Midgut-Specific Expression of P450 Gene Increases Deltamethrin Tolerance in The Fall Armyworm, Spodoptera Frugiperda

The piggyBac-based germline transformation system was recently established in a global agricultural pest, the fall armyworm (FAW), Spodoptera frugiperda. Tissue-specific promoters are needed to apply this transformation system to express transgenes in a tissue-specific manner. Highly expressed genes in the midgut were identified by RNA sequencing and RT-qPCR. Promoter regions of 11 genes highly expressed in the midgut were identified and cloned. Baculoviruses expressing the luciferase under the control of these promoters were produced and tested in the FAW. These baculoviruses did not show significant luciferase activity in the FAW midgut. Four transgenic FAW lines, expressing the luciferase under the control of SfSP38/P2000, SfCalphotin/P2000, SfMG17/P2000, and SfCPH38/P2000 promoters were generated using piggyBac-based germline transformation methods. Significantly higher luciferase activity was detected in the midgut than in other tissues of transgenic FAW. SfCPH38/P2000 promoter with the highest activity and midgut-specificity was used to drive the expression of a P450, SfCYP321A8 known to be involved in deltamethrin resistance. Higher mRNA levels of SfCYP321A8 and P450 activity were detected in the midgut of transgenic larvae than in wild-type larvae. Bioassays showed that the transgenic larvae expressing SfCYP321A8 in the midgut are tolerant to deltamethrin. Here, we presented methods for the identification of midgut-specific promoters in the FAW and used them to study the role of P450 overexpression in the midgut on insecticide resistance. These methods could also be used to identify other tissue-specific promoters for applications of piggyBac-based germline transformation in functional genomics in FAW and other non-model insects.

Improvement on the genetic engineering of an invasive agricultural pest insect, the cherry vinegar fly, Drosophila suzukii

Background The invasive fly Drosophila suzukii has become an established fruit pest in Europe, the USA, and South America with no effective and safe pest management. Genetic engineering enables the development of transgene-based novel genetic control strategies against insect pests and disease vectors. This, however, requires the establishment of reliable germline transformation techniques. Previous studies have shown that D. suzukii is amenable to transgenesis using the transposon-based vectors piggyBac and Minos, site-specific recombination (lox/Cre), and CRISPR/Cas9 genome editing. Results We experienced differences in the usability of piggyBac-based germline transformation in different strains of D. suzukii: we obtained no transgenic lines in a US strain, a single rare transgenic line in an Italian strain, but observed a reliable transformation rate of 2.5 to 11% in a strain from the French Alps. This difference in efficiency was confirmed by comparative examination of these three strains. In addition, we used an attP landing site line to successfully established φC31-integrase-mediated plasmid integration at a rate of 10% and generated landing site lines with two attP sequences to effectively perform φC31-Recombinase Mediated Cassette Exchange (φC31-RMCE) with 11% efficiency. Moreover, we isolated and used the endogenous regulatory regions of Ds nanos to express φC31 integrase maternally to generate self-docking lines for φC31-RMCE. Besides, we isolated the promoter/enhancer of Ds serendipity α to drive the heterologous tetracycline-controlled transactivator (tTA) during early embryonic development and generated a testes-specific tTA driver line using the endogenous beta-2-tubulin (β2t) promoter/enhancer. Conclusion Our results provide evidence that the D. suzukii strain AM derived from the French Alps is more suitable for piggyBac germline transformation than other strains. We demonstrated the feasibility of using φC31-RMCE in the cherry vinegar fly and generated a set of lines that can be used for highly efficient integration of larger constructs. The φC31-based integration will facilitate modification and stabilization of previously generated transgenic lines that carry at least one attP site in the transgene construction. An early embryo-specific and a spermatogenesis-specific driver line were generated for future use of the binary expression system tet-off to engineer tissue- and stage-specific effector gene expression for genetic pest control strategies.

The hAT-family transposable element, hopper, from Bactrocera dorsalis is a functional vector for insect germline transformation

Background The hopper hAT-family transposable element isolated from the Oriental fruit fly, Bactrocera dorsalis, is distantly related to both the Drosophila hobo element and the Activator element from maize. The original 3120 bp hopperBd-Kah element isolated from the Kahuku wild-type strain was highly degenerate and appeared to have a mutated transposase and terminal sequences, while a second 3131 bp element, hopperBd-we, isolated from a white eye mutant strain had an intact transposase reading frame and terminal sequences consistent with function. Results The hopperBd-we element was tested for function by its ability to mediate germline transformation in two dipteran species other than B. dorsalis. This was achieved by creating a binary vector/helper transformation system by linking the hopperBd-we transposase reading frame to a D. melanogaster hsp70 promoter for a heat-inducible transposase helper plasmid, and creating vectors marked with the D. melanogaster mini-white+ or polyubiquitin-regulated DsRed fluorescent protein markers. Conclusions Both vectors were successfully used to transform D. melanogaster, and the DsRed vector was also used to transform the Caribbean fruit fly, Anastrepha suspensa, indicating a wide range of hopper function in dipteran species and, potentially, non-dipteran species. This vector provides a new tool for insect genetic modification for both functional genomic analysis and the control of insect populations.