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June 26, 2004

PTC taste, balancing selection?

Long-time readers of the blog might know I have an interest in the genetics of taste. Here is an old post where I explore the question of whether variance in PTC-taster/non-taster frequency cross-culturally (that is, by population) might have a relationship to preference for spicy foods. The authors in the 1998 paper which I reference above note that:

  • The incidence of taste blindness to PTC/PROP varies around the world, from 3% in western Africa to >40% in India.
  • Capsaicin, the compound responsible for the oral burn of chili pepper, is more intensely hot to PROP tasters than to nontasters.

I tried to connect the fact that South Asians have the highest frequency of non-tasters regionally with the observation that they also traditionally consume spicy dishes. This was prompted by an article which correlated latitude/climate with how spicy local dishes were. The authors noted that there is often a strong antibacterial effect that comes with the inclusion of spices, and this might have strong fitness consequences in tropical climates, and less at higher latitudes where the pathogenic environment is less stringent. I really didn't get far, and gave up on this line of thought after Abiola noted that some West African cuisine was extremely spicy, the population with the lowest percentage of non-tasters.

But, I did not highlight that there were other correlations between various tastes and PTC/PROP status. From the paper cited above:

  • PROP and PTC are chemically related to compounds that are present in vegetables such as cabbage, broccoli, Brussel sprouts, turnips, and kale. In large quantities these compounds interfere with iodine metabolism and result in goiter. It has been noted that incidence of thyroid-deficiency disease is relatively rare among PTC tasters.
  • Taster children disliked raw broccoli, whereas nontaster children liked raw broccoli.
  • Tasters show greater sensitivity to a wide range of tastes, including bitter tastes, sweet tastes, oral irritants such as chili pepper, and the textural sensations of fats.
  • Solutions of caffeine, quinine, and isohumulones (the bittering agents in beer) are more bitter to tasters.
  • Tasters can distinguish fat levels in dressings far better than non-tasters.

So the key points from the above article(s):
1) Non-taster and taster individuals exist in all populations.
2) The frequency of the two groups seems to vary by population, sometimes to a rather large extent.
3) Non-taster and taster status have clear implications for one's perception of the taste of a wide range of foods.
4) Certain consumption patterns have clear relative fitness implications.

It was suggested in the paper above that PTC-PROP status is determined by a few genes, perhaps one primary locus and a secondary locus, and there could be more than two alleles at each locus. If the data on West Africa is correct, it seems that the alleles for non-taste exist at a rather low frequency, while in India, the alleles for non-taste exist at rather higher frequencies. I could offer a guess that perhaps India's history of elite vegetarianism meant that individuals who had fewer aversions to various vegetables had an advantage as far as nutrition went (in this case, there should be a positive correlation between the frequency of non-tasters and groups which have traditionally practiced vegetarianism, Jains, high caste Hindus and South Indians). Taking into account the issues with iodine deficiency and aversion to vegetables that might exacerbate this problem, there might also be a reason that people in certain regions are tasters.

In any case, this current post was prompted by this entry over at Science Blog on this paper that suggests that:

  • Balancing selection keeps both alleles in human populations, that is, the authors suggest that heterozygotes "are able to taste a broader range of toxic, bitter compounds, and have an evolutionary advantage".
  • There is no great variation in the frequencies across populations.

Of course, they are examining the "PTC gene" that was found recently. The previous authors had less information to go on, and hypothesized multiple genes. I'll have to do some digging on this, but even the summary over at Science Blog (much more readable than the abstract!) notes that there are 24 genes that effect the taste of bitter. The fact that the authors found no great regional variations in the PTC gene, but previous authors have noted variations in the phenotype, suggests to me there must be other genes at work here even in the specific case of PTC taste. I don't have access to the full paper, so if readers who do can clue me/us in to details I might have missed, that would be appreciated.

[1] This test is often given in high school biology courses using a piece of paper that has been covered with the chemical in question. Students are asked to put the paper on one's tongue, and tasters immediately react with disgust, while non-tasters tend to be oblivious.

Posted by razib at 10:54 PM