Human taste genetics

The molecular signature of selection underlying human adaptations

In the last decade, advances in human population genetics and comparative genomics have resulted in important contributions to our understanding of human genetic diversity and genetic adaptation. For the first time, we are able to reliably detect the signature of natural selection from patterns of DNA polymorphism. Identifying the effects of natural selection in this way provides a crucial piece of evidence needed to support hypotheses of human adaptation. This review provides a detailed description of the theory and analytical approaches used to detect signatures of natural selection in the human genome. We discuss these methods in relation to four classic human traits--skin color, the Duffy blood group, bitter-taste sensation, and lactase persistence. By highlighting these four traits we are able to discuss the ways in which analyses of DNA polymorphism can lead to inferences regarding past histories of selection. Specifically, we can infer the importance of specific regimes of selection (i.e. directional selection, balancing selection, and purifying selection) in the evolution of a trait because these different types of selection leave different patterns of DNA polymorphism. In addition, we demonstrate how these types of data can be used to estimate the time frame in which selection operated on a trait. As the field has advanced, a general issue that has come to the forefront is how specific demographic events in human history, such as population expansions, bottlenecks, and subdivision of populations, have also left a signature across the genome that can interfere with our detection of the footprint of selection at particular genes. Therefore, we discuss this general problem with respect to the four traits reviewed here, and describe the ways in which the signature of selection can be teased from a background signature of demographic history. Finally, we move from a discussion of analyses of selection motivated by a "candidate-gene" approach, in which a priori information led to the analysis of specific gene, to discussion of "genome-scanning" approaches that are directed at discovering new genes that have been under positive selection. Such scans can be designed to detect those genes that have been positively selected in our divergence from chimpanzees, as well as those genes that have been under selection as human populations have migrated, differentiated, and adapted to specific geographic environments. We predict that both approaches will be applied in the future, enabling a greater insight into human species-wide adaptations, as well as the specific adaptations of human populations.

Behavioral genetics and taste

This review focuses on behavioral genetic studies of sweet, umami, bitter and salt taste responses in mammals. Studies involving mouse inbred strain comparisons and genetic analyses, and their impact on elucidation of taste receptors and transduction mechanisms are discussed. Finally, the effect of genetic variation in taste responsiveness on complex traits such as drug intake is considered. Recent advances in development of genomic resources make behavioral genetics a powerful approach for understanding mechanisms of taste.

X-ray spectrometry and chemometrics in sugar classification, correlation with degree of sweetness and specific rotation of polarized light

This work presents correlations of conventional energy dispersive X-ray fluorescence spectra of common sugars with degrees of sweetness obtained via sensorial tests and specific rotations of polarized light, both data from the literature. Also, classifications of sugars are achieved based on their specific structures. Principal component analysis and partial least square chemometric tools are used to establish these modelings. Once again it is demonstrated that a common bench-top X-ray spectrometer can be used not only for inorganic analysis, but also shows potential in studies of organic constituents.

Functional variants in TAS2R38 and TAS2R16 influence alcohol consumption in high-risk families of African-American origin

BACKGROUND

A novel family of G protein-coupled receptors, TAS2Rs, has recently been characterized and linked to sensitivity to bitter taste compounds. We have previously reported that a missense mutation in the TAS2R16 gene reduces the sensitivity of the receptor to bitter-taste stimuli and that it is associated with risk for alcohol dependence. Other family-based studies on the genetic transmittance of taste perception have previously demonstrated a correlation between genetic variation in TAS2R38 and sensitivity to bitter-taste compounds such as phenylthiocarbamide (PTC) and 6-n-propylthiouracil (PROP). Haplotypes resulting from 3 common nonsynonymous coding single-nucleotide polymorphisms in the TAS2R38 gene have been shown to alter receptor functions and taste sensitivity to PTC and PROP. The perceived bitterness of PROP has also been associated with oral sensation and drinking behaviors.

METHODS

We used family-based association methods to test for association between TAS2R38 haplotypes and alcohol dependence as well as a measure of alcohol consumption (Maxdrinks) and age of onset of drinking behaviors in a sample of families densely affected with alcoholism. We have also extended our analysis of TAS2R16 to include the Maxdrinks phenotype.

RESULTS

A positive correlation was observed between TAS2R38 haplotypes and Maxdrinks in Collaborative Study on the Genetics of Alcoholism (COGA) high-risk women of African-American origin. The common taster haplotype is significantly associated with a lower mean Maxdrinks compared with the other haplotypes. Similarly, the allele of TAS2R16 that is associated with a lower risk for alcohol dependence is also associated with lower mean Maxdrinks scores in African-American families. In contrast to the previously reported significant association between TAS2R16 and alcohol dependence, we found no evidence that TAS2R38 haplotypes influence alcohol dependence in the COGA dataset.

CONCLUSION

Functional variants in both TAS2R16 and TAS2R38 correlate with alcohol consumption in African-American families.

Phenylthiocarbamide (PTC) perception in patients with schizophrenia and first-degree family members: relationship to clinical symptomatology and psychophysical olfactory performance

The inability to taste phenylthiocarbamide (PTC; "taste-blindness") has been associated with a number of medical and neurological illnesses not typically related to taste. We examined PTC sensitivity in 67 schizophrenia patients, 30 healthy controls, and 30 first-degree relatives to determine whether taster status could represent a simple vulnerability marker. A higher prevalence of non-tasters was seen in patients and family members relative to healthy controls. Among patients, non-tasters exhibited increased levels of negative and first-rank symptoms as well as poorer right nostril odor identification skills relative to PTC tasters. These differences were not explained by age, sex, education, smoking, or intensity differences. Phenotypic variation in PTC sensitivity is thought to be genetic in origin and suggests greater illness risk for those subjects with recessive taster alleles.

Transient receptor potential family members PKD1L3 and PKD2L1 form a candidate sour taste receptor

Animals use their gustatory systems to evaluate the nutritious value, toxicity, sodium content, and acidity of food. Although characterization of molecular identities that receive taste chemicals is essential, molecular receptors underlying sour taste sensation remain unclear. Here, we show that two transient receptor potential (TRP) channel members, PKD1L3 and PKD2L1, are coexpressed in a subset of taste receptor cells in specific taste areas. Cells expressing these molecules are distinct from taste cells having receptors for bitter, sweet, or umami tastants. The PKD2L1 proteins are accumulated at the taste pore region, where taste chemicals are detected. PKD1L3 and PKD2L1 proteins can interact with each other, and coexpression of the PKD1L3 and PKD2L1 is necessary for their functional cell surface expression. Finally, PKD1L3 and PKD2L1 are activated by various acids when coexpressed in heterologous cells but not by other classes of tastants. These results suggest that PKD1L3 and PKD2L1 heteromers may function as sour taste receptors.

Use it or lose it: molecular evolution of sensory signaling in primates

Sensory organs provide key and, in many cases, species-specific information that allows animals to effectively forage, find mates, and avoid hazards. The primary sensors for the vertebrate senses of vision, taste, and smell are G-protein-coupled receptors (GPCRs) expressed by sensory receptor cells that initiate intracellular signal transduction cascades in response to activation by appropriate stimuli. The identification of sensory GPCRs and their related downstream transduction components from a variety of species has provided an essential tool for understanding the molecular evolution of sensory systems. Expansion of the number of genes encoding sensory GPCRs has, in some cases, expanded the repertoire of signals that animals detect, allowing them to occupy new niches, while, in other cases, evolution has favored a reduction in the repertoire of receptors and their cognate signal transduction components when these signals no longer provide a selective advantage. This review will focus on recent studies that have identified molecular changes in vision, smell, taste, and pheromone detection during primate evolution.

Visual Artificial Tongue for Quantitative Metal-Cation Analysis by an Off-the-Shelf Dye Array

A chemical-probe array composed of 47 off-the-shelf dyes was prepared in solution format (New York Tongue 1: NYT-1) and was tested in the identification and quantitation of 47 cation analytes, including 44 metal ions, in addition to H(+), NH(4) (+), and tetrabutylammonium (TBA). The cation solutions were tested in a series of concentrations and the fold-change in effective absorbance was analyzed by principal-component analysis (PCA), hierarchical-cluster analysis (HCA), and nearest-neighbor decision to determine both identity and quantity of the analytes. Apart from alkali-metal ions (Na(+), K(+), Li(+), Cs(+), and Rb(+)), which behave very similarly to each other due mainly to their low response, most of the cations were clearly distinguishable at 10 mM concentration. The practical detection limit of each analyte was also determined by a sequential dilution and the nearest-neighbor decision method. In the finalized working analyte concentration range (approximately 10 mM down to 0.33 microM), by considering alkali metals as one analyte group, most of the analytes were correctly identified (99.4 %). Furthermore, the success rate at which the concentration of each analyte was correctly determined was also high (96.8 %).