Taste is a short-range contact chemosensation required by all animals to detect nutrient rich foods and avoid consuming toxic chemicals. In insects, it is also required to select mates and appropriate oviposition sites. Organization of the fruit fly Drosophila melanogaster taste system and availability of experimental tool box, makes Drosophila gustatory system an ideal model system for studying the perception of taste and taste elicited behaviors. Like humans, fruit flies also respond to wide range of taste chemical and can differentiate between different taste categories including sweet, bitter, sour, umami and salt. This chapter will present a research progress made in the field of taste using neuroanatomical, genetic, behavioral, molecular and cellular biology techniques in the fruit fly. The compiled survey will provide an outlook of taste research done in fruit fly and its comparison with human taste behavior.
- taste receptors
- taste behavior
The simplicity of the gustatory system of flies provides an ideal situation for comparative studies of taste perception and taste-elicited behaviors. The availability of the experimental tool box including high end imaging of neural circuits in the brain, simple behavioral assays, possibility of electrical recordings and ease of molecular- genetics analyses with the availability of transgenic and mutant flies makes fly a unique system to study taste. In addition, flies share the same molecular logic of taste as mammals.
Different members of gustatory receptor (GRs) genes expressed in gustatory neurons mediate the detection of taste compounds such as sugars and bitter compounds [3, 4, 5, 6, 7]. Expression patterns of taste receptors is based largely on transgenic GAL4 expression studies and suggest that different GRs are expressed in overlapping but non-identical subsets of sugar- and bitter-sensing neurons [6, 7, 8]. In addition, electrophysiological studies from taste neurons suggest heterogeneity among the responses of individual sugar- or bitter-sensing cells [9, 10, 11] suggesting diversity among the peripheral cell types that detect sugars or bitter compounds in
This chapter will present a research progress made in the field of taste perception in the fruit fly and will describe the anatomical properties of the
2. Mammalian taste system
In humans, taste receptors cells (TRC’s) helps in the detection of taste stimuli. TRC’s are present in taste buds and palate epithelium at the back and sides of the tongue (circumvallate and foliate papillae). The taste buds called fungiform are scattered across the front of the tongue and on the palate. Three morphologically distinct cell types (I, II and III) are present in a taste bud and constitute five functional classes of sensory cells, each specialized to detect one of the five basic taste qualities (bitter, sweet, umami, sour and salty). TRCs are epithelial cells that extend a process to the apical surface of the epithelium, where a taste pore allows direct contact with chemicals in the environment. The life of taste cells is short and they replenish from proliferative basal keratinocytes . TRCs can relay information of taste quality independent of cells relaying other taste qualities . Neurotransmitter receptors are present on taste cells. TRC’s release various neurotransmitters to communicate among cells in the taste bud to shape the output of the bud . Vertebrate TRCs do not possess an axon, and instead are innervated by pseudo unipolar neurons whose cell bodies reside in the petrosal and geniculate ganglia. The chorda tympani nerve that (innervates the anterior tongue) contain fungiform papillae and the glossopharyngeal nerve, (innervates the posterior tongue and most of the palate) carry most of the taste information. Neurons from taste ganglia project to the nucleus of the solitary tract, and from there information is relayed to the gustatory cortex .
3. Gustatory system of
Although the same taste preferences are shared between
A single MSN and several support cells are also present in the taste sensilla together with the GRNs (Figure 1) . These MSNs translate mechanical forces into electrical signals and mediate hearing, positional awareness, and the coordination of movements [27, 28]. The MSNs sense the hardness and viscosity of food  similar to the ability of the human tongue to determine the consistency and texture of foods.
3.1 Gustatory receptors of
Like mammals, GRs in
GRs within a subfamily detect structurally similar taste compounds. For example, the sugar receptor
Well established GAL4/UAS system, transgenic expression methods helped visualizing the expression of various
3.2 Types of taste receptors in mammals and
The three tastes, bitter, sweet and umami taste are mediated by taste-specific GPCRs, which are expressed in distinct subsets of taste receptor cells in mammals . These three taste employ a canonical G protein phosphoinositide-based pathway, where receptors activate a taste cell-specific G protein that activates PLCβ2, generating second messengers IP3, DAG and H+. IP3 acts on the IP3 receptor (IP3R3) to release Ca2+ from intracellular stores, and Ca2+ gates the membrane channel TRPM5 .
3.2.1 Sweet receptors
Highly concentrated sugars (100–500 mM), artificial sweeteners, and small number of sweet-tasting proteins elicit the sweet taste in mammals. The heterodimer of T1R2 and T1R3 constitutes the sweet receptor . Animals also sense energy-rich foods and various sugars through a mechanism similar to that used by pancreatic β-cells to detect blood glucose . According to this hypothesis, the metabolism of sugars by sweet cells produces ATP, which closes ATP-sensitive K+ channels leading to membrane depolarization .
Flies are attracted to many of the same sugars as humans [9, 51] although they respond most robustly to disaccharides (such as sucrose and maltose) and oligosaccharides . The fly sweet receptors belong to the same superfamily of receptors that includes most of the bitter receptors. In adult flies the three key receptors required for sensing sugars, except for fructose, are Gr5a, Gr64a and Gr64f [8, 40, 52, 53, 54]. These three receptors are co-expressed in the sugar-responsive GRNs in the labellum, along with five other related GRs that comprise the
Gr5a and Gr64a sense structurally different sugars. Gr64a participates in the response to sucrose and maltose [8, 52], while Gr5a detect trehalose and melezitose [8, 40, 41]. Gr64f might act as a co-receptor for the responses for all sugars tested except fructose, and functions in concert with Gr5a and Gr64a . Gr43a is the only receptor known to detect fructose .
3.2.2 Bitter receptors
Bitter taste allows animals to detect toxins in the environment and avoid consuming them. Compounds such as caffeine, cycloheximide (a protein synthesis inhibitor), denatonium (added to rubbing alcohol to discourage consumption), and quinine (a component of tonic water) taste bitter to humans, mice and flies. In vertebrates, bitter chemicals are detected by a small family of receptors (T2Rs), which are structurally related to rhodopsin, and range in number from 3 to 49, depending on the species [31, 34, 56]. In general, each bitter responsive taste receptor cell expresses multiple types of bitter receptors , but not all bitter receptors are expressed by every bitter cell , leading formally to the possibility that there are subclasses of bitter cells, as is the case in flies . The chemical receptive field of the bitter receptors fall into two classes—“specialists” that detect one or a few bitter chemicals and “generalists” that detect many .
In contrast to vertebrate bitter detection, flies employ a much more complex strategy to sample bitter chemicals. In flies, bitter sensitive GRNs have distinct sensitivities. Based on the response profile to a panel of 16 bitter compounds, the L-, I- and S-type sensilla on the labella are classified into five groups, four of which are sensitive to bitter chemicals (Figure 1) . Out of the four, two groups are narrowly tuned to distinct sets of bitter compounds (I-a and I-b). The other two groups respond broadly to bitter tastants but have variable patterns of activity (S-a, S-b). Analysis with a larger panel of bitter compounds may reveal more additional subgroups.
In flies, 33 out of 38
3.2.3 Salt taste receptors
Moderate levels of salt is necessary to maintain the important physiological functions such as muscle contraction, action potentials and many other functions while excessive salt intake is deleterious and can lead to hypertension. Salty taste is elicited by Na+ concentrations ranging from 10 to 500 mM. In humans, salt taste is amiloride-insensitive. The amiloride sensitive component of salt taste is selective for Na+ and Li+ over other monovalent cations such as K+, is sensitive to low concentrations of salts (<100 mM), and is generally appetitive . Amiloride-sensitive salt taste occurs only in the front of the tongue . Based on taste nerve recordings, there is a population of broadly tuned high-salt fibers that are insensitive to amiloride and activated by KCl and NaCl . These fibers innervate both the front and back of the tongue, in contrast to the amiloride-sensitive fibers that innervate only the front of the tongue. Epithelial Na+ channels (ENaCs) are composed of three subunits—α, β and γ and α subunit is absolutely essential and forms part of the pore . ENaC α has been suggested to be a component of the low salt sensor since a taste-cell specific knockout eliminates sensitivity and behavioral attraction to low concentrations of salt .
The cells that mediate the behavioral responses to high salts are not specifically dedicated to sensing high salt, but instead comprise at least two populations of cells with previously identified functions in sensing bitter and sour . Inactivation of TRPM5 or PLCβ2, expressed by bitter cells, eliminates a component of the high salt response, while silencing PKD2L1-expressing sour cells eliminates the remaining components . Remarkably, mice in which PKD2L1-expressing cells are silenced and TRPM5 is inactivated find high salt concentrations attractive, presumably due to activation of the amiloride-sensitive ENaC channels by high salt .
Salt taste preferences in
3.2.4 Sour taste
Acidic pH and organic acids such as acetic acid evokes sour taste in the tongue. A subset of taste receptor cells in the tongue and palate epithelium that respond to acidic pH and weak organic acids with electrical activity detects the sour taste [78, 79]. PKD2L1-expressing cells respond are required for sensory response to acids [72, 78, 80] which is mediated by an unusual proton-selective ion channel . Proton selectivity allows the cells to respond to acids without interference from Na+, which may vary independently in concentration. The taste of carbonation (CO2) is also detected by PKD2L1-expressing cells. This response is dependent on a membrane anchored carbonic anhydrase isoform 4, Car4 , which interconverts CO2 + H2O to H+ + HCO3−. How Car4 contributes to the activation of sour cells is still not known.
Fruit flies reject foods that are too acidic and prefer the ones which are slightly acidic, such as carbonated water. Carbonated water triggers Ca2+ influx in the region of the SEZ innervated by taste peg GRNs, suggesting these neurons are involved in CO2 detection . Fruit flies avoid many carboxylic acids with a low pH. Behavioral and physiological analysis reveals that the avoidance to carboxylic acid is mainly mediated by a subset of bitter GRNs . The ionotropic receptor Ir7a has been shown for rejecting foods laced with high levels of acetic acid suggesting flies discriminate foods on the basis of acid composition rather than just pH .
3.2.5 Amino acid receptors
Umami (amino acid taste) is the sensation elicited by glutamate. In humans, umami is only elicited by glutamate, while mice are sensitive to a wider range of L-amino acids . Addition of the nucleotides IMP or GMP potentiates the umami response, distinguishing it from a more general sensing of glutamate . T1R1/T1R3 is widely recognized as the umami receptor .
Fruit flies can taste amino acids too, although their preference is enhanced when raised on a food source devoid of amino acids . Female fruit flies show greatest preference for cysteine, phenylalanine, threonine and tyrosine, while males prefer leucine and histidine. None of the 18 standard amino acids tested stimulates action potentials in GRNs in sugar responsive sensilla  raising the possibility of taste pegs in sensing amino acids. Another amino acid, L-canavanine is toxic and elicits an avoidance response in flies  and is sensed by GRNs in a subset of S-type sensilla . Gr8a and Gr66a are both required for L-canavanine avoidance . An ionotropic receptor Ir76b has been shown recently for amino acid taste in flies .
Activation of fly GRNs by sweet substances, bitter compounds and the amino acid, L-canavanine occur through direct activation of ion channels and G-protein signaling pathways. G proteins subunits Gγ, Goα, Gsα and Gqα are implicated in sugar signaling [90, 91, 92, 93]. PLCβ is an effector for Gqα. Knockdown in sugar-responsive GRNs of
3.3 Taste coding
Taste in flies and mammals use labeled line model of coding (in which each cell represents a distinct taste quality and communicates essentially without interruption to the central nervous system). Single taste neurons in flies can detect multiple taste qualities having the same valence (behavioral output) supported by the results that some GRNs are activated by sugars, and low levels of fatty acids, both of which promote feeding  while other GRNs are activated by bitter compounds and high concentrations of salt and suppress feeding . In addition, a subset of bitter GRNs is also activated by low pH carboxylic acids, which are feeding deterrents . A complex model for salt coding in flies that combinatorially integrates inputs from across cell types to afford robust and flexible salt behaviors .
The taste system of mice also uses a variant of the labeled line model. In mice, taste receptors are segregated into distinct populations such that bitter, sweet, sour and low concentrations of salt are detected by non-overlapping sets of cells [1, 58]. Whether this principle applies to sweet and umami is presently unclear. Recent evidence suggest that aversive high concentrations of salt are not detected by a separate subset of cells, but are instead detected by the populations of cells that detect bitter and sour  suggesting that the mammalian taste system is relatively hard-wired to behavior, as is the case in flies.
3.4 Taste modulation in
Modulation of taste neuron activity prior to the first relay has been suggested. Presence of multiple molecular and cellular mechanisms by which tastant information is integrated in primary taste neurons has been proposed . Various studies suggest that aversive tastants such as bitter compounds and acids, can inhibit the activity of appetitive taste circuits in adult flies and larvae [83, 98, 99]. The reduction of the firing rate of sweet neurons in mixtures of sucrose and the aversive tastants is independent of the activity of the deterrent neuron [83, 98, 99]. Bitter compounds suppress feeding by activating bitter—GRNs and by inhibiting sugar—sensitive GRNs . The suppression of sugar GRNs depends on a odorant binding proteins” (OBP), OBP49a, which is expressed in gustatory organs or indirectly via GABAergic interneurons that connect bitter taste neuron activity to that of sweet taste neurons [100, 101]. Accessory cells synthesized OBP49a and release it into endolymph fluid bathing the GRNs, which then acts non-cell autonomously on sugar activated GRNs. OBP49a binds directly to bitter compounds, and later interacts with the sugar receptor, Gr64a, on the cell surface of the GRNs to suppress its activity . Such non-cell autonomous modulation of the sugar response ensures that bitter compounds in sugar-laden foods are not consumed. Low concentrations of acid tastants have also been observed to modulate detection of bitter compounds in the context of both sweet and deterrent neurons, suppressing their inhibitory effect in the former and their excitatory effect in the latter . Although the mechanisms by which carboxylic acids or low pH inhibit taste neurons remains to be determined.
Internal state can change the gustatory sensitivity as well: starvation potentiates the responses of sweet GRN and suppresses bitter GRN responses; mating increases taste peg GRN sensitivity to polyamines and behavioral responses to low salt in females; and protein deprivation sensitizes taste peg GRNs to yeast and increases behavioral sensitivity to amino acids [86, 103, 104, 105, 106, 107, 108]. Taste neuron sensitivity is also modulated by prior dietary experience. Response to camphor (non-toxic bitter compound) decrease after exposing flies to camphor for long . An E3 ubiquitin ligase-regulated decline in the levels of Trpl caused the change in sensitivity. No calories diet also cause increase activity in the
3.5 Non-canonical taste qualities
Vertebrates can sense a variety of other important taste qualities such as wetness and fattiness. Olfaction and somatosensation helps in the detection of fats, and they elicit post-ingestive effects that promote consumption. It has been shown that mice prefer water spiked with free fatty acids supports a role for the taste system in detecting this rich source of calories . A fatty acid transporter (CD36) and two fat-sensitive GPCRs—GPR40 and GPR120 are putative receptors for fat taste including K+ channels that are sensitive to polyunsaturated fatty acids . GPR120 is required for preference to fatty acids in mice  and is expressed in human TRCs as well .
In flies, sweet GRN activation requires the function of the three
3.5.2 Calcium taste
Ca2+, an ion is required for a vast array of cellular functions. Ca2+-deprived animals show attraction and Ca2+-sated animals show rejection. The aversive response to Ca2+requires a functioning T1R3 receptor, a subunit of the umami and sweet receptor . In human subjects an attenuation of the taste of Ca2+ by the T1R3 blocker lactisole has been shown .
Fruit flies avoids toxic levels of calcium. This repulsion is mediated by two mechanisms—activation of a specific class of GRNs that suppresses feeding, and inhibition of sugar-activated GRNs, which normally stimulates feeding. The distaste for Ca2+, and electrophysiological responses to Ca2+ require three members of the variant ionotropic receptor family Ir25a, Ir62a and Ir76b. The high concentrations of Ca2+ show decrease survival in flies .
No water receptor has been identified in vertebrate so far. The somatosensory system of animals can detect wetness across their body and also contribute to the sensing of aqueous solutions in the oral cavity. Various tastes have been ascribed to distilled water, from bitter to salty and sweet. Notably, application of water after exposure to some artificial sweeteners, such as saccharin, elicits a sweet taste .
A member of the Degenerin/Epithelial Sodium Channel family, ppk28 (an osmosensitive ion channel) mediates the cellular and behavioral response to water in flies.
3.6 Taste signal processing and taste sensory maps in the
In flies, after evaluation of taste input, the information translates into an appropriate behavioral response such as feeding, cessation of feeding, search for alternative food source, courtship, or egg-laying. Detection of sweet compounds by labellum GRs induces a sucking response and sugar detection by the tarsi induces extension of proboscis. It is a requirement to understand the flow of information from peripheral activation of GRNs to behavioral output to gain insight into the neuronal wiring of the taste at each level of information processing.
Unlike mammalian taste cells, fly GRNs from labellum and pharynx send projections of axons directly to the SEZ area of the brain. GRNs in the ovipositor, wings, and some leg sensilla send projections to the thoracic ganglia [24, 25]. Taste neurons send their axons to loosely defined, widely circumscribed zones in the SEZ or thoracic ganglia . Labial palp
The SEZ is a primary gustatory center, the higher brain centers where taste information is conveyed from the SEZ are unknown. Recently, sweet second order projection neurons that relay sweet taste information from the SEZ to the antennal and mechanosensory motor center (AMMC) in the deutocerebrum were described . The results support the role of AMMC (generally receives input from mechanosensory and olfactory neurons) in processing multisensory information. Various other studies have identified interneurons that impinge on taste circuits and feeding behavior routines, including a feeding promoting command neuron, feeding promoting dopaminergic neurons, bitter sensitive projection interneurons, feeding restrain GABAergic neurons and neurons in the ventral nerve cord that balance feeding and locomotion [121, 122, 123, 124, 125, 126].
The taste representations in the mushroom bodies (MB) (sites for associative learning) examined recently and found that input to the main calyx continues to be segregated according to taste modality and the location that taste information originates from. The bitter and sweet stimuli activate distinct areas, and stimuli from different taste organs activate partially overlapping but distinct patterns . The information about water and sweet qualities, as well as nutritive and non-nutritive sugars is also separated in MB [128, 129]. Unraveling taste circuits, therefore, will be important not only for understanding how sensory input is translated to behavioral output, but also how taste associations are formed in reward and aversive learning .
This work is supported by Wellcome trust/DBT India Alliance Fellowship (grant number IA/I/15/2/502074) awarded to PK.
Conflict of interest
The authors declare no conflict of interest.
Criteria for authorship
SK and PK both substantially contributed to the conception and design of the work. Both participated in drafting and revising the work, made the figures, wrote the chapter and approved the final version for publication.
|TRCs||Taste receptors cells|
|GRNs||Gustatory receptor neurons|
|SEZ||Sub esophageal zone|
|GPCRs||G-protein coupled receptors|
|OR genes||Olfactory receptor genes|
|GFP||Green fluorescence protein|
|UAS||Upstream activating sequence|
|TRP||Transient receptor potential|
|TRPL||Transient receptor potential-like|
|ENaCs||Epithelial Na+ channels|
|OBP||Odorant binding proteins|
|AMMC||Antennal and mechanosensory motor center|
|IP3 receptor||Inositol 1,4,5-triphosphate receptor|
|norpA||No receptor potential A|
|PKD2L1||Polycystic kidney disease 2-like 1|
|Car4||Carbonic anhydrase isoform 4|
|sNPF||Short neuropeptide F|