Does bitter taste predict toxicity?
All animals are equipped with taste receptors which enable them to predict if food is nutritious or contains potential toxicants. While humans have only about 30 taste receptors, almost all of these receptors are sensitive to aversive or bitter chemicals and only one is sensitive to sugars. In insects, a similar trend is observed. In addition, appetitive receptors are inhibited by some aversive chemicals. This suggests that detecting aversive molecules directly or indirectly is essential to the survival of animals and that a link exists between aversiveness and toxicity.
In order to test this hypothesis,. we evaluated the aversiveness and toxicity of a panel of 7 aversive molecules in Drosophila. Adults were exposed to food mixed with these molecules and we monitored their feeding activities and survival, as well as their survival following injections of these molecules into their abdomen. While all substances induced a dose-dependent reduction of survival correlated with a reduction of feeding, the relative potency of these substances changed with the method of exposure.
Our results suggest that a strict correlation does not exist between toxicity and bitterness for this panel of molecules. In retrospect, such a correlation would be astonishing since sensory receptors detect extracellular molecules through membrane receptors while toxicity often involves interactions with intracellular targets and a variety of mechanisms occurring at different timescale. Therefore, we postulate that if a link exists between bitterness and toxicity, it is established through evolution.
1 Ferreira EA, Lambert S, Verrier T, Marion-Poll F, & Yassin A (2021) Soft selective sweep on chemosensory genes correlates with ancestral preference for toxic noni in a specialist Drosophila population. Genes 12(1):13.
2 Marion-Poll F (2020) The Gustatory System in Insects. The Senses. A Comprehensive Reference. Volume 3, The Senses. A Comprehensive Reference, eds Fritzsch B & Meyerhof W (Elsevier), Second Ed, pp 148-168.
3 French AS, et al. (2015) Dual mechanism for bitter avoidance in Drosophila. J. Neurosci. 35(9):3990-4004.