BackgroundSymbiotic bacteria affect insect physiology and ecology. They may also mediate insecticide resistance within their hosts and thereby impact pest and vector control practices. Here, we document a novel mechanism of insecticide resistance in which a gut symbiont of the tephritid pest fruit fly Bactrocera dorsalis enhances resistance to the organophosphate insecticide trichlorphon.ResultsWe demonstrated that the gut symbiont Citrobacter sp. (CF-BD) plays a key role in the degradation of trichlorphon. Based on a comparative genomics analysis with other Citrobacter species, phosphatase hydrolase genes were identified in CF-BD. These CF-BD genes had higher expression when trichlorphon was present. Bactrocera dorsalis inoculated with isolated CF-BD obtained higher trichlorphon resistance, while antibiotic-treated flies were less resistant confirming the key role of CF-BD in insecticide resistance.ConclusionsOur findings suggest that symbiont-mediated insecticide resistance can readily develop in B. dorsalis and may represent a more widely relevant insecticide resistance mechanism than previously recognized.Electronic supplementary materialThe online version of this article (doi:10.1186/s40168-017-0236-z) contains supplementary material, which is available to authorized users.

Eco-evolutionary dynamics of microbiotas at the macroscale level are largely driven by ecological variables. The diet and living environment of the oriental fruit fly, Bactrocera dorsalis, diversify during development, providing a natural system to explore convergence, divergence, and repeatability in patterns of microbiota dynamics as a function of the host diet, phylogeny, and environment. Here, we characterized the microbiotas of 47 B. dorsalis individuals from three distinct populations by 16S rRNA amplicon sequencing. A significant deviation was found within the larvae, pupae, and adults of each population. Pupae were characterized by an increased bacterial taxonomic and functional diversity. Principal components analysis showed that the microbiotas of larvae, pupae, and adults clearly separated into three clusters. Acetobacteraceae, Lactobacillaceae, and Enterobacteriaceae were the predominant families in larval and adult samples, and PICRUSt analysis indicated that phosphoglycerate mutases and transketolases were significantly enriched in larvae, while phosphoglycerate mutases, transketolases, and proteases were significantly enriched in adults, which may support the digestive function of the microbiotas in larvae and adults. The abundances of Intrasporangiaceae, Dermabacteraceae (mainly Brachybacterium) and Brevibacteriaceae (mainly Brevibacterium) were significantly higher in pupae, and the antibiotic transport system ATP-binding protein and antibiotic transport system permease protein pathways were significantly enriched there as well, indicating the defensive function of microbiotas in pupae. Overall, differences in the microbiotas of the larvae, pupae, and adults are likely to contribute to differences in nutrient assimilation and living environments.

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To accurately assess gene expression levels, it is essential to normalize real-time quantitative PCR (RT-qPCR) data with suitable internal reference genes. For the red imported fire ant, Solenopsis invicta, reliable reference genes to assess the transcript expression levels of the target genes have not been previously investigated. In this study, we examined the expression levels of five candidate reference genes (rpl18, ef1-beta, act, GAPDH, and tbp) in different developmental stages, castes and tissues of S. invicta. To evaluate the suitability of these genes as endogenous controls, three software-based approaches (geNorm, BestKeeper and NormFinder) and one web-based comprehensive tool (RefFinder) were used to analyze and rank the tested genes. Furthermore, the optimal number of reference gene(s) was determined by the pairwise variation value. Our data showed that two of the five candidate genes, rpl18 and ef1-beta, were the most suitable reference genes because they have the most stable expression among different developmental stages, castes and tissues in S. invicta. Although widely used as reference gene in other species, in S. invicta the act gene has high variation in expression and was consequently excluded as a reliable reference gene. The two validated reference genes, rpl18 and ef1-beta, can be widely used for quantification of target gene expression with RT-qPCR technology in S. invicta.

Chemosensory proteins (CSPs) have been predicted to be involved in development; however, direct evidence for their involvement is lacking, and genetic basis is largely unknown. To determine the function of the chemosensory protein 9 (Si-CSP9) gene in Solenopsis invicta, we used RNA interference to silence Si-CSP9 in 3rd-instar larvae. The 3rd-instar larvae failed to shed their cuticle after being fed Si-CSP9-directed siRNA, and expression profiling of RNAi-treated and untreated control larvae showed that 375 genes were differentially expressed. Pathway enrichment analysis revealed that 4 pathways associated with larval development were significantly enriched. Blast analysis revealed that one fatty acid amide hydrolase (FAAH) gene was up-regulated and 4 fatty acid synthase (FAT) genes and one protein kinase DC2 gene (PKA) were down-regulated in the enriched pathways. Significantly higher expression of these genes was found in 4th-instar larvae, and Pearson correlation analysis of the expression patterns revealed significant relationships among Si-CSP9, PKA, FAAH, and FAT1-4. Moreover, we confirmed that expression levels of Si-CSP9, FAAH, and FAT1-4 were significantly reduced and that the development of 3rd-instar larvae was halted with PKA silencing. These results suggest that Si-CSP9 and PKA may be involved in the network that contributes to development of 3rd-instar larvae.