**1. Introduction**

Animals including *Drosophila melanogaster* use their chemosensory system to monitor the chemical world around them. The chemosensory system includes olfactory system to detect volatile chemicals and gustatory system to detect soluble compounds. The olfactory and visual system helps in food detection and the taste system controls the food acceptance or rejection behavior by helping animals detect nutrient-rich food and avoid toxic substances. The quality and concentration of taste compounds help animals to make such an assessment.

*Drosophila* and mammals are able to detect basic taste modalities including sugars, bitter compounds, salt, acids, and amino acids [1]. The taste qualities are detected by taste cells present in the periphery. The activation of different taste cells provides a simple mechanism to encode modality. Like mammals' fly taste cells also show dose dependent activation providing the potential to encode different concentrations. The taste system of a fly is distributed over the whole-body, proboscis or labial palps being the main taste organ. It is located on the distal end of the labellum. Like other insects, the taste sensilla are present on labellum, legs, wings and on the female genitalia (**Figure 1**) [2].

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

#### **Figure 1.**

*Taste system of adult Drosophila. (A) Taste neurons are located on different body parts of fly namely labellum, pharynx, legs, wings and ovipositor as shown with red and green dots. (B) Three different types of taste sensillum (Large-L, Intermediate-I and small-S) present on the labellum (mouth part of a fly). (C) Taste sensillum structure showing pore at the tip and different types of taste neurons in a taste sensillum namely sweet (green), bitter (magenta), salt (yellow) and water (sky blue) neurons together with one mechanosensory neuron (black) surrounded by support cells (gray).*

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 [3–7]. 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 [6–8]. In addition, electrophysiological studies from taste neurons suggest heterogeneity among the responses of individual sugar- or bitter-sensing cells [9–11] suggesting diversity among the peripheral cell types that detect sugars or bitter compounds in *Drosophila.* This organization provides the potential for different taste cell types to be activated by different compounds within a taste modality and the possibility for intra modality discrimination.

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 *fly* gustatory system. We shall then review taste perception mainly from a molecular genetic perspective that includes the results from behavioral, electrophysiological and imaging analyses. The parallels between the flies and human taste system will provide insight into how the detection of taste compounds regulates feeding decisions.

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*Understanding Taste Using* Drosophila melanogaster *DOI: http://dx.doi.org/10.5772/intechopen.89643*

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 [12]. TRCs can relay information of taste quality independent of cells relaying other taste qualities [13]. 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 [14]. 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

Although the same taste preferences are shared between *Drosophila* and mam-

mals, the organization of their gustatory systems are rather different. Unlike humans, flies have wide distribution of taste cells over much of the body including many peripheral organs like labellum, legs, wings and genitalia (**Figure 1**). Such a distribution of taste cells enables the fly to gather contact chemosensory information from many reference points that may make contact with their body enabling detection of potential calorie rich foods or toxic compounds [16]. The presence of taste-sensing cells in other tissues provides the safety benefits allowing evaluation of chemicals without the potential hazard of accidental ingestion. The gustatory sensillum or taste bristle are the main sensory unit of all taste organs housing two to four primary gustatory receptor neurons (GRNs) as well as a single mechanosensory neuron (MSN) [2]. The labellum is the main taste organ in *Drosophila* located at the end of the proboscis (equivalent to human tongue). Labial palps contain 31 bristles (sensilla) each that are arranged in a stereotyped pattern. The sensilla are morphologically classified into three types long, intermediate and short (L, I, and S type) based on their shape and location (**Figure 1**) [2, 17, 18]. L- and S-type sensillum house dendrites of four GRNs, and the I type are associated with two GRNs. Electrophysiological investigations suggest each GRN is thought to respond exclusively to either sugar, water, low salt concentration, or high salt concentration and bitter compounds [11, 19–22]. The terminal pore at the tip of the taste bristle (**Figure 1**) allows taste stimuli access to the dendrite of the GRN, which extends into the bristle shaft [23]. In addition to the peripheral taste sensillum on the palps, legs, and wings, taste neurons are also semi-internally or internally located. The first group consists of row of taste pegs that line the inside of the labial palps and are exposed to foods when the fly 'opens' its palps and readies itself for 'sucking up' foods. The internally located group consists of three sensillum clusters that line the pharynx (**Figure 1**). They allow re-evaluation of the food as it passes and enters

**2. Mammalian taste system**

is relayed to the gustatory cortex [15].

**3. Gustatory system of** *Drosophila*
