Evolutionary insights into umami, sweet, and bitter taste receptors in amphibians

Relationships between Bitter Taste Receptor Gene Evolution, Diet, and Gene Repertoire in Primates

Bitter taste perception plays a critical role in deterring animals from consuming harmful and toxic substances. To characterize the evolution of primate Tas2r, test the generality of Tas2r duplication in Cercopithecidae species, and examine whether dietary preferences have shaped the Tas2r repertoire of primate species, we identified Tas2r in the genomes of 35 primate species, including 16 Cercopithecidae, 6 Hominidae, 4 Cebidae, 3 Lemuridae, and 6 other species. The results showed that the total number of primate Tas2r ranged from 27 to 51, concentrating on 2 to 4 scaffolds of each species. Closely related genes were tandemly duplicated in the same scaffold. Phylogenetic construction revealed that Tas2r can be divided into 21 clades, including anthropoid-, Strepsirrhini-, and Cercopithecidae-specific Tas2r duplications. Phylogenetically independent contrast analysis revealed that the number of intact Tas2r significantly correlated with feeding preferences. Altogether, our data support diet as a driver of primate Tas2r evolution, and Cercopithecidae species have developed some specific Tas2r duplication during evolution. These results are probably because most Cercopithecidae species feed on plants containing many toxins, and it is necessary to develop specialized Tas2r to protect them from poisoning.

A singular shark bitter taste receptor provides insights into the evolution of bitter taste perception

Many animal and plant species synthesize toxic compounds as deterrent; thus, detection of these compounds is of vital importance to avoid their ingestion. Often, such compounds are recognized by taste 2 receptors that mediate bitter taste in humans. Until now, bitter taste receptors have only been found in bony vertebrates, where they occur as a large family already in coelacanth, a "living fossil" and the earliest-diverging extant lobe-finned fish. Here, we have revisited the evolutionary origin of taste 2 receptors (T2Rs) making use of a multitude of recently available cartilaginous fish genomes. We have identified a singular T2R in 12 cartilaginous fish species (9 sharks, 1 sawfish, and 2 skates), which represents a sister clade to all bony fish T2Rs. We have examined its ligands for two shark species, a catshark and a bamboo shark. The ligand repertoire of bamboo shark represents a subset of that of the catshark, with roughly similar thresholds. Amarogentin, one of the most bitter natural substances for humans, also elicited the highest signal amplitudes with both shark receptors. Other subsets of ligands are shared with basal bony fish T2Rs indicating an astonishing degree of functional conservation over nearly 500 mya of separate evolution. Both shark receptors respond to endogenous steroids as well as xenobiotic compounds, whereas separate receptors exist for xenobiotics both in early- and late-derived bony vertebrates (coelacanth, zebrafish, and human), consistent with the shark T2R reflecting the original ligand repertoire of the ancestral bitter taste receptor at the evolutionary origin of this family.

The evolution of bitter taste receptor gene in primates: Gene duplication and selection

Bitter taste perception plays an important role in preventing animals from digesting poisonous and harmful substances. In primates, especially the Cercopithecidae species, most species feed on plants; thus, it is reasonable to speculate that most of the bitter taste receptor genes (T2Rs) of primates are under purifying selection to maintain the functional stability of bitter taste perception. Gene duplication has happened in T2Rs frequently, and what will be the fate of T2Rs copies is another question we are concerned about. To answer these questions, we selected the T2Rs of primates reported in another study and conducted corresponding selective pressure analyses to determine what kind of selective pressure was acting on them. Further, we carried out selective pressure analyses on gene copies and their corresponding ancestors by considering several possible situations. The results showed that among the 25 gene groups examined here, 15 groups are subject to purifying selection and others are under relaxed selection, with many positively selected sites detected. Gene copies existed in several groups, but only some groups (clade1_a1-b2, clade1_c-c2, clade1_d1-d3, clade1_f1-f2, T2R10, T2R13, and T2R42) have positively selected sites, inferring that they may have some relation to functional divergence. Taken together, T2Rs in primates are under diverse selective pressures, and most gene copies are subject to the same selective pressures. In such cases, the copies may be just to keep the function conservative, and more copies can increase the quantity of the bitter taste receptor, raise the efficiency of bitter substance recognition, and finally enhance the fitness of feeding during the evolutionary course of primates. This study can improve our understanding of T2Rs evolution in primates.

Evolution of bitter receptor genes and ontogenetic dietary shift in a frog

Vertebrate Tas2r taste receptors detect bitter compounds that are potentially poisonous. Previous studies found substantial variation in the number of Tas2r genes across vertebrates, with some frog species carrying the largest number. Peculiar among vertebrates, frogs undergo metamorphosis, often associated with a dietary shift between tadpoles and adults. A possible explanation for the large size of frog Tas2r families could be that distinct sets of Tas2r genes are required for tadpoles and adults, suggesting differential expression of Tas2r genes between tadpoles and adults. To test this hypothesis, we first examined 20 amphibian genomes and found that amphibians generally possess more Tas2r genes than do other vertebrate clades. We next focused on the American bullfrog (Lithobates catesbeianus) to examine the expression of its Tas2r genes in herbivorous tadpoles and insectivorous adult frogs. We report that close to one fifth of its 180 Tas2r genes are differentially expressed (22 genes enriched in adults and 11 in tadpoles). Tuning properties were determined for a subset of differentially expressed genes by a cell-based functional assay, with the adult-enriched Tas2r gene set covering a larger range of ligands compared to the tadpole-enriched subset. These results suggest a role of Tas2r genes in the ontogenetic dietary shift of frogs and potentially initiate a new avenue of ontogenetic analysis of diet-related genes in the animal kingdom.

Role of feeding specialization in taste receptor loss: insights from sweet and umami receptor evolution in Carnivora

Controversy and misunderstanding surround the role of feeding specialization in taste receptor loss in vertebrates. We refined and tested the hypothesis that this loss is caused by feeding specializations. Specifically, feeding specializations were proposed to trigger time-dependent process of taste receptor loss through deprivation of benefit of using the receptor's gustatory function. We propose that this process may be accelerated by abiotic environmental conditions or decelerated/stopped because of extragustatory functions of the receptor's protein(s). As test case we used evolution of the sweet (TAS1R2+TAS1R3) and umami (TAS1R1+TAS1R3) receptors in Carnivora (dogs, cats, and kin). We predicted these receptors' absence/presence using data on presence/absence of inactivating mutations in these receptors' genes and data from behavioral sweet/umami preference tests. We identified 20 evolutionary events of sweet (11) or umami (9) receptor loss. These events affected species with feeding specializations predicted to favor sweet/umami receptor loss (27 and 22 species, respectively). All species with feeding habits predicted to favor sweet/umami receptor retention (11 and 24, respectively) were found to retain that receptor. Six species retained the sweet (5) or umami (1) receptor despite feeding specialization predicted to favor loss of that receptor, which can be explained by the time dependence of sweet/umami receptor loss process and the possible decelerating effect of TAS1R extragustatory functions so that the sweet/umami receptor process is ongoing in these species. Our findings support the idea that feeding specialization leads to taste receptor loss and is the main if not only triggering factor for evolutionary loss of taste receptors.