research team behind the study.
"Because Africa is the site of origin of all modern humans, Africans are going to have a large amount of diversity and non-Africans are going to have a subset of that diversity," said Professor Sarah Tishkoff, the study's senior author. "In Africa, you get an opportunity to observe how these genetic variants are influencing phenotypes that you wouldn't have if you were only studying non-Africans."
"We're starting to understand that these taste receptors are involved in so many functions other than just oral sensory perception," explained lead author Michael Campbell from the University of Pennsylvania in the US.
Writing in the journal Molecular Biology and Evolution, Campbell and his team noted that their research represents the first time that the bitter-taste sensing gene, TAS2R16, has been studied in a large set of ethnically and culturally diverse African populations.
To better understand the patterns of variation at TAS2R16 in humans globally, the team collected DNA from 595 people in 74 populations across Africa with diverse lifestyles, including pastoralism, hunting-gathering and agriculture. They then sequenced the stretch of DNA encompassing the TAS2R16 gene in all of these individuals and also examined previously collected DNA from 94 non-Africans from the Middle East, Europe, East Asia and the Americas.
Tishkoff and her colleagues found 15 variants of the bitter taste receptor in total - most of which were only found in Africa.
The TAS2R16 gene codes for a molecular receptor that binds salicin - a compound found naturally in willow bark, which is the source of aspirin, explained the team - who asked 296 of the Africans sampled to perform 'taste tests' of progressively more concentrated solutions of salicin and report when they could detect a bitter taste.
"The taste testing shows that the mutations in TAS2R16 had functional significance for the bitter taste perception system," explained Paul Breslin of Rutgers University and Monell Chemical Senses Center - who also worked on the study. "In this case, the mutation caused a gain of taste function."
When the researchers mapped individuals' genetic profiles onto their tasting ability, they found a strong correlation between one of the 15 variants and an increased sensitivity to salicin. On a population level, the team found that this 'high-sensitivity' variant for salicin was more prevalent in individuals from East Africa than in those from West Central or Central Africa, while non-Africans possessed only the 'high-sensitivity' version of the gene.
What's more, in East Africans this high-sensitivity variant, which arose roughly 1.1 million years ago, showed signs of being under a force of natural selection in humans, suggesting it conferred an evolutionary advantage at some point during our past, said the authors.
"That's another sign that this variant must be tremendously important for human survival because it evolved in our human ancestors so long ago and carried over to us," Campbell suggested.
The authors added that the geographic structure of TAS2R16 variants revealed in the new study contrasts with previous work on TAS2R38 variants - which did not appear to fall into any clear geographic pattern.
These differences between two genes that both relate to bitter taste perception offer more support to the idea that taste was not the only force driving the evolution of this gene, they said.
"The types of populations we're studying are diverse and they have diverse diets, suggesting that there is likely something else going on here," said Tishkoff.
"By getting a handle on how much variation is in these populations, where it is located and what are the particular signatures of selection, it might start giving us clues as to what we should be looking at in terms of the biomedical or physiological significance of these genes."
Source: Molecular Biology and Evolution
Published online ahead of print, doi: 10.1093/molbev/mst211
" Origin and Differential Selection of Allelic Variation at TAS2R16 Associated with Salicin Bitter Taste Sensitivity in Africa"
Authors: Michael C. Campbell, Alessia Ranciaro, Daniel Zinshteyn, et al