Lowered sensitivity of bitter taste receptors to β-glucosides in bamboo lemurs: an instance of parallel and adaptive functional decline in TAS2R16?

The Remarkable Diversity of Vertebrate Bitter Taste Receptors: Recent Advances in Genomic and Functional Studies

Bitter taste perception is crucial for animal survival. By detecting potentially harmful substances, such as plant secondary metabolites, as bitter, animals can avoid ingesting toxic compounds. In vertebrates, this function is mediated by taste receptors type 2 (T2Rs), a family of G protein-coupled receptors (GPCRs) expressed on taste buds. Given their vital roles, T2Rs have undergone significant selective pressures throughout vertebrate evolution, leading to frequent gene duplications and deletions, functional changes, and intrapopulation differentiation across various lineages. Recent advancements in genomic and functional research have uncovered the repertoires and functions of bitter taste receptors in a wide range of vertebrate species, shedding light on their evolution in relation to dietary habits and other ecological factors. This review summarizes recent research on bitter taste receptors and explores the mechanisms driving the diversity of these receptors from the perspective of vertebrate ecology and evolution.

Whole Genome Analysis Reveals Evolutionary History and Introgression Events in Bale Monkeys

Background/Objective: The Bale monkey (Chlorocebus djamdjamensis) is a threatened primate species endemic to Ethiopia and, in contrast to other members of the genus Chlorocebus, lives at high altitudes and feeds mainly on bamboo. Two populations of the species are present, one in continuous bamboo forest (CF) in the eastern part of the species' range, and the other in fragmented forest (FF) in the western part. Based on mitochondrial DNA and phenotypic characteristics, previous studies have suggested introgression by parapatric congeners into the FF population but not into the CF population. The objective of this study was to gain insights into the evolutionary history of Bale monkeys and their potential genetic adaptations to high altitudes and for bamboo consumption. Methods: We sequenced the whole genomes of individuals from both populations and compared their genomes with those of the other five Chlorocebus species. We applied phylogenetic methods and conducted population demographic simulations to elucidate their evolutionary history. A genome-wide analysis was conducted to assess gene flow and identify mutations potentially associated with adaptations to high altitudes and for bamboo metabolism. Results: Our analyses revealed Bale monkeys as the sister clade to Chlorocebus aethiops and showed that gene flow occurred between C. aethiops and FF but not between C. aethiops and CF. In addition, we detected non-synonymous mutations in genes potentially associated with the adaptation to high altitudes (EPAS1) in both populations and with the adaptation for bamboo metabolism (TAS2R16, MPST, and TST) mainly in the CF population. Conclusions: Our study provides insights into the evolutionary history of a threatened primate species and reveals the genetic basis for its adaptions to unique environments and for diet specialization.

Reduction of bitter taste receptor gene family in folivorous colobine primates relative to omnivorous cercopithecine primates

Bitter taste perception is important in preventing animals from ingesting potentially toxic compounds. Whole-genome assembly (WGA) data have revealed that bitter taste receptor genes (TAS2Rs) comprise a multigene family with dozens of intact and disrupted genes in primates. However, publicly available WGA data are often incomplete, especially for multigene families. In this study, we employed a targeted capture (TC) approach specifically probing TAS2Rs for ten species of cercopithecid primates with diverse diets, including eight omnivorous cercopithecine species and two folivorous colobine species. We designed RNA probes for all TAS2Rs that we modeled to be intact in the common ancestor of cercopithecids ("ancestral-cercopithecid TAS2R gene set"). The TC was followed by short-read and high-depth massive-parallel sequencing. TC retrieved more intact TAS2R genes than found in WGA databases. We confirmed a large number of gene "births" at the common ancestor of cercopithecids and found that the colobine common ancestor and the cercopithecine common ancestor had contrasting trajectories: four gene "deaths" and three gene births, respectively. The number of intact TAS2R genes was markedly reduced in colobines (25-28 detected via TC and 20-26 detected via WGA analysis) as compared with cercopithecines (27-36 via TC and 19-30 via WGA). Birth or death events occurred at almost every phylogenetic-tree branch, making the composition of intact genes variable among species. These results show that evolutionary change in intact TAS2R genes is a complex process, refute a simple general prediction that herbivory favors more TAS2R genes, and have implications for understanding dietary adaptations and the evolution of detoxification abilities.

Dietary flexibility of the greater bamboo lemur (Prolemur simus), a specialized feeder, in eastern Madagascar

The degree of dietary flexibility in primates is species specific; some incorporate a wider array of resources than others. Extreme interannual weather variability in Madagascar results in seasonal resource scarcity which has been linked to specialized behaviors in lemurs. Prolemur simus, for example, has been considered an obligate specialist on large culm bamboo with >60% of its diet composed of woody bamboos requiring morphological and physiological adaptations to process. Recent studies reported an ever-expanding list of dietary items, suggesting that this species may not be an obligate specialist. However, long-term quantitative feeding data are unavailable across this species' range. To explore the dietary flexibility of P. simus, we collected data at two northern sites, Ambalafary and Sahavola, and one southern site, Vatovavy, from September 2010 to January 2016 and May 2017 to September 2018, respectively. In total, we recorded 4022 h of behavioral data using instantaneous sampling of adult males and females from one group in Ambalafary, and two groups each in Sahavola and Vatovavy. We recorded 45 plant species eaten by P. simus over 7 years. We also observed significant differences in seasonal dietary composition between study sites. In Ambalafary, bamboo was the most frequently observed resource consumed (92.2%); however, non-bamboo resources comprised nearly one-third of the diet of P. simus in Sahavola and over 60% in Vatovavy. Consumption of all bamboo resources increased during the dry season at Ambalafary and during the wet season at Vatovavy, but never exceeded non-bamboo feeding at the latter. Culm pith feeding was only observed at Ambalafary, where it was more common during the dry season. We identify P. simus as a bamboo facultative specialist capable of adjusting its feeding behavior to its environment, indicating greater dietary flexibility than previously documented, which may enable the species to survive in increasingly degraded habitats.

Inhibitors of human bitter taste receptors from the five-flavour berry, Schisandra chinensis

The human bitter taste 2 receptor member 16 (TAS2R16) is one of 25 class A G-protein-coupled receptors (GPCRs) and responds to a variety of molecules responsible for the bitter taste sensation perceived in humans. TAS2R16 can be activated by β-glucopyranosides, and its activation can be inhibited by probenecid, a synthetic drug compound used to treat gout. In this study we describe naturally derived compounds which can inhibit the activation of TAS2R16 by salicin in vitro. These compounds belong to the lignan class derived from the fruit of Schisandra chinensis, which is commonly known as the five-flavour berry. We further tested other analogs with this lignan scaffold, found their differential inhibitory activities towards TAS2R16 in vitro, and sought to rationalize the activity using molecular docking of these lignans on a computationally modelled structure of TAS2R16. Selected lignans with inhibitory activity against other TAS2Rs reveal sub-millimolar inhibitory activity towards TAS2R10, TAS2R14, and TAS2R43 in cell-based assays. These compounds with demonstrated in vitro inhibition of bitter taste receptors may serve as tool compounds to investigate the molecular mechanisms of hTAS2Rs biology in gustatory and non-gustatory tissues.

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.

The sensory ecology of primate food perception, revisited

Twenty years ago, Dominy and colleagues published "The sensory ecology of primate food perception," an impactful review that brought new perspectives to understanding primate foraging adaptations. Their review synthesized information on primate senses and explored how senses informed feeding behavior. Research on primate sensory ecology has seen explosive growth in the last two decades. Here, we revisit this important topic, focusing on the numerous new discoveries and lines of innovative research. We begin by reviewing each of the five traditionally recognized senses involved in foraging: audition, olfaction, vision, touch, and taste. For each sense, we provide an overview of sensory function and comparative ecology, comment on the state of knowledge at the time of the original review, and highlight advancements and lingering gaps in knowledge. Next, we provide an outline for creative, multidisciplinary, and innovative future research programs that we anticipate will generate exciting new discoveries in the next two decades.

Functional Diversity and Evolution of Bitter Taste Receptors in Egg-Laying Mammals

Egg-laying mammals (monotremes) are a sister clade of therians (placental mammals and marsupials) and a key clade to understand mammalian evolution. They are classified into platypus and echidna, which exhibit distinct ecological features such as habitats and diet. Chemosensory genes, which encode sensory receptors for taste and smell, are believed to adapt to the individual habitats and diet of each mammal. In this study, we focused on the molecular evolution of bitter taste receptors (TAS2Rs) in monotremes. The sense of bitter taste is important to detect potentially harmful substances. We comprehensively surveyed agonists of all TAS2Rs in platypus (Ornithorhynchus anatinus) and short-beaked echidna (Tachyglossus aculeatus) and compared their functions with orthologous TAS2Rs of marsupial and placental mammals (i.e., therians). As results, the agonist screening revealed that the deorphanized monotreme receptors were functionally diversified. Platypus TAS2Rs had broader receptive ranges of agonists than those of echidna TAS2Rs. While platypus consumes a variety of aquatic invertebrates, echidna mainly consumes subterranean social insects (ants and termites) as well as other invertebrates. This result indicates that receptive ranges of TAS2Rs could be associated with feeding habits in monotremes. Furthermore, some orthologous receptors in monotremes and therians responded to β-glucosides, which are feeding deterrents in plants and insects. These results suggest that the ability to detect β-glucosides and other substances might be shared and ancestral among mammals.

The Investigation of Phenylalanine, Glucosinolate, Benzylisothiocyanate (BITC) and Cyanogenic Glucoside of Papaya Fruits (Carica papaya L. cv. ‘Tainung No. 2’) under Different Development Stages between Seasons and Their Correlation with Bitter Taste

Papaya fruit is one of economic crops in Taiwan, mostly eaten as table fruits. In some Asian countries, unripe papaya fruit is eaten as salad and this led to trends in Taiwan as well. However, unripe papaya fruit may taste bitter during cool seasons. Glucosinolate and cyanogenic glucoside are among the substances that cause bitter taste in many plants, which can also be found in papaya. However, there is still no report about the relationship between seasons and bitter taste in papaya fruits. Thus, the purpose of this study is to investigate the glucosinolate biosynthesis and its correlation between bitterness intensity during cool and warm seasons. The bitterness intensity was highest at the young fruit stage and decreased as it developed. In addition, the bitterness intensity in cool season fruits is higher than in warm season fruits. Cyanogenic glucoside and BITC content showed negative correlation with bitterness intensity (r = −0.54 ***; −0.46 ***). Phenylalanine showed positive correlation with bitterness intensity (r = 0.35 ***), but its content did not reach the bitterness threshold concentration, which suggested that phenylalanine only acts as cyanogenic glucoside and glucosinolate precusors. Glucosinolate content showed positive correlation with bitterness intensity at different developmental stages (r = 0.805 ***). However, the correlation value in different lines/cultivars decreased (0.44 ***), suggesting that glucosinolate was not the only substance that caused bitter taste in immature papaya fruits.

Lowered sensitivity of bitter taste receptors to β-glucosides in bamboo lemurs: an instance of parallel and adaptive functional decline in TAS2R16?

Bitter taste facilitates the detection of potentially harmful substances and is perceived via bitter taste receptors (TAS2Rs) expressed on the tongue and oral cavity in vertebrates. In primates, TAS2R16 specifically recognizes β-glucosides, which are important in cyanogenic plants' use of cyanide as a feeding deterrent. In this study, we performed cell-based functional assays for investigating the sensitivity of TAS2R16 to β-glucosides in three species of bamboo lemurs (Prolemur simus, Hapalemur aureus and H. griseus), which primarily consume high-cyanide bamboo. TAS2R16 receptors from bamboo lemurs had lower sensitivity to β-glucosides, including cyanogenic glucosides, than that of the closely related ring-tailed lemur (Lemur catta). Ancestral reconstructions of TAS2R16 for the bamboo-lemur last common ancestor (LCA) and that of the Hapalemur LCA showed an intermediate sensitivity to β-glucosides between that of the ring-tailed lemurs and bamboo lemurs. Mutagenetic analyses revealed that P. simus and H. griseus had separate species-specific substitutions that led to reduced sensitivity. These results indicate that low sensitivity to β-glucosides at the cellular level—a potentially adaptive trait for feeding on cyanogenic bamboo—evolved independently after the Prolemur–Hapalemur split in each species.