Gut physiology and host-microbe interactions of the rotation-resistant variant of the western corn rootworm (Diabrotica virgifera virgifera LeConte) (Coleoptera: Chrysomelidae)

The western corn rootworm (WCR; Diabrotica virgifera virgifera LeConte) is an important chrysomelid pest of corn (Zea mays) in the US. WCR larvae feed on corn roots, while adult beetles may consume aboveground corn tissues. WCR larvae are nutritionally dependent on corn roots; completion of the univoltine WCR lifecycle relies on the nearby availability of root tissues for the larvae. Close access to the host plant is generally secured by strong adult ovipositional fidelity to cornfields. Annual crop rotation between corn and soybean (Glycine max) has been the main strategy for controlling the WCR in the eastern Corn Belt; WCR larvae emerging in rotated soybean fields cannot survive. However, this method selected for a “rotation-resistant” (RR) variant with reduced ovipositional fidelity to cornfields. Although genetically diagnostic markers differentiating RR individuals from wild-type (WT) WCR are currently unavailable, characterizations of RR-WCR populations showed that they exhibit greater mobility and different responses to corn phenology. In addition, gene transcripts sharing sequence similarity with immune genes were found differentially expressed in WT and RR populations. When feeding on soybean, RR-WCR populations also exhibit greater tolerance of soybean diets than WT populations, which may indicate an adaptation to soybean protease inhibitors. Since transcriptional analysis of several protease genes could not fully explain these differences, possible contributions from other genes or gut microbes have been proposed. Despite these findings, interactions amongst RR-WCR gut physiology, immune system, genetic regulation, and differences between those of WT-WCR are not well understood. In search of explanations for the distinct physiological traits and gene expression profiles documented in RR-WCR populations, host-microbe interactions in adult RR- and WT-WCR guts were characterized and tested for their contribution to WCR gut physiology (Chapter 2). Microbial community analyses across multiple field populations of RR- and WT-WCR identified shifts in gut bacterial community structure associated with WCR adaptation to brief soybean herbivory. Suppression of gut bacteria using antibiotic treatments reduced RR-WCR tolerance of soybean diets to the level of WT-WCR, whereas WT-WCR receiving the same treatment were unaffected, suggesting that gut bacteria may facilitate WCR adaptation to rotated landscapes. To examine whether the functional importance of genes correlated with RR-WCR’s physiological traits differed between phenotypes, a cysteine protease gene (DvRS5) and an immune gene (att1) were targeted for RNA interference (RNAi) in three field-sampled WCR populations. WT and RR populations that exhibit different levels of tolerance of soybean herbivory, gut cysteine protease activity, and immune gene expression (Chapter 3) were subjected to these treatments and compared to control groups (treated with Drosophila double-stranded RNA). In all three populations, RNAi treatments targeting DvRS5 reduced gut cysteine protease activity. However, the proportion of the cysteine protease activity that was inhibited was substantially smaller in the RR populations than in a WT population. When att1 was targeted for RNAi, different changes in survival among RR and WT populations on soybean occurred. In both DvRS5 and att1, the sequences targeted for RNAi were identical across all three populations, suggesting that the functional effects of these genes on WCR may differ depending on the physiological background in which they are expressed. To further characterize genetic mechanisms underlying the physiological changes observed in RR-WCR populations, a transcriptomic survey of the adult digestive tracts of phenotypically characterized RR- and WT-WCR populations was executed (Chapter 4). A de novo assembly of the WCR adult gut transcriptome was constructed and used to conduct RNA-sequencing (RNA-seq) analyses on RNA libraries from different WCR phenotypes fed with corn or soybean diets. Global gene expression profiles of WT- and RR-WCR were similar on corn diets but different when fed on soybean diets. Network-based analyses of the sequencing data identified gene “modules” that are transcriptionally correlated with the RR phenotype. Gene ontology enrichment analyses on these modules indicated that they were related to metabolic processes, biological adhesion, growth, immune responses, and other processes/functions that could correlate to physiological traits of RR populations. Overall, results from this series of studies suggested that host-microbe interactions, gut physiology, and related genetic regulation differed between RR- and WT-WCR. Whether these differences contribute to RR-WCR behavioral changes, are pleiotropic effects or subsequent adaptations due to the reduced ovipositional fidelity to corn is uncertain. However, it is likely that RR-WCR have undergone physiological adaptations that improve their fitness in areas where annual crop rotation is practiced. Consequently, these findings provide a broader understanding of how insects could be affected by ecological disturbances.Ope