The Odorant Binding Protein, SiOBP5, Mediates Alarm Pheromone Olfactory Recognition in the Red Imported Fire Ant, Solenopsis invicta

Abstract

Olfaction is crucial in mediating various behaviors of social insects such as red imported fire ants, Solenopsis invicta Buren. Olfactory receptor (OR) complexes consist of odor-specific ORs and OR co-receptors (Orco). Orcos are highly conserved across insect taxa and are widely co-expressed with ORs. Odorant binding proteins (OBPs) can transport semiochemicals to ORs as protein carriers and thus constitute the first molecular recognition step in insect olfaction. In this study, three OBP genes highly expressed in S. invicta antenna, OBP1, OBP5, OBP6, and Orco were partially silenced using RNA interference (RNAi). RNAi SiOBP5- and Orco-injected ants showed significantly lower EAG (electroantennography) responses to fire ant alarm pheromones and the alkaloid, 2,4,6-trimethylpyridine than water- or GFP-injected ants 72 h post injection. Subsequent qRT-PCR analysis demonstrated that the transcript level of the OBP1, OBP5, OBP6, and Orco significantly decreased 72 h after ants were injected with dsRNAs; however, there were no transcript level or EAG changes in ants fed dsRNAs. Our results suggest that S. invicta Orco and SiOBP5 are crucial to fire ants for their responses to alarm pheromones. RNAi knocking down SiOBP5 can significantly disrupt alarm pheromone communication, suggesting that disrupting SiOBP5 and Orcos could be potentially useful in the management of red imported fire ants.

Sublethal Exposure to Cadmium Induces Chemosensory Dysfunction in Fire Ants

“…On the contrary, undecane elicited a weak EAG response in the control fire ants, but Cd exposure significantly activated the EAG response after two, four, and 6 weeks (Figure D). The alkaloid, 2,4,6-trimethylpyridine, from the lady beetle, was previously demonstrated to evoke strong EAG responses in S. invicta . , However, exposure to 100 mg Cd kg –1 for 2 weeks deprived the EAG response to this odorant . Analogously, the electro-olfactogram (EOG) responses of goldfish to food-related odorants and postovulatory pheromone were weakened after 10 min exposure to 0.1 mM copper sulfate .…”

“…Although the sequences encoding putative OBPs in S. invicta have been identified and the temporal and spatial expression of SiOBPs has been profiled, the functions of SiOBPs in semiochemical detection of S. invicta remain largely unknown. A previous study reported that SiOBP5 is crucial for the perception of 2-ethyl-3,5(6)-dimethylpyrazine, the alarm pheromone of S. invicta . To explore the function of SiOBP7 and SiOBP15 in the olfactory perception of S. invicta , we determined the behavioral responses of fire ants to these odorants with higher binding affinities after knocking down SiOBP7 or SiOBP15.…”

“…The red imported fire ant, Solenopsis invicta Buren, is a notorious invasive and eusocial ant species . The chemosensory genes of S. invicta contribute to the evolution of their sophisticated eusociality and remarkable environmental adaptability. , Small molecular weight soluble proteins, which are known as odorant binding proteins (OBPs) and chemosensory proteins (CSPs), in the olfactory sensilla of fire ants play indispensable roles in the realization and modulation of their social behavior such as necrophoresis and immunization, − suggesting that SiOBPs and SiCSPs are essential to olfactory signal transduction (OST).…”

Olfactory Strategies in the Defensive Behaviour of Insects

“…Activity in Or49a is carried on by PNs to the lateral protocerebrum, which behaviourally mediates the avoidance behaviour of adult flies [ 6 ]. Similarly, a recent study showed that specific odorant-binding proteins mediate the olfactory recognition of the alarm pheromone in the fire ant Solenopsis invicta [ 112 ]. Also, the “flee” response observed in aphids that drop from their host plant when detecting either the predator or the alarm pheromone from conspecifics is initiated by a single compound [ 52 ].…”

Recent Insights into Insect Olfactory Receptors and Odorant-Binding Proteins

“…Together, these data are consistent with a combination of the carrier and presentation models for OBP function and implicate a direct interaction between the LUSH and Or67d–Orco pheromone receptors on the dendrites. Following these studies, many groups have demonstrated specific roles for OBP members in the detection of volatile ligands in many species [ 79 , 80 , 81 , 82 , 83 , 84 , 85 , 86 , 87 , 88 ], and even for tastants and humidity [ 89 , 90 ]. Finally, in Drosophila , an odorant-binding protein has been implicated in blocking sugar taste detection when sucrose is present with bitter compounds [ 91 ].…”