Abstract
Ammonia is both a building block and a breakdown product of amino acids and is found widely in the environment. The odor of ammonia is attractive to many insects, including insect vectors of disease. The olfactory response of Drosophila to ammonia has been studied in some detail, but the taste response has received remarkably little attention. Here, we show that ammonia is a taste cue for Drosophila. Nearly all sensilla of the major taste organ of the Drosophila head house a neuron that responds to neutral solutions of ammonia. Ammonia is toxic at high levels to many organisms, and we find that it has a negative valence in two paradigms of taste behavior, one operating over hours and the other over seconds. Physiological and behavioral responses to ammonia depend at least in part on Gr66a+ bitter-sensing taste neurons, which activate a circuit that deters feeding. The Amt transporter, a critical component of olfactory responses to ammonia, is widely expressed in taste neurons but is not required for taste responses. This work establishes ammonia as an ecologically important taste cue in Drosophila, and shows that it can activate circuits that promote opposite behavioral outcomes via different sensory systems.
Highlights
Ammonia is both a building block and a breakdown product of amino acids and is found widely in the environment
The concentration of ammonia in fly culture vials may approach 30 mM45, near the threshold of the physiological responses we have found for ammonia
It seems likely that the taste response to ammonia warns the fly of the toxicity of food sources with high ammonia levels and inhibits their consumption
Summary
Ammonia is both a building block and a breakdown product of amino acids and is found widely in the environment. The olfactory response of Drosophila to ammonia has been studied in some detail, but the taste response has received remarkably little attention. Most sensilla of the major taste organ of the Drosophila head house a neuron that responds to neutral solutions of ammonia. The Amt transporter, a critical component of olfactory responses to ammonia, is widely expressed in taste neurons but is not required for taste responses. Ammonia is an essential building block of amino acids, and it is a breakdown product of amino acids It is found widely in the environment: in air, in waters, in soils, and in all forms of life[2]. Ammonia is an olfactory cue for many organisms, including insect vectors of human disease. The ammonia in human sweat might elicit an appetitive response via the taste organs of mosquitoes. The legs, pharynx, and wings contain taste neurons[24,28,29,30]
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