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Reproduction and immune defense are costly functions, and they are expected to tradeoff with each other to drive evolution. The brown planthopper (BPH), Nilaparvata lugens Stål (Hemiptera, Delphacidae), is a global superpest that mostly damages rice crops. Yeast-like symbionts (YLS) exist in the abdominal fat body tissue and are tightly associated with the development, growth, and reproduction of BPH. Our previous research demonstrated that mating behavior promotes the release of YLS from the fat body into the hemolymph in the BPH, thereby triggering an immune response. Additionally, the fitness costs related to life-history traits of BPH (such as survival rate) have a strong dependence on the relative abundance of YLS. However, the possible relationship between reproduction and the immune response in BPH has not been identified. In this study, an omics-based approach was used to analyze the transcriptome of fat body tissues in mated and unmated BPH at 72 h post-eclosion, from which two antimicrobial peptide genes, NlDefensin A (NlDfA) and NlDefensin B (NlDfB), were selected since they were highly expressed in mated BPH. Subsequently, the full-length cDNA sequences of the NlDfA and NlDfB genes were cloned and analyzed. qPCR results showed up-regulation of the NlDfA and NlDfB genes in mated BPH when compared to unmated BPH. Spatial-temporal expression analysis indicated that the NlDfA and NlDfB genes were expressed in all tissues and developmental stages, and they were most highly expressed in the fat body at 24 h post-eclosion. Moreover, the symbionts in BPH were significantly inhibited by the in vitro expression of the NlDfA and NlDfB proteins. Furthermore, RNA interference (RNAi)-mediated suppression of NlDfA and NlDfB dramatically increased the relative abundance of YLS in the fat body, while YLS in the hemolymph decreased significantly. These BPH also displayed some fitness disadvantages in survival, fecundity, hatchability, and possibly the vertical transmission of YLS from hemolymph to egg. Our results indicated that mating could heighten the immunity of BPH by up-regulating the expression of the NlDfA and NlDfB genes, which protect the host from pathogen challenge during reproduction. However, the reduced content of YLS may act as a fitness disadvantage in dictating the life-history traits of BPH. This work has significant theoretical and practical implications for the precise green control technology that involves crucial gene targeting, as well as for the "endosymbionts for pest control" strategy in insects.
Sun et al. (Fri,) studied this question.