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. 2010 Jun 18:4:20.
doi: 10.3389/fncel.2010.00020. eCollection 2010.

Molecular and cellular designs of insect taste receptor system

Affiliations

Molecular and cellular designs of insect taste receptor system

Kunio Isono et al. Front Cell Neurosci. .

Abstract

The insect gustatory receptors (GRs) are members of a large G-protein coupled receptor family distantly related to the insect olfactory receptors. They are phylogenetically different from taste receptors of most other animals. GRs are often coexpressed with other GRs in single receptor neurons. Taste receptors other than GRs are also expressed in some neurons. Recent molecular studies in the fruitfly Drosophila revealed that the insect taste receptor system not only covers a wide ligand spectrum of sugars, bitter substances or salts that are common to mammals but also includes reception of pheromone and somatosensory stimulants. However, the central mechanism to perceive and discriminate taste information is not yet elucidated. Analysis of the primary projection of taste neurons to the brain shows that the projection profiles depend basically on the peripheral locations of the neurons as well as the GRs that they express. These results suggest that both peripheral and central design principles of insect taste perception are different from those of olfactory perception.

Keywords: Drosophila; feeding behavior; gustatory receptor (GR); insect; subesophageal ganglion complex (SOG); taste ligand; taste neuron; taste sensillum.

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Figures

Figure 1
Figure 1
Gain and loss events of Gr5a, a sugar receptor, based on phylogenetic analysis of Gr5a orthologs from 15 dipteran insects in which the whole genome assemblies are available. The phylogenetic tree is modified from BLAST homepage of FlyBase (http://flybase.org/blast/). Species names in red and blue italics illustrate species with or without Gr5a orthologs, respectively. Red and blue circles represent gain and loss events of Gr5a orthologs, respectively. A green box represents a functional mutation from ancestral Thr218 to Ala218 that occurred only in Drosophila melanogaster branch.
Figure 2
Figure 2
Increasing magnitude of proboscis extension response (from up to down) in Drosophila.
Figure 3
Figure 3
An example of feeding choice test to behaviorally evaluate taste sensitivity and feeding preference in Drosophila. (A,B) Hungry flies after 20 h of food deprivation ingest a maximum amount of 100 mM sucrose + 1% agar solutions mixed with red or blue food dyes, respectively, and can be visually inspected after feeding. (C) A microtiter dish containing two different solutions containing the two food dyes to test for the feeding preference. (D) A video analysis simultaneously monitoring the locomotor traces of three individual flies in the choice of 100 mM sucrose + 1% agar solution (marked with a blue dye) and a plain 1% agar solution (marked with a red dye). The traces show that more frequent visits or stays were made on wells containing 100 mM sucrose than on wells containing plain water. The video analysis was provided courtesy of Dr. M. Koganezawa).
Figure 4
Figure 4
Electrophysiological recordings from a single l-type taste sensillum of the labellum in Drosophila. Different types of neurons are activated by different taste stimulations. See explanations in the text. Recordings provided courtesy of Dr. N. Tanabe.
Figure 5
Figure 5
Examples showing taste sensilla and labeled taste neurons in Drosophila. (A,B) Taste sensilla along the peripheral labella of a transformant fly expressing a cytoplasmic GFP marker protein driven by Gr5a promotor-Gal4 (Gr5a-Gal4/Gr5a-Gal4, UAS-2xEGFP/UAS-2xEGFP). Pictures were taken under microscope by transmission light (A) or by fluorescence microscope (B). Arrows in (C) and (D) show other types of taste sensilla along the tarsal segments of the distal legs (C) and wing margins in (D).
Figure 6
Figure 6
A schematic drawing of the primary projection areas in the subesophageal ganglion (SOG) of sugar taste neurons (left) and bitter taste neurons (right). Frontal views of the anterior (blue), medial (pink) and posterior (yellow) projection areas in the SOG are superimposed in the same plane. Upward direction is dorsal. An oval represents the location of the esophagus. A subset of sugar receptor genes are indicated for each projection (Dahanukar et al., ; Wang et al., 2004). Note distinct projection areas for sugar and bitter neurons.

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