Abstract

In the desert locust Schistocerca gregaria (S. gregaria), pheromones are considered to be crucial for governing important behaviors and processes, including phase transition, reproduction, aggregation and swarm formation. The receptors mediating pheromone detection in olfactory sensory neurons (OSNs) on the antenna of S. gregaria are unknown. Since pheromone receptors in other insects belong to the odorant receptor (OR) family and are typically co-expressed with the “sensory neuron membrane protein 1” (SNMP1), in our search for putative pheromone receptors of S. gregaria, we have screened the OR repertoire for receptor types that are expressed in SNMP1-positive OSNs. Based on phylogenetic analyses, we categorized the 119 ORs of S. gregaria into three groups (I–III) and analyzed a substantial number of ORs for co-expression with SNMP1 by two-color fluorescence in situ hybridization. We have identified 33 ORs that were co-expressed with SNMP1. In fact, the majority of ORs from group I and II were found to be expressed in SNMP1-positive OSNs, but only very few receptors from group III, which comprises approximately 60% of all ORs from S. gregaria, were co-expressed with SNMP1. These findings indicate that numerous ORs from group I and II could be important for pheromone communication. Collectively, we have identified a broad range of candidate pheromone receptors in S. gregaria that are not randomly distributed throughout the OR family but rather segregate into phylogenetically distinct receptor clades.

Highlights

  • Chemical signals from the environment trigger important behaviors in insects, such as foraging, reproduction, or aggregation [1,2]

  • I odorant receptor (OR) with sensory neuron membrane protein 1” (SNMP1); yet, testing several SgreORs that do not belong to group I, five receptors were found to be co-expressed with SNMP1 [24]

  • In search for candidate pheromone receptors, we have analyzed about two-thirds of the S. gregaria

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Summary

Introduction

Chemical signals from the environment trigger important behaviors in insects, such as foraging, reproduction, or aggregation [1,2]. The recognition and primary responses to suitable compounds take place in cuticular hair-like structures, called sensilla, on the surface of chemosensory organs. Within such sensilla, olfactory sensory neurons (OSNs) project their dendritic process into the fluid-filled sensillar cavity, and the dendritic membrane is endowed with receptors that render these neurons responsive to distinct substances [1,3]. Pheromones, signal molecules for intraspecific communication, are detected by specialized OSNs that are usually characterized by the expression of pheromone receptors and the olfactory receptor co-receptor (Orco), and by the “sensory neuron membrane protein 1” (SNMP1) [4,5,6,7,8,9]. More recent studies have shown that SNMP1 is located in close proximity to odorant receptor (OR) proteins and Insects 2019, 10, 350; doi:10.3390/insects10100350 www.mdpi.com/journal/insects

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