Bitter taste receptors: Genes, evolution and health

Bitter taste perception plays vital roles in animal behavior and fitness. By signaling the presence of toxins in foods, particularly noxious defense compounds found in plants, it enables animals to avoid exposure. In vertebrates, bitter perception is initiated by TAS2Rs, a family of G protein-coupled receptors expressed on the surface of taste buds. There, oriented toward the interior of the mouth, they monitor the contents of foods, drinks and other substances as they are ingested. When bitter compounds are encountered, TAS2Rs respond by triggering neural pathways leading to sensation. The importance of this role placed TAS2Rs under selective pressures in the course of their evolution, leaving signatures in patterns of gene gain and loss, sequence polymorphism, and population structure consistent with vertebrates' diverse feeding ecologies. The protective value of bitter taste is reduced in modern humans because contemporary food supplies are safe and abundant. However, this is not always the case. Some crops, particularly in the developing world, retain surprisingly high toxicity and bitterness remains an important measure of safety. Bitter perception also shapes health through its influence on preference driven behaviors such as diet choice, alcohol intake and tobacco use. Further, allelic variation in TAS2Rs is extensive, leading to individual differences in taste sensitivity that drive these behaviors, shaping susceptibility to disease. Thus, bitter taste perception occupies a critical intersection between ancient evolutionary processes and modern human health.

Expression profiles and functional characterization of common carp (Cyprinus carpio) T2Rs

Bitter taste perception is mediated by a family of G protein-coupled receptors (T2Rs) in vertebrates. Common carp (Cyprinus carpio), which has experienced an additional round of whole genome duplication during the course of evolution, has a small number of T2R genes similar to zebrafish, a closely related cyprinid fish species, and their expression pattern at the cellular level or their cognate ligands have not been elucidated yet. Here, we showed through in situ hybridization experiments, that three common carp T2R (ccT2R) genes encoding ccT2R200-1, ccT2R202-1, and ccT2R202-2, were specifically expressed in the subsets of taste receptor cells in the lips and gill rakers. ccT2R200-1 was co-expressed with genes encoding downstream signal transduction molecules, such as PLC-β2 and Gαia. Heterologous expression system revealed that each ccT2R showed narrowly, intermediately, or broadly tuned ligand specificity, as in the case of zebrafish T2Rs. However, ccT2Rs showed different ligand profiles from their orthologous zebrafish T2Rs previously reported. Finally, we identified three ccT2Rs, namely ccT2R200-1, ccT2R200-2, and ccT2R203-1, to be activated by natural bitter compounds, andrographolide and/or picrotoxinin, which elicited no response to zebrafish T2Rs, in a dose-dependent manner. These results suggest that some ccT2Rs may have evolved to function in the oral cavity as taste receptors for natural bitter compounds found in the habitats in a species-specific manner.

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Common carp T2R (ccT2R) gene was co-expressed with genes encoding downstream signal transduction molecules in subsets of taste receptor cells, similar to zebrafish.

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Each ccT2R showed narrowly, intermediately, or broadly tuned ligand specificity, as in the case of zebrafish T2Rs; however, ccT2Rs showed different ligand profiles from their orthologous zebrafish T2Rs previously reported.

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Some ccT2Rs may have evolved to function in the oral cavity as taste receptors for natural bitter compounds found in the habitats in a species-specific manner.

Differences in behavior between surface and cave Astyanax mexicanus may be mediated by changes in catecholamine signaling

Astyanax mexicanus is a teleost fish that is in the process of allopatric speciation. Ancestral Astyanax are found in surface rivers and derived blind forms are found in cave systems. Adaptation to life in nutrient poor caves without predation includes the evolution of enhanced food seeking behaviors and loss of defensive responses. These behavioral adaptations may be mediated by changes in catecholaminergic control systems in the brain. We examined the distribution of tyrosine hydroxylase, a conserved precursor for the synthesis of the catecholamines dopamine and noradrenaline, in the brains of surface and cave Astyanax using immunohistochemistry. We found differences in tyrosine hydroxylase staining in regions that are associated with nonvisual sensory perception, motor control, endocrine release, and attention. These differences included significant increases in the diameters of tyrosine hydroxylase immunoreactive soma in cave Astyanax in the olfactory bulb, basal telencephalon, preoptic nuclei, ventral thalamus, posterior tuberculum, and locus coeruleus. These increases in modulation by dopamine and noradrenaline likely indicate changes in behavioral control that underlie adaptations to the cave environment.

Functional evolution of vertebrate sensory receptors

Sensory receptors enable animals to perceive their external world, and functional properties of receptors evolve to detect the specific cues relevant for an organism's survival. Changes in sensory receptor function or tuning can directly impact an organism's behavior. Functional tests of receptors from multiple species and the generation of chimeric receptors between orthologs with different properties allow for the dissection of the molecular basis of receptor function and identification of the key residues that impart functional changes in different species. Knowledge of these functionally important sites facilitates investigation into questions regarding the role of epistasis and the extent of convergence, as well as the timing of sensory shifts relative to other phenotypic changes. However, as receptors can also play roles in non-sensory tissues, and receptor responses can be modulated by numerous other factors including varying expression levels, alternative splicing, and morphological features of the sensory cell, behavioral validation can be instrumental in confirming that responses observed in heterologous systems play a sensory role. Expression profiling of sensory cells and comparative genomics approaches can shed light on cell-type specific modifications and identify other proteins that may affect receptor function and can provide insight into the correlated evolution of complex suites of traits. Here we review the evolutionary history and diversity of functional responses of the major classes of sensory receptors in vertebrates, including opsins, chemosensory receptors, and ion channels involved in temperature-sensing, mechanosensation and electroreception.

Massive Loss of Olfactory Receptors But Not Trace Amine-Associated Receptors in the World's Deepest-Living Fish (Pseudoliparis swirei)

Olfactory receptor repertoires show highly dynamic evolution associated with ecological adaptations in different species. The Mariana snailfish (Pseudoliparis swirei) living below a depth of 6000 m in the Mariana Trench evolved degraded vision and occupies a specific feeding habitat in a dark, low-food environment. However, whether such adaptations involve adaptive changes in the chemosensory receptor repertoire is not known. Here, we conducted a comparative analysis of the olfactory receptor (OR) and trace amine-associated receptor (TAAR) gene repertoires in nine teleosts with a focus on the evolutionary divergence between the Mariana snailfish and its shallow-sea relative, Tanaka's snailfish (Liparis tanakae). We found many fewer functional OR genes and a significantly higher fraction of pseudogenes in the Mariana snailfish, but the numbers of functional TAAR genes in the two species were comparable. Phylogenetic analysis showed that the expansion patterns of the gene families were shared by the two species, but that Mariana snailfish underwent massive gene losses in its OR repertoire. Despite an overall decreased size in OR subfamilies and a reduced number of TAAR subfamilies in the Mariana snailfish, expansion of certain subfamilies was observed. Selective pressure analysis indicated greatly relaxed selective strength in ORs but a slightly enhanced selective strength in TAARs of Mariana snailfish. Overall, our study reveals simplified but specific OR and TAAR repertoires in the Mariana snailfish shaped by natural selection with respect to ecological adaptations in the hadal environment. This is the first study on the chemosensation evolution in vertebrates living in the hadal zone, which could provide new insights into evolutionary adaptation to the hadal environment.

To see or not to see: molecular evolution of the rhodopsin visual pigment in neotropical electric fishes

Functional variation in rhodopsin, the dim-light-specialized visual pigment, frequently occurs in species inhabiting light-limited environments. Variation in visual function can arise through two processes: relaxation of selection or adaptive evolution improving photon detection in a given environment. Here, we investigate the molecular evolution of rhodopsin in Gymnotiformes, an order of mostly nocturnal South American fishes that evolved sophisticated electrosensory capabilities. Our initial sequencing revealed a mutation associated with visual disease in humans. As these fishes are thought to have poor vision, this would be consistent with a possible sensory trade-off between the visual system and a novel electrosensory system. To investigate this, we surveyed rhodopsin from 147 gymnotiform species, spanning the order, and analysed patterns of molecular evolution. In contrast with our expectation, we detected strong selective constraint in gymnotiform rhodopsin, with rates of non-synonymous to synonymous substitutions lower in gymnotiforms than in other vertebrate lineages. In addition, we found evidence for positive selection on the branch leading to gymnotiforms and on a branch leading to a clade of deep-channel specialized gymnotiform species. We also found evidence that deleterious effects of a human disease-associated substitution are likely to be masked by epistatic substitutions at nearby sites. Our results suggest that rhodopsin remains an important component of the gymnotiform sensory system alongside electrolocation, and that photosensitivity of rhodopsin is well adapted for vision in dim-light environments.