Friday, November 15, 2013

Bitter Taste Sensitivity in Africa

Origin and Differential Selection of Allelic Variation at TAS2R16 Associated with Salicin Bitter Taste Sensitivity in Africa 

Bitter taste perception influences human nutrition and health, and the genetic variation underlying this trait may play a role in disease susceptibility. To better understand the genetic architecture and patterns of phenotypic variability of bitter taste perception, we sequenced a 996 bp region, encompassing the coding exon of TAS2R16, a bitter taste receptor gene, in 595 individuals from 74 African populations, and in 94 non-Africans from 11 populations. We also performed genotype-phenotype association analyses of threshold levels of sensitivity to salicin, a bitter anti-inflammatory compound, in 296 individuals from Central and East Africa. In addition, we characterized TAS2R16 mutants in vitro to investigate the effects of polymorphic loci identified at this locus on receptor function. Here, we report striking signatures of positive selection, including significant Fay and Wu's H statistics predominantly in East Africa, indicating strong local adaptation, and greater genetic structure among African populations than expected under neutrality. Furthermore, we observed a “star-like” phylogeny for haplotypes with the derived allele at polymorphic site 516 associated with increased bitter taste perception that is consistent with a model of selection for “high-sensitivity” variation. In contrast, haplotypes carrying the “low-sensitivity” ancestral allele at site 516 showed evidence of strong purifying selection. We also demonstrated, for the first time, the functional effect of nonsynonymous variation at site 516 on salicin phenotypic variance in vivo in diverse Africans, and showed that most other nonsynonymous substitutions have weak or no effect on cell surface expression in vitro, suggesting that one main polymorphism at TAS2R16 influences salicin recognition. Additionally, we detected geographic differences in levels of bitter taste perception in Africa not previously reported, and infer an East African origin for high salicin sensitivity in human populations.

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From the Press:

“Because Africa is the site of origin of all modern humans,” said study author Sarah Tishkoff, a professor in the University of Pennsylvania’s Department of Genetics, “Africans are going to have a large amount of diversity and non-Africans are going to have a subset of that diversity. 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.”

“The taste testing shows that the mutations in TAS2R16 had functional significance for the bitter taste perception system,” said study author Paul Breslin, an experimental psychologist from Rutgers University. “In this case, the mutation caused a gain of taste function.”

“The types of populations we’re studying are diverse and they have diverse diets,” Tishkoff said, “suggesting that there is likely something else going on here. 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.”  


  1. It's interesting, thanks, although I am always overly cautious about "selective" explanations (almost invariably other more random explanations are equally possible and researchers seldom bother to properly justify the supposed extra fitness of the trait). In general I would expect greater diversity in Eastern (or Eastern-Central) Africa without need to appeal to selection, just because it seems to be the cradle of African Humankind (as well as of the extra-African one), but also because, in addition to that, it also seems the area with greater backflow from nearly all directions.

  2. I don't terribly like Breslin's characterization of the mutation as a "gain in taste function." Really, to the extent that you have selection for bitter taste sensitivity you are balancing an ability to avoid poisons before having a fatal dose v. the ability to be omnivorous and avoid starvation or malnutrition (stereotypically not eating bitter vegetables like brocolli that are healthy).

    It would be interesting to know what bitter natural poison(s) in East Africa could have had that selective impact. One time frame where that kind of thing might have a selective effect distinct from any that should be shared by all modern humans as Maju suggests could be in the proto-farming stage where there is organized gathering of a non-domesticated plant that has a mimic that is poisonous - particularly in arid highland areas of East Africa.