Recent research from the Department of Biological Chemistry and Neuroscience Center for Sensory Biology at the Hopkins School of Medicine has found that certain gustatory receptors (GRs) in fruit flies are essential for taste and preventing courtship between males.
Research in the gustatory system, the sensory system of taste, is especially applicable to humans because from an evolutionary standpoint, it allows us to know what food is safe or harmful, although nowadays this ability is taken for granted.
Gustatory receptors are commonly known as receptors of taste. In the fruit fly, although there are some GRs whose functions have been discovered, such as those that prevent the ingestion of caffeine, receptors that trigger avoidance of harmful, nonvolatile chemicals are relatively unknown.
A current Hopkins study reveals the nature of another gustatory receptor, Gr33a, which is expressed widely in gustatory receptor neurons (GRNs) that respond to aversive chemicals, such as bug repellents.
"Since many botanical compounds function as naturally occurring insect repellents and pesticides, the identification of Gr33a offers potential for devising mechanisms for reducing interactions between insect pests and plants," Craig Montell, a professor of Biological Chemistry at Hopkins and the senior researcher of the study, said.
His study shows that Gr33a may play a fundamental role in the avoidance in fruit flies. In tests of this hypothesis, flies with mutant versions of Gr33a were found to have difficulty avoiding chemicals ranging from quinine (an anti-malarial drug) to denatonium (the most bitter chemical compound known).
"We reported the identification of a receptor, Gr33a, which is expressed in all gustatory receptor neurons (GRNs) that respond to avoidance compounds," Montell said. "We generated a mutation in Gr33a and found that the mutant flies were unable to detect every repellent compound tested that is sensed through contact chemosensation."
The research suggests that Gr33a may be a co-receptor required for sensing all nonvolatile repulsive chemicals in the fruit fly, crucial for the functioning of other gustatory receptors. Therefore, Gr33a may be fundamental for the survival of flies.
"Gr33a is the first receptor identified in an insect which is broadly required for avoiding noxious compounds," Montell said.
These taste receptors in the fruit fly have significance for multiple reasons. First, chemosensation is essential in the survival of insects. These include evasive behaviors such as suppressing the consumption of repelling compounds and inhibiting male-male courtship. Second, knowledge of these gustatory receptors may revolutionize how humans can affect the interaction between insects and plants.
After the discovery that Gr33a mutant flies were unresponsive to all aversive compounds tested, the lab also investigated whether it was also required for inhibiting male-to-male courtship, since behavior is suppressed by an inhibitory pheromone present on the male cuticle.
Gr33a-mutant males also displayed increased male-to-male courtship, implying that the receptor functions in the detection of repulsive male pheromones, chemical messages that are released and can be detected by fruit flies.
"Gr33a is also expressed in GRNs in leg tarsi [the end segment of the leg] that are necessary in males for detection of inhibitory pheromones released by other males," Montell said. "Consistent with this expression pattern, Gr33a mutant males showed increased male-male courtship behavior."
To test this hypothesis, the lab combined Gr33a-mutant males with normal wild-type males. The male-male courtship was recorded as vibration of the wings, licking and attempted copulation. The finding revealed the expected; furthermore, Gr33a male flies even displayed increased courtship toward passive, decapitated males.
The finding that Gr33a affects male-male courtship raises the possibility that this receptor is required for sensing an inhibitory male pheromone.
Along these lines, Montell's lab outlines their future perspective of this study. The identification of Gr33a as the first molecular target required broadly for detection of all harmful chemicals provides the possibility of screening for molecules that could act as Gr33a inhibitors. These inhibitors would promote the intake of noxious chemicals and as a result, create more effective insect repellents that target the very biological makeup of insects.
Lastly, because many gustatory receptors such as Gr33a are divergent between the fruit fly and other insects, such inhibitors have the incredible potential to selectively target GRs in disease vectors, such as mosquitoes that carry malaria parasites, but not in insects that would be of benefit to maintain. This could potentially lead to a new method for controlling insect-borne diseases.
