Comparing sensory experience in bitter taste perception of phenylthiocarbamide within and between human twins and singletons: intrapair differences in thresholds and genetic variance estimates

BACKGROUND: Quantitative genetic studies revealed that not all of the phenotypic variance in PTC taste perception is heritable. AIM: To study quantitative variations in PTC tasting ability in twins and to estimate heritability of PTC taste perception on the taste of twin data on males and females sexes separately. SUBJECTS AND METHODS: The data for PTC taste sensitivity following the classic method of Harris & Kalmus (1949) were collected on a sample of 141 twin pairs (66 MZ and 75 DZ) and 275 singletons (128 males and 147 females) from Chandigarh, India. Genetic analyses were performed following Christian (1979), Donner (1986) and Sham (1998). RESULTS: Frequency of non-tasters was similar in twins (33 %) and singletons (32 %), but significant sex differences were observed. No differences were found between zygosities for mean thresholds. Similarly, no evidence of variance heterogeneity and environmental covariance was seen between zygosities. Since no basic assumption of the twin method was found violated, within-pair estimates of genetic variance would be unbiased. These estimates were highly significant in both males and females. However, dominance and additive components of genetic variance were found to differ between sexes. CONCLUSION: PTC thresholds do not seem to be significantly affected by environmental factors as no variance inequality was observed between twin zygosities. Intensity of bitterness (scalar dimensions) of PTC is a separate trait having no commonality with the genetic basis of recognition threshold for PTC tasting ability. The receptors recognizing bitter taste are different from the receptors determining intensity of taste. The absolute difference between co-twins in PTC thresholds can be used as a simple tool in the twin zygosity diagnosis. The results show that none of the MZ co-twins had manifested difference of more than 3 in their PTC threshold.     

Physical growth and development, neurological maturation, and behavioral functioning in two Ecuadorian Andean communities in which goiter is endemic. II. PTC taste sensitivity and neurological maturation

PTC (phenylthiocarbamide) taste sensitivity was correlated with a measure of visual-motor maturation in children six to 15 years of age in two Ecuadorian Andean communities in which goiter (enlargement of the thyroid gland) and cretinism are endemic. The children in one population received injections of iodine in oil IM in 1966 while the children in the other population did not. The correlations between PTC taste sensitivity and visual-motor maturation were significant in both sex groups in the non-iodized population, but not in the iodized population. There was also a significant increase in taste sensitivity with age in the non-iodized, but not the iodized, population. The data support the hypothesis that the sensitive “tasters” of PTC limit their ingestion of naturally occurring goitrogens and are at an adaptive advantage with respect to the less sensitive “tasters” and “non-tasters” in this area in which iodine intake is low and the consumption of food crops containing naturally occurring goitrogens is high.     

Defective colour vision and taste sensitivity to PTC in the Maharashtrian population

Three caste groups of the Maharashtrian population, namely Brahmin (N=58), Maratha (N=989) and Scheduled caste (N=1073), were studied for defective colour vision and for the ability to taste phenylthiocarbamide (PTC). Comparisons of defective colour vision and PTC taste sensitivity were made with other Maharashtrian populations studied previously by various authors. The incidence of defective colour vision is lowest among the Brahmins (3.44%) and highest in the Scheduled caste people (4.28%). Taste sensitivity to PTC is highest among the Brahmins (72.5%) with a value which is close to the Vednagar Brahmins (73.3%) who show the highest frequency of the ‘T’ gene recorded so far in Maharashtra.     

Natural selection and molecular evolution in PTC, a bitter-taste receptor gene

The ability to taste phenylthiocarbamide (PTC) is a classic phenotype that has long been known to vary in human populations. This phenotype is of genetic, epidemiologic, and evolutionary interest because the ability to taste PTC is correlated with the ability to taste other bitter substances, many of which are toxic. Thus, variation in PTC perception may reflect variation in dietary preferences throughout human history and could correlate with susceptibility to diet-related diseases in modern populations. To test R. A. Fisher's long-standing hypothesis that variability in PTC perception has been maintained by balancing natural selection, we examined patterns of DNA sequence variation in the recently identified PTC gene, which accounts for up to 85% of phenotypic variance in the trait. We analyzed the entire coding region of PTC (1,002 bp) in a sample of 330 chromosomes collected from African (n=62), Asian (n=138), European (n=110), and North American (n=20) populations by use of new statistical tests for natural selection that take into account the potentially confounding effects of human population growth. Two intermediate-frequency haplotypes corresponding to "taster" and "nontaster" phenotypes were found. These haplotypes had similar frequencies across Africa, Asia, and Europe. Genetic differentiation between the continental population samples was low (FST=0.056) in comparison with estimates based on other genes. In addition, Tajima's D and Fu and Li's D and F statistics demonstrated a significant deviation from neutrality because of an excess of intermediate-frequency variants when human population growth was taken into account (P<.01). These results combine to suggest that balancing natural selection has acted to maintain "taster" and "nontaster" alleles at the PTC locus in humans.     

Bitter taste study in a sardinian genetic isolate supports the association of phenylthiocarbamide sensitivity to the TAS2R38 bitter receptor gene

Recently, a major locus on chromosome 7q was found in association with the taste sensitivity to phenylthiocarbamide (PTC) in humans. This region contains the TAS2R38 gene that encodes a member of the TAS2R bitter taste receptor family. Three SNPs within this gene demonstrated a strong association with taster status in Utah families and in an additional sample of 85 unrelated individuals. We studied a small isolated village in eastern Sardinia and carried out a genome-wide scan to map the genetic basis of PTC perception in this population. We performed both qualitative and quantitative PTC-taste linkage analysis. Qualitative analysis was carried out by defining a cut-off from the bimodal distribution of the trait and classifying subjects as tasters and non-tasters (75 and 25%, respectively). Linkage analysis on 131 subjects belonging to a unique large multi-generation pedigree comprising 239 subjects confirmed significant evidence for linkage at 7q35 also in our population. Haplotype analyses of the three SNPs inside the PTC gene allowed us to identify only two haplotypes that were associated with the non-taster phenotype (80% AVI homozygous) and to taster phenotype (40% PAV homozygous and 56% PAV/AVI heterozygous). Sex, age and haplotype effect explained 77.2 % of the total variance in PTC sensitivity.     

Evolution of functionally diverse alleles associated with PTC bitter taste sensitivity in Africa

Although human bitter taste perception is hypothesized to be a dietary adaptation, little is known about genetic signatures of selection and patterns of bitter taste perception variability in ethnically diverse populations with different diets, particularly from Africa. To better understand the genetic basis and evolutionary history of bitter taste sensitivity, we sequenced a 2,975 bp region encompassing TAS2R38, a bitter taste receptor gene, in 611 Africans from 57 populations in West Central and East Africa with diverse subsistence patterns, as well as in a comparative sample of 132 non-Africans. We also examined the association between genetic variability at this locus and threshold levels of phenylthiocarbamide (PTC) bitterness in 463 Africans from the above populations to determine how variation influences bitter taste perception. Here, we report striking patterns of variation at TAS2R38, including a significant excess of novel rare nonsynonymous polymorphisms that recently arose only in Africa, high frequencies of haplotypes in Africa associated with intermediate bitter taste sensitivity, a remarkably similar frequency of common haplotypes across genetically and culturally distinct Africans, and an ancient coalescence time of common variation in global populations. Additionally, several of the rare nonsynonymous substitutions significantly modified levels of PTC bitter taste sensitivity in diverse Africans. While ancient balancing selection likely maintained common haplotype variation across global populations, we suggest that recent selection pressures may have also resulted in the unusually high level of rare nonsynonymous variants in Africa, implying a complex model of selection at the TAS2R38 locus in African populations. Furthermore, the distribution of common haplotypes in Africa is not correlated with diet, raising the possibility that common variation may be under selection due to their role in nondietary biological processes. In addition, our data indicate that novel rare mutations contribute to the phenotypic variance of PTC sensitivity, illustrating the influence of rare variation on a common trait, as well as the relatively recent evolution of functionally diverse alleles at this locus.     

Hydrogen cyanide and phenylthiocarbamide sensitivity, mid-phalangeal hair and color blindness in Yucatán, Mexico

Approximately 1450 persons, 800 of them unrelated, of both sexes from age eigth upward from the town of Ticul, Yucatán, Mexico, formed an American Indian (Maya) sample with apparently little Caucasian admixture for four genetic tests: ability to taste PTC and smell HCN, presence of mid-phalangeal hair, and color blindness. A modified PTC sorting test indicated a nontaster allele (t) frequency of 0.29 in Ticul, a relatively high result for American Indians but compatible with previously reported Mayan data. A color vision deficiency frequency of 3.6% was found in the total male sample, and a subsample of unrelated males had a rate of 2.8%. No color blind females were detected. The Ticuleños exhibited a high rate of midphalangeal hairlessness, characteristic of American Indians, with a notable sex differential: 75.9% for males, 87.1% for females. Previous studies of the inability to smell HCN, most of which suggest an X-linked recessive inheritance with an allele frequency of about 0.2, are reviewed. The Yucatán material, with a nonsmeller prevalence around 50% and no significant difference between the sexes, contradicts the X-linked recessive hypothesis on the basis of both population and family analyses. No simple Mendelian scheme appears to fit the Ticul HCN sensitivity test results.     

Anthropological studies in Assam, India. 1. Observations on five Mongoloid populations

Five Mongoloid population groups from Upper Assam (Ahom, Chutia, Deuri, Mishing and Moran) have been investigated for the distribution of anthropometric and dermatoglyphic traits as well as for that of ABO blood groups and PTC taste sensitivity. The results are discussed with special reference to extent and causes of intergroup variability.     

Ability to test phenylthiocarbamide (PTC-test) among the population of Kirghizia]

The distribution of the taste sensitivity to phenylthiocarbamide (PTC) among 320 Kirghiz boys, 320 Kirghiz girls, 45 Russian boys and 200 Russian girls was studied. The measurements of taste sensitivity to PTC was carried out by the standard method with a scale of PTC dilution to 29.     

Taste sensitivity to phenylthiourea (PTC) in the rhesus monkey (Macaca mulatta)

The hypothesis that the distribution of thresholds of taste sensitivity to phenylthiourea (PTC) is unimodal was tested in a group of 367 rhesus monkeys. The distribution was found to be unimodal and highly skewed. The regression of taste thresholds of offspring on that of their father was 0.840±0.34. This estimate is consistent with an hypothesis of essentially no environmental influence. It was concluded that phenotypic variability at the PTC tasting locus in rhesus monkeys is almost totally determined by the non-additive influences of several genes on taste sensitivity.     

Effect of high altitude on sensitivity to the taste of phenylthiocarbamide

Sensitivity to the taste of phenylthiocarbamide (PTC) was studied using the Harris-Kalmus method in healthy human volunteers at sea level and then subsequently at an altitude of 3500 m over a period of 3 weeks, after which they were brought back to sea level. Blood sugar, insulin and blood cortisol levels were estimated weekly. The results indicated that, out of 51 subjects studied, 26 (55%) were PTC tasters at sea level. Eight of those unable to taste PTC at sea level tested as tasters at high altitude, and 2 of them reverted to being non-tasters on return to sea level. In the blood, an increase in cortisol and blood insulin levels was seen without any significant change in sugar levels. All the changes recorded at high altitude tended to return to basal values after re-induction to sea level. The study suggests that high-altitude hypoxia in some way, possibly involving changes in hormonal profile among other factors, causes an alteration in sensitivity to the taste of PTC, resulting in some of the individuals shifting to lower PTC sensitivity. Received: 24 November 1999 / Accepted: 25 January 2000     

Taste sensitivities to PROP and PTC vary independently in mice

Mammals use common mechanisms to detect, transduce and process taste stimulus information. For example, they share families of receptors that respond to amino acids, and sweet- and bitter-tasting stimuli. Nonetheless, it also clear that different species exhibit unique taste sensitivities that may reflect specific genetic variations. In humans, sensitivities to the chemically similar, bitter-tasting compounds 6-n-propylthiouracil (PROP) and phenylthiocarbamide (PTC) are heritable and strongly correlated, suggesting a common genetic basis. However, it is unknown whether PROP and PTC taste sensitivities are similarly correlated in mice. Here we report that PROP and PTC taste sensitivities vary independently between two inbred strains of mice. In brief-access taste tests C3HeB/FeJ (C3) and SWR/J (SW) mice possess similar taste sensitivity to PTC, while SW mice are significantly more sensitive to PROP than are C3 mice. In two-bottle preference tests, however, SW mice display greater aversion to both compounds. This discrepancy may be explained by the observation that SW mice consumed taste solutions at a greater rate during the intake test than did C3 mice. Therefore, PTC avoidance is correlated with the amount of PTC consumed in the intake tests rather than the concentration of PTC tested. These findings suggest that post-ingestive factors play a significant role in PTC avoidance during intake tests and highlight an important advantage of brief-access tests over intake tests in resolving the gustatory and post-ingestive contributions to taste-related behaviors. Most strikingly, these results demonstrate that in mice, unlike in humans, PTC and PROP taste sensitivities vary independently, thereby suggesting a subtle functional diversity of bitter-taste mechanisms across mammalian species.     

Effects of age and sex on taste sensitivity to phenylthiocarbamide (PTC) in the Berkeley Guidance sample

Using a modification of the Harris and Kalmus method for ascertainingthe taste threshold for phenylthiocarbamide (PTC), data werecollected on 424 Caucasian individuals from the three generationallongitudinal study of the Berkeley Guidance subjects in Berkeley,CA. Analysis of the data indicates the distribution of thresholdsis significantly different from that in the London sample ofHarris and Kalmus in that the frequency of nontasters is abouthalf that found in London. The difference is not attributedto the modifications made in the testing procedure but to areal difference in gene frequency. Both the PTC taster and nontasterphenotypes display a large amount of variation in expressionbetween individuals. They are differently expressed in malesand females, the latter being more sensitive. As people age,the expressions of their genotypes change. Neither age nor sexseem to affect the penetrance of the PTC genes which remainsthe same within each sex at all ages unless some other environmentalfactor, such as radiation, impinges. Consideration of theserelationships is necessary for the investigation of family relationships,the mechanisms responsible for maintaining this ancient geneticpolymorphism and the degree of polytypy present in the species.     

草地贪夜蛾核型多角体病毒对我国草地贪夜蛾不同地理种群毒力的比较分析

草地贪夜蛾Spodoptera frugiperda(J. E. Smith)入侵我国多个地区,逐渐形成地理种群。在草地贪夜蛾核型多角体病毒Spodoptera frugiperda multiple nucleopolyhedrovirus(SfMNPV)生物农药的生产过程中发现,不同地理来源的宿主对SfMNPV的敏感性和产量存在明显差异,显著影响了SfMNPV的生产效率。为解析其敏感性差异及其产生的原因,本研究首先对云南德宏、广东广州、广西钦州、西藏林芝4个地区草地贪夜蛾种群基因型进行了鉴定,采用生物测定法测试SfMNPV对4龄幼虫的口服毒力,然后,通过向幼虫体内注射草地贪夜蛾核多角体病毒出芽型病毒粒子(budded virions, BVs)的方式,越过口服感染中肠的过程,分析敏感性差异发生的阶段。最后比较了高敏感和低敏感种群中肠肠液pH,并基于16S rDNA测序测定了肠道菌群组成。结果表明,广西种群属于纯合玉米型,其余种群为带有水稻型COI标记的杂合玉米型。广西种群对SfMNPV的口服敏感性最低,西藏种群的敏感性最高,但两者注射BV后死亡率差异无统计学意义,暗示病毒敏感性差异...     

甘肃、青海十个民族中苯硫脲尝味能力测定

本文用Harris和Kalmus改良阈值法对甘肃、青海两省汉、回、东乡、保安、藏、裕固、蒙古、撒拉、土、哈萨克等十个民族共计7008人进行了测定。以三号液为味盲界线。结果表明:l)各民族味盲发生率差异极显著(P<0.001),其中保安族味盲发生率是到目前为止我国大陆各民族中发现的仅高于黎族的民族;味盲总发生率与日本人相近。2)各民族总计中男性与女性味盲百分率无显著差异,但尝味阈值女性显著低于男性。3)色觉异常患者与尝味能力的关系与国内外其它学者结果不同,即味盲率和尝味阈值与色觉异常无关。     

Anthropological studies in Assam, India. 4. Observations on the Kalitas

Five regional groups of the Kalitas, an Assamese caste group, have been investigated for the distribution of anthropometric and dermatoglyphic traits as well as for the distribution of ABO blood groups and PTC taste sensitivity. The intergroup variability is to some extent considerable. The possible reasons for that are discussed.     

Anthropological studies in Assam, India. 3. Observations on three Brahmin groups

Three Brahmin groups from Lower, Middle and Upper Assam (Kamrup, Darrang, Sibsagar) have been investigated for the distribution of anthropometric and dermatoglyphic traits as well as for that of ABO blood groups and PTC taste sensitivity. The intergroup variability is discussed.     

Anthropological studies in Assam, India

Four Assamese caste groups--Jogis, Hiras, Kumars and Kaibartas--have been analysed for the distribution of anthropometric and dermatoglyphic traits as well as for the distribution of ABO blood groups and PTC taste sensitivity. The differences among these four caste groups are statistically mostly significant, which can be connected with the history of these groups and their genetic isolation from each other.     

PTC and PROP behave differently in tests of discrimination from their solvents

Better discrimination was possible between phenylthiocarbamide (PTC) solutions and the pure solvent when the solvent was a tasteless low- concentration NaCl solution to which the subject had adapted than when the solvent was purified water. The reverse was true for 6-n- propylthiouracil (PROP). The differences in discrimination for PROP and PTC in the different solvents were caused by differences in the intensity and persistence of aftertastes, rather than a more intense perception of the PTC and PROP tastes per se. This has consequences for traditional approaches to measuring taste sensitivity, as well as indicating that PTC and PROP are not necessarily equivalent indicators of 'taster' versus 'non-taster' status.      

Anthropological studies in Assam, India. 2. Observations on Muslims

Six Muslim groups from various parts of Assam (Goalpara, Kamrup, Darrang, Nowgong, Sibsagar and Dibrugarh) have been analysed for the distribution of anthropometric and dermatoglyphic traits as well as for that of AB0 blood groups and PTC taste sensitivity. The intergroup variability in the distribution of all these anthropological characters is considerable. The possible reasons therefore are discussed.