Characterization of Queen Supergene Pheromone in the Red Imported Fire Ant Using Worker Discrimination Assays

Ants use chemical signals to communicate for various purposes related to colony function. Social organization in the red imported fire ant, Solenopsis invicta, is determined by the Sb supergene, with colonies of the monogyne (single-queen) form lacking the element and colonies of the polygyne (multiple-queen) form possessing it. Polygyne workers accept new reproductive queens in their nest, but only those carrying Sb; young winged queens lacking this genetic element are executed as they mature sexually in their natal nest or as they attempt to enter a foreign nest to initiate reproduction after mating and shedding their wings. It has been suggested that queen supergene genotype status is signaled to workers by unsaturated cuticular hydrocarbons, while queen reproductive status is signaled by piperidines (venom alkaloids). We used high-throughput behavioral assays to study worker acceptance of paper dummies dosed with fractions of extracts of polygyne queens, or blends of synthetic counterparts of queen cuticular compounds. We show that the queen supergene pheromone comprises a blend of monoene and diene unsaturated hydrocarbons. Our assays also reveal that unsaturated hydrocarbons elicit discrimination by polygyne workers only when associated with additional compounds that signal queen fertility. This synergistic effect was obtained with a polar fraction of queen extracts, but not by the piperidine alkaloids, suggesting that the chemical(s) indicating queen reproductive status are compounds more polar than cuticular hydrocarbons but are not the piperidine alkaloids. Our results advance understanding of the role of chemical signaling that is central to the regulation of social organization in an important invasive pest and model ant species.

Reactive carbon fiber ionization-mass spectrometry for characterization of unsaturated hydrocarbons from plant aroma

Carbon fiber ionization (CFI)-mass spectrometry (MS) is an ambient technique that can be used to detect samples in gas, liquid, and solid forms simply by using a piece of carbon fiber as the ionization emitter. Reactive MS can be performed to selectively detect target analytes by conducting fast reactions during ionization. Most ambient ionization MS techniques used to monitor chemical reactions are limited to liquid-phase reactions. Herein, we develop reactive CFI-MS to be a suitable tool for monitoring of reaction products derived from volatile unsaturated hydrocarbons in the gas phase. Hydroamination is a fast reaction that can form a carbon-nitrogen bond through the addition of an amine to unsaturated hydrocarbons. In this study, reactive CFI-MS was used to selectively characterize aroma molecules, which are unsaturated hydrocarbons derived from plants, through hydroamination. A piece of carbon fiber was placed close (~ 1 mm) to the inlet of the mass spectrometer and deposited with dried methylamine. The sample in either liquid or solid form was placed underneath the carbon fiber. The volatiles derived from the sample reacted with amine on the carbon fiber were simultaneously determined once the mass spectrometer was switched on. For proof of concept, ethylene glycol dimethacrylate, which has double bonds and is highly volatile, was initially selected as the model sample to demonstrate the feasibility of using reactive CFI-MS to detect its hydroamination derivative. Banana, garlic, and ginger, which possess aroma molecules with unsaturated hydrocarbons, were selected as real-world samples. Graphical abstract.