Sunday, August 14, 2005

A bitterly positive sweep   posted by Razib @ 8/14/2005 02:06:00 AM

Long time readers will know that I've followed the genetics of taste for a while now (see here, here, here and here). So I am really interested in a new paper that just came out in Current Biology, titled Positive Selection on a High-Sensitivity Allele of the Human Bitter-Taste Receptor TAS2R16:

We detected signatures of positive selection, indicated by an excess of evolutionarily derived alleles at the nonsynonymous site K172N and two linked sites and significant values of Fay and Wu’s H statistics in 19 populations. The estimated age range for the common ancestor of the derived N172 variant is 78,700–791,000 years, placing it in the Middle Pleistocene and before the expansion of early humans out of Africa. Using calcium imaging in cells expressing different receptor variants, we showed that N172 is associated with an increased sensitivity to salicin, arbutin, and five different cyanogenic glycosides....

A nonsynonymous mutation means that the amino acid encoded by the codon has been altered, so there is often (though not always) a functional implication downstream. Positive selection, suggested by an excess of nonsynonymous vs. synonymous (functionally neutral) mutations, would imply that the fitness of the mutant is greater than the ancestral variant. The term "cyanogenic" should give you a clue to the possible reasons why sensitivity of perception to various chemicals can shift the selection coefficienct: strong unpleasant taste associations may serve as a hinderance to poisoning. This seems to have been very beneficial indeed, here is a map of the frequencies of the ancestral alleles (re-edited for ease of viewing):

The lighter regions indicate higher frequencies (click on the map for better resolution). It seems that the ancestral low sensitivity variants are predominantly found in Africa.

Here are aggregate frequencies of the two ancestral alleles by region: Africa 13.8%, Middle East 2.4%, Central/South East Asia 0.2%, Europe 0%, Americans 0%, East Asia 0% and Oceania 0%. The sample numbers were between 100 and 500 individuals (closer to 150-300 in most cases), so the total lack of ancestral haplotypes in much of the world seems to be a testament to the power of selection (there were a small number of "Other" rare haplotypes, though usually on the order of 1-8%). The authors do note that there seems to be a correlation within Africa itself of malarial regions and a high frequency of the ancestral less bitter sensitive allele. They note that "Chronic low-level ingestion of cyanogenic foods has been linked with an increased protection against malaria."

Taste perception is a complex trait. We all know the importance of smell in augmenting and adding greater dimensionality to it. Nevertheless, it is an important part of our day to day life, and, it affects how we interact with others. A great deal of it is due to upbringing and fortitude in the pursuit of acquired discernment, but, it isn't a level playing field, some people have further to go to tolerate certain flavors (the supertasters) while others must coax every ounce of sensory input to perceive any flavor (the nontasters). It is interesting that though less than 15% of the African population exhibit the ancestral genotype, it is likely that 25-35% of African Americans do, at least based on their source populations in this study (ie; Yoruba, South African Bantu, etc.).