Abstract
Diabrotica virgifera virgifera LeConte, the western corn rootworm (WCR) is one of the most destructive pests in the U.S. Corn Belt. Transgenic maize lines expressing various Cry toxins from Bacillus thuringiensis have been adopted as a management strategy. However, resistance to many Bt toxins has occurred. To investigate the mechanisms of Bt resistance we carried out RNA-seq using Illumina sequencing technology on resistant, eCry3.1Ab-selected and susceptible, unselected, whole WCR neonates which fed on seedling maize with and without eCry3.1Ab for 12 and 24 hours. In a parallel experiment RNA-seq experiments were conducted when only the midgut of neonate WCR was evaluated from the same treatments. After de novo transcriptome assembly we identified differentially expressed genes (DEGs). Results from the assemblies and annotation indicate that WCR neonates from the eCry3.1Ab-selected resistant colony expressed a small number of up and down-regulated genes following Bt intoxication. In contrast, unselected susceptible WCR neonates expressed a large number of up and down-regulated transcripts in response to intoxication. Annotation and pathway analysis of DEGs between susceptible and resistant whole WCR and their midgut tissue revealed genes associated with cell membrane, immune response, detoxification, and potential Bt receptors which are likely related to eCry3.1Ab resistance. This research provides a framework to study the toxicology of Bt toxins and mechanism of resistance in WCR, an economically important coleopteran pest species.
Highlights
The Bt (Bacillus thuringiensis) Cry[3] δ-endotoxin superfamily is known for its specificity to coleopteran species[1,2]
Our results suggest that aminopeptidase N (APN) is a potential eCry3.1Ab target and the reduced expression level under intoxication might contribute to resistance in western corn rootworm (WCR)
Since dynein is a cytoskeletal motor protein involved in intracellular transportation and the movement of chromosomes, we propose that selected-WCR many have a stronger activity in either endocytosis or cell mitosis to remove the attached eCry3.1Ab molecules[47], or to repair damaged epithelial cells[48]
Summary
The Bt (Bacillus thuringiensis) Cry[3] δ-endotoxin superfamily is known for its specificity to coleopteran species[1,2]. Www.nature.com/scientificreports proteases, binding to brush border receptors, toxin insertion into the epithelial cell membrane, pore formation by oligomerization, and destruction of midgut epithelial cells[17,18] followed by fatal sepsis[19]. Anything interfering with these processes (e.g. decreased activation, reduced receptor affinity, increased toxin degradation, or increased repair of midgut epithelial cells) could cause resistance[20]. Toxin dose is a very important component of what has been coined the ‘high-dose/refuge strategy’ to delay resistance[14] In this theory the ‘high-dose’ refers to Bt toxins that cause very high mortality to pest populations. The expression patterns of different populations under these conditions reveal how WCR respond to eCry3.1Ab and provide clues of resistance mechanisms
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