Taste preferences of the common vampire bat (Desmodus rotundus)

Loss of sweet taste despite the conservation of sweet receptor genes in insectivorous bats

The sense of taste provides key information on diet, but evolution of taste receptor genes in vertebrates is sometimes unable to predict their feeding ecology. Here we use behavioral experiments and functional assays to demonstrate the loss of sweet taste despite the conservation of sweet receptor genes in insectivorous bats. Although sweet taste receptor genes were highly conserved between frugivorous and insectivorous bats at the sequence level, our behavioral experiments revealed dramatic divergence in two bat species with distinct diets: the insectivorous bat showed no preference for natural sugars, whereas the frugivorous bat showed strong preferences for sucrose and fructose. Our cell-based assays from multiple representative bat species across the phylogeny further supported the behavioral preference tests.

The evolution of taste perception is usually associated with the ecology and dietary changes of organisms. However, the association between feeding ecology and taste receptor evolution is unclear in some lineages of vertebrate animals. One example is the sweet taste receptor gene Tas1r2. Previous analysis of partial sequences has revealed that Tas1r2 has undergone equally strong purifying selection between insectivorous and frugivorous bats. To test whether the sweet taste function is also important in bats with contrasting diets, we examined the complete coding sequences of both sweet taste receptor genes (Tas1r2 and Tas1r3) in 34 representative bat species. Although these two genes are highly conserved between frugivorous and insectivorous bats at the sequence level, our behavioral experiments revealed that an insectivorous bat (Myotis ricketti) showed no preference for natural sugars, whereas the frugivorous species (Rousettus leschenaultii) showed strong preferences for sucrose and fructose. Furthermore, while both sweet taste receptor genes are expressed in the taste tissue of insectivorous and frugivorous bats, our cell-based assays revealed striking functional divergence: the sweet taste receptors of frugivorous bats are able to respond to natural sugars whereas those of insectivorous bats are not, which is consistent with the behavioral preference tests, suggesting that functional evolution of sweet taste receptors is closely related to diet. This comprehensive study suggests that using sequence conservation alone could be misleading in inferring protein and physiological function and highlights the power of combining behavioral experiments, expression analysis, and functional assays in molecular evolutionary studies.

Bitter taste receptors of the common vampire bat are functional and show conserved responses to metal ions in vitro

The bitter taste sensation is important to warn mammals of the ingestion of potentially toxic food compounds. For mammals, whose nutrition relies on highly specific food sources, such as blood in the case of vampire bats, it is unknown if bitter sensing is involved in prey selection. By contrast to other bat species, vampire bats exhibit numerous bitter taste receptor pseudogenes, which could point to a decreased importance of bitter taste. However, electrophysiological and behavioural studies suggest the existence of functional bitter taste transmission. To determine the agonist spectra of the three bitter taste receptors that are conserved in all three vampire bat species, we investigated the in vitro activation of Desmodus rotundus T2R1, T2R4 and T2R7. Using a set of 57 natural and synthetic bitter compounds, we were able to identify agonists for all three receptors. Hence, we confirmed a persisting functionality and, consequently, a putative biological role of bitter taste receptors in vampire bats. Furthermore, the activation of the human TAS2R7 by metal ions is shown to be conserved in D. rotundus.

Molecular evolution and deorphanization of bitter taste receptors in a vampire bat

Bats represent the largest dietary radiation in a single mammalian order, and have become an emerging model group for studying dietary evolution. Taste receptor genes have proven to be molecular signatures of dietary diversification in bats. For example, all 3 extant species of vampire bats have lost many bitter taste receptor genes (Tas2rs) in association with their dietary shift from insectivory to sanguivory. Indeed, only 8 full-length Tas2rs were identified from the high-quality genome of the common vampire bat (Desmodus rotundus). However, it is presently unknown whether these bitter receptors are functional, since the sense of taste is less important in vampire bats, which have an extremely narrow diet and rely on other senses for acquiring food. Here, we applied a molecular evolutionary analysis of Tas2rs in the common vampire bat compared with non-vampire bats. Furthermore, we provided the first attempt to deorphanize all bitter receptors of the vampire bat using a cell-based assay. We found that all Tas2r genes in the vampire bat have a level of selective pressure similar to that in non-vampire bats, suggesting that this species must have retained some bitter taste functions. We demonstrated that 5 of the 8 bitter receptors in the vampire bat can be activated by some bitter compounds, and observed that the vampire bat generally can not detect naturally occurring bitter compounds examined in this study. Our study demonstrates functional retention of bitter taste in vampire bats as suggested by cell-based functional assays, calling for an in-depth study of extra-oral functions of bitter taste receptors.

Lineage-specific duplication and adaptive evolution of bitter taste receptor genes in bats

By generating raw genetic material and diverse biological functions, gene duplication represents a major evolutionary mechanism that is of fundamental importance in ecological adaptation. The lineage-specific duplication events of bitter taste receptor genes (Tas2rs) have been identified in a number of vertebrates, but functional evolution of new Tas2r copies after duplication remains largely unknown. Here, we present the largest data set of bat Tas2rs to date, identified from existing genome sequences of 15 bat species and newly sequenced from 17 bat species, and demonstrate lineage-specific duplications of Tas2r16, Tas2r18 and Tas2r41 that only occurred in Myotis bats. Myotis bats are highly speciose and represent the only mammalian genus that is naturally distributed on every continent except Antarctica. The occupation of such diverse habitats might have driven the Tas2r gene expansion. New copies of Tas2rs in Myotis bats have shown molecular adaptation and functional divergence. For example, three copies of Tas2r16 in Myotis davidii showed differential sensitivities to arbutin and salicin that may occur in their insect prey, as suggested by cell-based functional assays. We hypothesize that functional differences among Tas2r copies in Myotis bats would increase their survival rate through preventing the ingestion of an elevated number of bitter-tasting dietary toxins from their insect prey, which may have facilitated their adaptation to diverse habitats. Our study demonstrates functional changes of new Tas2r copies after lineage-specific duplications in Myotis bats and highlights the potential role of taste perception in exploiting new environments.

Vampire bats exhibit evolutionary reduction of bitter taste receptor genes common to other bats

The bitter taste serves as an important natural defence against the ingestion of poisonous foods and is thus believed to be indispensable in animals. However, vampire bats are obligate blood feeders that show a reduced behavioural response towards bitter-tasting compounds. To test whether bitter taste receptor genes (T2Rs) have been relaxed from selective constraint in vampire bats, we sampled all three vampire bat species and 11 non-vampire bats, and sequenced nine one-to-one orthologous T2Rs that are assumed to be functionally conserved in all bats. We generated 85 T2R sequences and found that vampire bats have a significantly greater percentage of pseudogenes than other bats. These results strongly suggest a relaxation of selective constraint and a reduction of bitter taste function in vampire bats. We also found that vampire bats retain many intact T2Rs, and that the taste signalling pathway gene Calhm1 remains complete and intact with strong functional constraint. These results suggest the presence of some bitter taste function in vampire bats, although it is not likely to play a major role in food selection. Together, our study suggests that the evolutionary reduction of bitter taste function in animals is more pervasive than previously believed, and highlights the importance of extra-oral functions of taste receptor genes.

Genetics of taste receptors

Taste receptors function as one of the interfaces between internal and external milieus. Taste receptors for sweet and umami (T1R [taste receptor, type 1]), bitter (T2R [taste receptor, type 2]), and salty (ENaC [epithelial sodium channel]) have been discovered in the recent years, but transduction mechanisms of sour taste and ENaC-independent salt taste are still poorly understood. In addition to these five main taste qualities, the taste system detects such noncanonical "tastes" as water, fat, and complex carbohydrates, but their reception mechanisms require further research. Variations in taste receptor genes between and within vertebrate species contribute to individual and species differences in taste-related behaviors. These variations are shaped by evolutionary forces and reflect species adaptations to their chemical environments and feeding ecology. Principles of drug discovery can be applied to taste receptors as targets in order to develop novel taste compounds to satisfy demand in better artificial sweeteners, enhancers of sugar and sodium taste, and blockers of bitterness of food ingredients and oral medications.

Do polymorphisms in the TAS1R1 gene contribute to broader differences in human taste intensity?

The TAS1R genes encode heterodimeric receptors that mediate umami (hTAS1R1 + hTAS1R3) and sweet (hTAS1R2 + hTAS1R3) sensations. The question of interest for this study is if TAS1R1 variation associates with differences in overall taste intensity. We leveraged an existing database of adults (n = 92, primarily European American) to test associations between 2 TAS1R1 single nucleotide polymorphisms (SNPs) (intronic rs17492553, C/T and exonic rs34160967, G/A) and intensity of 4 prototypical tastants (NaCl, sucrose, citric acid, and quinine), applied regionally to fungiform and circumvallate loci, and sampled with the whole mouth. Both SNPs were associated with modest shifts in perceived intensities across all taste qualities. Three genotype groups were represented for the intronic SNP-minor allele homozygotes (TT) averaged 40% lower intensities than did CC homozygotes for all regionally applied tastants, as well as whole-mouth NaCl and citric acid. Similar, but less pronounced, intensity differences were seen for the exonic SNP (GG homozygotes reported greater intensities than did the AA/AG group). Our predominantly European American cohort had a low frequency of AA homozygotes, which may have attenuated the SNP-related differences in perceived intensity. These preliminary findings, if replicated, could add TAS1R1 polymorphisms to the repertoire of genotypic and phenotypic markers of heightened taste sensation.

From the ultrasonic to the infrared: molecular evolution and the sensory biology of bats

Great advances have been made recently in understanding the genetic basis of the sensory biology of bats. Research has focused on the molecular evolution of candidate sensory genes, genes with known functions [e.g., olfactory receptor (OR) genes] and genes identified from mutations associated with sensory deficits (e.g., blindness and deafness). For example, the FoxP2 gene, underpinning vocal behavior and sensorimotor coordination, has undergone diversification in bats, while several genes associated with audition show parallel amino acid substitutions in unrelated lineages of echolocating bats and, in some cases, in echolocating dolphins, representing a classic case of convergent molecular evolution. Vision genes encoding the photopigments rhodopsin and the long-wave sensitive opsin are functional in bats, while that encoding the short-wave sensitive opsin has lost functionality in rhinolophoid bats using high-duty cycle laryngeal echolocation, suggesting a sensory trade-off between investment in vision and echolocation. In terms of olfaction, bats appear to have a distinctive OR repertoire compared with other mammals, and a gene involved in signal transduction in the vomeronasal system has become non-functional in most bat species. Bitter taste receptors appear to have undergone a "birth-and death" evolution involving extensive gene duplication and loss, unlike genes coding for sweet and umami tastes that show conservation across most lineages but loss in vampire bats. Common vampire bats have also undergone adaptations for thermoperception, via alternative splicing resulting in the evolution of a novel heat-sensitive channel. The future for understanding the molecular basis of sensory biology is promising, with great potential for comparative genomic analyses, studies on gene regulation and expression, exploration of the role of alternative splicing in the generation of proteomic diversity, and linking genetic mechanisms to behavioral consequences.

Major taste loss in carnivorous mammals

Mammalian sweet taste is primarily mediated by the type 1 taste receptor Tas1r2/Tas1r3, whereas Tas1r1/Tas1r3 act as the principal umami taste receptor. Bitter taste is mediated by a different group of G protein-coupled receptors, the Tas2rs, numbering 3 to ∼66, depending on the species. We showed previously that the behavioral indifference of cats toward sweet-tasting compounds can be explained by the pseudogenization of the Tas1r2 gene, which encodes the Tas1r2 receptor. To examine the generality of this finding, we sequenced the entire coding region of Tas1r2 from 12 species in the order Carnivora. Seven of these nonfeline species, all of which are exclusive meat eaters, also have independently pseudogenized Tas1r2 caused by ORF-disrupting mutations. Fittingly, the purifying selection pressure is markedly relaxed in these species with a pseudogenized Tas1r2. In behavioral tests, the Asian otter (defective Tas1r2) showed no preference for sweet compounds, but the spectacled bear (intact Tas1r2) did. In addition to the inactivation of Tas1r2, we found that sea lion Tas1r1 and Tas1r3 are also pseudogenized, consistent with their unique feeding behavior, which entails swallowing food whole without chewing. The extensive loss of Tas1r receptor function is not restricted to the sea lion: the bottlenose dolphin, which evolved independently from the sea lion but displays similar feeding behavior, also has all three Tas1rs inactivated, and may also lack functional bitter receptors. These data provide strong support for the view that loss of taste receptor function in mammals is widespread and directly related to feeding specializations.

Genomic and genetic evidence for the loss of umami taste in bats

Umami taste is responsible for sensing monosodium glutamate, nucleotide enhancers, and other amino acids that are appetitive to vertebrates and is one of the five basic tastes that also include sour, salty, sweet, and bitter. To study how ecological factors, especially diets, impact the evolution of the umami taste, we examined the umami taste receptor gene Tas1r1 in a phylogenetically diverse group of bats including fruit eaters, insect eaters, and blood feeders. We found that Tas1r1 is absent, unamplifiable, or pseudogenized in each of the 31 species examined, including the genome sequences of two species, suggesting the loss of the umami taste in most, if not all, bats regardless of their food preferences. Most strikingly, vampire bats have also lost the sweet taste receptor gene Tas1r2 and the gene required for both umami and sweet tastes (Tas1r3), being the first known mammalian group to lack two of the five tastes. The puzzling absence of the umami taste in bats calls for a better understanding of the roles that this taste plays in the daily life of vertebrates.

Evolution of the sweet taste receptor gene Tas1r2 in bats

Taste perception is an important component of an animal's fitness. The identification of vertebrate taste receptor genes in the last decade has enabled molecular genetic studies of the evolution of taste perception in the context of the ecology and dietary preferences of organisms. Although such analyses have been conducted in a number of species for bitter taste receptors, a similar analysis of sweet taste receptors is lacking. Here, we survey the sole sweet taste-specific receptor gene Tas1r2 in 42 bat species that represent all major lineages of the order Chiroptera, one of the most diverse groups of mammals in terms of diet. We found that Tas1r2 is under strong purifying selection in the majority of the bats studied, with no significant difference in the strength of the selection between insect eaters and fruit eaters. However, Tas1r2 is a pseudogene in all three vampire bat species and the functional relaxation likely started in their common ancestor, probably due to the exclusive feeding of vampire bats on blood and their reliance on infrared sensors rather than taste perception to locate blood sources. Our survey of available genome sequences, together with previous reports, revealed additional losses of Tas1r2 in horse, cat, chicken, zebra finch, and western clawed frog, indicating that sweet perception is not as conserved as previously thought. Nonetheless, we found no common dietary pattern among the Tas1r2-lacking vertebrates, suggesting different causes for the losses of Tas1r2 in different species. The complexity of the ecological factors that impact the evolution of Tas1r2 calls for a better understanding of the physiological roles of sweet perception in different species.