A subset of chemosensory genes differs between two populations of a specialized leaf beetle after host plant shift

Antennal Transcriptome Evaluation and Analysis for Odorant-Binding Proteins, Chemosensory Proteins, and Suitable Reference Genes in the Leaf Beetle Pest Diorhabda rybakowi Weise (Coleoptera: Chrysomelidae)

Diorhabda rybakowi Weise is one of the dominant pests feeding on Nitraria spp., a pioneer plant used for windbreaking and sand fixation purposes, and poses a threat to local livestock and ecosystems. To clarify the key olfactory genes of D. rybakowi and provide a theoretical basis for attractant and repellent development, the optimal reference genes under two different conditions (tissue and sex) were identified, and the bioinformatics and characterization of the tissue expression profiles of two categories of soluble olfactory proteins (OBPs and CSPs) were investigated. The results showed that the best reference genes were RPL13a and RPS18 for comparison among tissues, and RPL19 and RPS18 for comparison between sexes. Strong expressions of DrybOBP3, DrybOBP6, DrybOBP7, DrybOBP10, DrybOBP11, DrybCSP2, and DrybCSP5 were found in antennae, the most important olfactory organ for D. rybakowi. These findings not only provide a basis for further in-depth research on the olfactory molecular mechanisms of host-specialized pests but also provide a theoretical basis for the future development of new chemical attractants or repellents using volatiles to control D. rybakowi.

Genetic mechanisms modulating behaviour through plastic chemosensory responses in insects

The ability to transition between different behavioural stages is a widespread phenomenon across the animal kingdom. Such behavioural adaptations are often linked to changes in the sensitivity of those neurons that sense chemical cues associated with the respective behaviours. To identify the genetic mechanisms that regulate neuronal sensitivity, and by that behaviour, typically *omics approaches, such as RNA- and protein-sequencing, are applied to sensory organs of individuals displaying differences in behaviour. In this review, we discuss these genetic mechanisms and how they impact neuronal sensitivity, summarize the correlative and functional evidence for their role in regulating behaviour and discuss future directions. As such, this review can help interpret *omics data by providing a comprehensive list of already identified genes and mechanisms that impact behaviour through changes in neuronal sensitivity.

Smelling in the dark: Phylogenomic insights into the chemosensory system of a subterranean beetle

The chemosensory system has experienced relevant changes in subterranean animals, facilitating the perception of specific chemical signals critical to survival in their particular environment. However, the genomic basis of chemoreception in cave-dwelling fauna has been largely unexplored. We generated de novo transcriptomes for antennae and body samples of the troglobitic beetle Speonomus longicornis (whose characters suggest an extreme adaptation to a deep subterranean environment) in order to investigate the evolutionary origin and diversification of the chemosensory gene repertoire across coleopterans through a phylogenomic approach. Our results suggested a diminished diversity of odourant and gustatory gene repertoires compared to polyphagous beetles that inhabit surface habitats. Moreover, S. longicornis showed a large diversity of odourant-binding proteins, suggesting an important role of these proteins in capturing airborne chemical cues. We identified a gene duplication of the ionotropic coreceptor IR25a, a highly conserved single-copy gene in protostomes involved in thermal and humidity sensing. In addition, no homologous genes to sugar receptors or the ionotropic receptor IR41a were detected. Our findings suggest that the chemosensory gene repertoire of this cave beetle may result from adaptation to the highly specific ecological niche it occupies, and that gene duplication and loss may have played an important role in the evolution of gene families involved in chemoreception. Altogether, our results shed light on the genomic basis of chemoreception in a cave-dwelling invertebrate and pave the road towards understanding the genomic underpinnings of adaptation to the subterranean lifestyle at a deeper level.

Genomic content of chemosensory receptors in two sister blister beetles facilitates characterization of chemosensory evolution

Background

More than 2500 species belong to the Meloidae family (Coleoptera: Tenebrionoidea), members of which produce the potent defensive blistering agent cantharidin and are commonly known as blister beetles or Spanishflies. Cantharidin has recently been used for cancer therapy. Hycleus cichorii and Hycleus phaleratus have been used in traditional Chinese medicine for more than 2000 years due to their ability to biosynthesize cantharidin. To understand the role of the chemosensory system in beetle evolution, we comparatively analysed the chemosensory receptor families of both blister beetle species and compared them with those of other beetles.

Results

We identified 89 odorant receptors (ORs), 86 gustatory receptors (GRs), and 45 ionotropic receptors (IRs) in H. phaleratus and 149 ORs, 102 GRs and 50 IRs in H. cichorii. Nine groups of beetle ORs were recovered, and a similar pattern of ORs in Coleoptera emerged. Two evident expanded clades in Hycleus (Groups 5A and 3) were reconstructed in the phylogenetic tree. Four of eight genes with evidence of positive selection were clustered in the expanded clades of Group 5A. Three, eight and three orthologous pairs of CO₂, sugar and fructose receptors, respectively, were identified in both blister beetles. Two evident expanded clades of putative bitter GRs in Hycleus were also found, and the GR in one clade had notably low divergence. Interestingly, IR41a was specifically expanded in blister beetles compared to other insects identified to date, and IR75 was also clearly expanded in both blister beetles based on our phylogenetic tree analysis. Moreover, evidence of positive selection was detected for eight ORs, three GRs and two IRs, half of which were from five duplicate clades.

Conclusions

We first annotated the chemosensory receptor families in a pair of sister beetle genomes (Meloidae: Hycleus), which facilitated evolutionary analysis of chemosensory receptors between sibling species in the Coleoptera group. Our analysis suggests that changes in chemosensory receptors have a possible role in chemical-based species evolution in blister beetles. Future studies should include more species to verify this correlation, which will help us understand the evolution of blister beetles.

Identification and Expression Profile of Olfactory Receptor Genes Based on Apriona germari (Hope) Antennal Transcriptome

Insects’ olfactory receptor plays a central role in detecting chemosensory information from the environment. Odorant receptors (ORs) and ionotropic receptors (IRs) are two types of olfactory receptors, and they are essential for the recognition of ligands at peripheral neurons. Apriona germari (Hope) (Coleoptera: Cerambycidae) is one of the most serious insect pests that cause damage to economic trees and landscaping trees, resulting in massive environmental damages and economic losses. Olfactory-based management strategy has been suggested as a promising strategy to control this wood-boring beetle. However, the olfactory perception mechanism in A. germari is now almost unknown. In the present study, RNA sequencing analysis was used to determine the transcriptomes of adult A. germari antennae. Among 36,834 unigenes derived from the antennal assembly, we identified 42 AgerORs and three AgerIRs. Based on the tissue expression pattern analysis, 27 AgerORs displayed a female-biased expression. Notably, AgerOR3, 5, 13, 33, and 40 showed a significant female-biased expression and were clustered with the pheromone receptors of Megacyllene caryae in the phylogenetic tree, suggesting that these AgerORs could be potential pheromone receptors for sensing male-produced sex pheromones in A. germari. The AgerIRs expression profile demonstrated that AgerIR2 had high expression levels in male labial palps, suggesting that this receptor may function to detect female-deposited trail-sex pheromone blend of A. germari. In addition, the phylogenetic tree showed that the Orco gene of five cerambycidae species was highly conservative. These results provide a foundation for further studies on the molecular mechanisms of olfactory chemoreception in A. germari apart from suggesting novel targets for the control of this pest in the future.