Parasitoid insects manipulate their hosts’ physiology by injecting various factors such as venom, polydnavirus (PDV) and other virus-like particles into their host upon parasitization. Most of these factors interfere with host immunity and development. In this study, we used next generation deep sequencing technology to explore the impact of Diadegma semiclausum parasitization on its host, diamondback moth Plutella xylostella L. (Lepidoptera, Plutellidae), a notorious pest of cruciferous plants.
Transcriptome profiling of parasitized larvae helps us to obtain a better understanding of subsequent related cellular activities in the host including growth, development, and immune defense. The first P. xylostella transcriptome profiles from naive and parasitized larvae were produced using Illumina Genome Analyzer IIx next generation sequencing. De novo assembly of cDNA sequence data generated 172,660 contigs between 100 and 10,000 bp in length; with 35% of >200 bp in length. Parasitization had significant impacts on expression levels of 928 identified insect host transcripts. Gene ontology data illustrated that the majority of the differentially expressed genes are involved in binding, catalytic activity, and metabolic and cellular processes. The expression levels of many membrane proteins and cellular receptors such as scavenger receptors (SR) were significantly altered in parasitized larvae. Expression of D. semiclausum ichnovirus (DsIV) genes were also detected in parasitized larvae with 19 unique sequences identified from five PDV gene families. Transient expression of two DsIV genes, Vankyrin1 and Repeat element 4 (Rep4), in P. xylostella Px cells led to significant down-regulation of SR transcript levels.
Our next approach to study host-parasitoid interaction at the gene expression level was to investigate the expression profile of host small non-coding RNAs in response to parasitization using a deep sequencing approach. MicroRNAs (miRNAs) as small non-coding RNAs play important roles in many biological processes such as development, cell signaling and immune response. Combining the deep sequencing data and bioinformatics, 235 miRNAs were identified from P. xylostella. Differential expression of host cellular miRNAs in response to parasitism was examined by making small RNA libraries from parasitized and naive second instar larvae of P. xylostella. Bantam, miR-276*, miR-10, miR-31 and miR-184 were recognized as the five most abundant miRNAs in both libraries and 96 miRNAs were identified that were differentially expressed after parasitization. We also showed that pxy-miR-8 positively regulates the transcript levels of the serine protease inhibitor Serpin 27, which has been shown to regulate activation of the Toll pathway and prophenoloxidase involved in the melanization response in insects. Interestingly, miR-8 is down-regulated following parasitization leading to significant declines in Serpin 27 transcript levels.
This study provides detailed information on differential expression of P. xylostella larval genes following parasitization. By combining the data from the miRNA expression profiles and host genes transcriptome, our current understanding of this host-parasitoid interaction has been improved and offered an important resource for further studies to get better insights into possible involvement of small RNAs in insect immune response to parasitism.