RNAi-mediated silencing of an immune gene of Spodoptera littoralis (Lepidoptera, Noctuidae) alters its embryonic development

BThe immune gene named 102 is highly expressed in haemocytes and encodes a precursor protein that gives rise to amyloid fibrils essential for the cellular immune response in both Heliothis virescens and Spodoptera littoralis noctuid moths (Lepidoptera, Noctuidae). RNAi-mediated silencing of this gene (Sl102) in S. littoralis larvae results in an immunosuppressed phenotype that increases the susceptibility of this pest to pathogen infections and provides the basis for developing new insect control strategies. Here, we investigate the effects of Sl102 gene silencing on embryonic development, by soaking S. littoralis eggs in a dsRNA solution. Time-course analysis showed that the Sl102 gene is expressed throughout embryonic development, from egg laying to hatching, and the transcript levels are strongly reduced after treatment with specific dsRNA molecules (dsSl102), compared to control embryos treated with dsRNA targeting GFP (dsGFP). Interestingly, the gene silencing is associated with a drastic reduction in egg hatching and a very high mortality rate of newly hatched larvae. Structural and ultrastructural analyses showed a dramatic delay in the development of silenced embryos, which presented morphological alterations that point out an important role played by the Sl102 gene during embryogenesis. This study suggests that targeting eggs for disrupting embryogenesis represents a promising strategy for Noctuidae control. Further studies will be aimed at understanding Sl102 role in embryogenesis and developing a proper delivery system for dsRNA targeting lepidoptera eggs.