SNMP1 and odorant receptors are co-expressed in olfactory neurons of the labial and maxillary palps from the desert locust Schistocerca gregaria (Orthoptera: Acrididae)

Sensilla on the maxillary palp of cave and surface-dwelling species of the genus Tachycines (Orthoptera: Rhaphidophoridae)

Due to caves' environmental features (e.g., darkness and food limitation), cave-dwelling insects have evolved well-developed sensory systems. The habitats of the camel crickets of Tachycines are diverse, found both in cave and surface ecosystems. This study aims to clarify the sensilla types and distribution on the maxillary palp in male adults of the surface-dwelling Tachycines huaxi and cave-dwelling Tachycines shuangcha. The morphology and sensilla on the maxillary palp were observed using scanning electron microscopy. The maxillary palps of the two Tachycines species consist of five segments with varying lengths. Seven types of sensilla were recorded on maxillary palp of both species: sensilla chaetica (Sc.1-3), sensilla trichodea (St.1-2), sensilla palmatum (Sp), Böhm's bristles (Bb), sensilla campaniformia (Sca), sensilla basiconica (Sb.1-4), and sensilla coeloconica (Sco). The sensilla are primarily located on the fifth palpomere ofmaxillary palp. Sb.2 were found exclusively on maxillary palp of the species T. huaxi. The distribution of sensilla was similar between T. shuangcha and T. huaxi, but sensilla of the two species differed in length, diameter, and number. The potential functions of these sensilla, and possible morphological adaptations to the cave environments exhibited by the maxillary palp of the cavernicolous T. shuangcha are discussed.

A chemical defense deters cannibalism in migratory locusts

Many animals engage in cannibalism to supplement their diets. Among dense populations of migratory locusts, cannibalism is prevalent. We show that under crowded conditions, locusts produce an anticannibalistic pheromone called phenylacetonitrile. Both the degree of cannibalism and the production of phenylacetonitrile are density dependent and covary. We identified the olfactory receptor that detects phenylacetonitrile and used genome editing to make this receptor nonfunctional, thereby abolishing the negative behavioral response. We also inactivated the gene underlying phenylacetonitrile production and show that locusts that lack this compound lose its protection and are more frequently exposed to intraspecific predation. Thus, we reveal an anticannibalistic feature built on a specifically produced odor. The system is very likely to be of major importance in locust population ecology, and our results might therefore provide opportunities in locust management.

The specific expression patterns of sensory neuron membrane proteins are retained throughout the development of the desert locust Schistocerca gregaria

The desert locust Schistocerca gregaria detects odorants through olfactory sensory neurons (OSNs) that are surrounded by non-neuronal support cells (SCs). OSNs and SCs are housed in cuticle structures, named sensilla found abundantly on the antenna in all developmental stages of the hemimetabolic insect. In insects, multiple proteins expressed by OSNs and SCs are indicated to play a pivotal role in the detection of odorants. This includes insect-specific members of the CD36 family of lipid receptors and transporters called sensory neuron membrane proteins (SNMPs). While the distribution pattern of the SNMP1 and SNMP2 subtypes in OSNs and SCs across different sensilla types has been elucidated for the adult S. gregaria antenna, their localization in cells and sensilla of different developmental stages is unclear. Here, we determined the SNMP1 and SNMP2 expression topography on the antenna of the first, third and fifth instar nymphs. Through FIHC experiments we found that in all developmental stages SNMP1 is expressed in OSNs and SCs of the trichoid and basiconic sensilla while SNMP2 is restricted to the SCs of the basiconic and coeloconic sensilla thus resembling the adult arrangement. Our results demonstrate that both SNMP types have defined cell- and sensilla-specific distribution patterns established already in the first instar nymphs and retained into the adult stage. This conserved expression topography underlines the importance of SNMP1 and SNMP2 in olfactory processes throughout the development of the desert locust.

Identification and analysis of odorant receptors expressed in the two main olfactory organs, antennae and palps, of Schistocerca americana

Locusts depend upon their sense of smell and provide useful models for understanding olfaction. Extending this understanding requires knowledge of the molecular and structural organization of the olfactory system. Odor sensing begins with olfactory receptor neurons (ORNs), which express odorant receptors (ORs). In insects, ORNs are housed, in varying numbers, in olfactory sensilla. Because the organization of ORs within sensilla affects their function, it is essential to identify the ORs they contain. Here, using RNA sequencing, we identified 179 putative ORs in the transcriptomes of the two main olfactory organs, antenna and palp, of the locust Schistocerca americana. Quantitative expression analysis showed most putative ORs (140) are expressed in antennae while only 31 are in the palps. Further, our analysis identified one OR detected only in the palps and seven ORs that are expressed differentially by sex. An in situ analysis of OR expression suggested ORs are organized in non-random combinations within antennal sensilla. A phylogenetic comparison of OR predicted protein sequences revealed homologous relationships among two other Acrididae species. Our results provide a foundation for understanding the organization of the first stage of the olfactory system in S. americana, a well-studied model for olfactory processing.

The Sensilla-Specific Expression and Subcellular Localization of SNMP1 and SNMP2 Reveal Novel Insights into Their Roles in the Antenna of the Desert Locust Schistocerca gregaria

Simple Summary

The desert locust, Schistocerca gregaria, can form gigantic swarms of millions of individuals that devastate the vegetation of invaded landscapes. Locust food search, reproduction, and aggregation behaviors are triggered and controlled by complex olfactory signals. Insects detect odorants through different types of olfactory sensilla on the antenna that house olfactory sensory neurons and associated support cells, both of which express the proteins required for olfactory signaling. Among these proteins, two members of the CD36 lipid transporter/receptor family, named sensory neuron membrane proteins 1 and 2 (SNMP1 and SNMP2), are indicated to be of vital importance. Towards a better understanding of the role of the two SNMPs in the olfactory system of S. gregaria, we have analysed their antennal topography and subcellular localization using specific antibodies. The results indicate sensilla type- and cell type-specific distribution patterns of the SNMP proteins. SNMP1 was located in the receptive dendrites of subpopulations of olfactory sensory neurons as well as in the microvilli of associated support cells, suggesting a dual function of this protein, both in olfactory signal detection and in sensillum lymph maintenance, respectively. In contrast, SNMP2 was found solely in support cells and their microvilli membranes, suggesting a role limited to sensillum lymph recovery processes.

Abstract

Insect olfactory sensilla house olfactory sensory neurons (OSNs) and supports cells (SCs). The olfactory sensory processes require, besides the odorant receptors (ORs), insect-specific members of the CD36 family, named sensory neuron membrane proteins (SNMPs). While SNMP1 is considered to act as a coreceptor in the OR-mediated detection of pheromones, SNMP2 was found to be expressed in SCs; however, its function is unknown. For the desert locust, Schistocerca gregaria, we previously visualized mRNA for SNMP1 in OSNs and SNMP2 mRNA in cells associated with OSN clusters. Towards an understanding of their functional implication, it is imperative to explore the cellular and the subcellular localization the SNMP proteins. Therefore, we have generated polyclonal antibodies against SNMP1 and SNMP2 and used fluorescence immunohistochemistry (FIHC) to visualize the SNMP proteins. We found SNMP1 in the somata and respective dendrites of all OSNs in trichoid sensilla and in subsets of OSNs in basiconic sensilla. Notably, SNMP1 was also detected in SCs of these sensilla types. In contrast, SNMP2 protein was only visualized in SCs of basiconic and coeloconic sensilla, but not of trichoid sensilla. Exploring the subcellular localization by electron microscopy using anti-SNMP1-ab and anti-SNMP2-ab revealed an immunogold labelling of SC microvilli bordering the sensillum lymph. Together our findings suggest a dual role of SNMP1 in the antenna of S. gregaria, in some OSN subpopulations in odor detection as well as in functions of some SCs, whereas the role of SNMP2 is limited to the functions of support cells.

Description of Chemosensory Genes in Unexplored Tissues of the Moth Spodoptera littoralis

Illumina-based transcriptome sequencing of chemosensory organs has become a standard in deciphering the molecular bases of chemical senses in insects, especially in non-model species. A plethora of antennal transcriptomes is now available in the literature, describing large sets of chemosensory receptors and binding proteins in a diversity of species. However, little is still known on other organs such as mouthparts, legs and ovipositors, which are also known to carry chemosensory sensilla. This is the case of the noctuid Spodoptera littoralis, which has been established as a model insect species in molecular chemical ecology thanks to the description of many—but not all—chemosensory genes. To fulfill this gap, we present here an unprecedented transcriptomic survey of chemosensory tissues in this species. RNAseq from male and female proboscis, labial palps, legs and female ovipositors allowed us to annotate 115 putative chemosensory gene transcripts, including 30 novel genes in this species. Especially, we doubled the number of candidate gustatory receptor transcripts described in this species. We also evidenced ectopic expression of many chemosensory genes. Remarkably, one third of the odorant receptors were found to be expressed in the proboscis. With a total of 196 non-overlapping chemosensory genes annotated, the S. littoralis repertoire is one of the most complete in Lepidoptera. We further evaluated the expression of transcripts between males and females, pinpointing sex-specific transcripts. We identified five female-specific transcripts, including one odorant receptor, one gustatory receptor, one ionotropic receptor and one odorant-binding protein, and one male-specific gustatory receptor. Such sex-biased expression suggests that these transcripts participate in sex-specific behaviors, such as host choice for oviposition in females and/or mating partner recognition in both sexes.

The role of SNMPs in insect olfaction

The sense of smell enables insects to recognize olfactory signals crucial for survival and reproduction. In insects, odorant detection highly depends on the interplay of distinct proteins expressed by specialized olfactory sensory neurons (OSNs) and associated support cells which are housed together in chemosensory units, named sensilla, mainly located on the antenna. Besides odorant-binding proteins (OBPs) and olfactory receptors, so-called sensory neuron membrane proteins (SNMPs) are indicated to play a critical role in the detection of certain odorants. SNMPs are insect-specific membrane proteins initially identified in pheromone-sensitive OSNs of Lepidoptera and are indispensable for a proper detection of pheromones. In the last decades, genome and transcriptome analyses have revealed a wide distribution of SNMP-encoding genes in holometabolous and hemimetabolous insects, with a given species expressing multiple subtypes in distinct cells of the olfactory system. Besides SNMPs having a neuronal expression in subpopulations of OSNs, certain SNMP types were found expressed in OSN-associated support cells suggesting different decisive roles of SNMPs in the peripheral olfactory system. In this review, we will report the state of knowledge of neuronal and non-neuronal members of the SNMP family and discuss their possible functions in insect olfaction.

A Subset of Odorant Receptors from the Desert Locust Schistocerca gregaria Is Co-Expressed with the Sensory Neuron Membrane Protein 1

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.