Antiviral Treatment Reveals a Cooperative Pathogenicity of Baculovirus and Iflavirus in Spodoptera exigua, a Lepidopteran Insect

Lymantria Dispar Iflavirus 1 RNA Comprises a Large Proportion of RNA in Adult L. dispar Moths

The spongy moth virus Lymantria dispar iflavirus 1 (LdIV1), originally identified from a Lymantria dispar cell line, was detected in 24 RNA samples from female moths of four populations from the USA and China. Genome-length contigs were assembled for each population and compared with the reference genomes of the first reported LdIV1 genome (Ames strain) and two LdIV1 sequences available in GenBank originating from Novosibirsk, the Russian Federation. A whole-genome phylogeny was generated for these sequences, indicating that LdIV1 viruses observed in North American (flightless) and Asian (flighted) spongy moth lineages indeed partition into clades as would be expected per their host's geographic origin and biotype. A comprehensive listing of synonymous and non-synonymous mutations, as well as indels, among the polyprotein coding sequences of these seven LdIV1 variants was compiled and a codon-level phylogram was computed using polyprotein sequences of these, and 50 additional iflaviruses placed LdIV1 in a large clade consisting mostly of iflaviruses from other species of Lepidoptera. Of special note, LdIV1 RNA was present at very high levels in all samples, with LdIV1 reads accounting for a mean average of 36.41% (ranging from 1.84% to 68.75%, with a standard deviation of 20.91) of the total sequenced volume.

Four antimicrobial peptides of Asian gypsy moth respond to infection of its viral pathogen, nucleopolyhedrovirus (LdMNPV)

Antimicrobial peptides (AMPs) play essential roles in defending against various invading pathogens. Although antibacterial or antifungal properties of AMPs have been well characterized, the contribution of AMPs to immune defenses against viruses especially baculoviruses is still unclear. In this study, four full-length AMP genes (Ldcec, Ldatt, Ldglo and Ldmor) that encode the cecropin, attacin, gloverin and moricin, respectively, were characterized in Lymantria dispar (Asian gypsy moth). All four AMPs were cationic peptides and exhibited hydrophilicity. Structural analysis showed that the Ldcec and Ldmor were α-helical peptides. Tissue-specific Ldcec expression was the highest in fat body, while expression of Ldatt, Ldglo and Ldmor was the highest in epidermis. All four AMP genes were expressed during all developmental stages with the highest expression in the pupa and adult. Compared to mock infection, expression of these four AMP genes were significantly induced following Lymantria dispar multiple nucleopolyhedrovirus (LdMNPV) challenge and sharply increased at 72 h post infection. After Ldglo gene silencing, the DNA replication levels of LdMNPV in L. dispar larvae significantly increased at 48 and 72 h post infection, indicating that the Ldglo could suppress the DNA replication of LdMNPV. Our results suggest that four AMPs of L. dispar may play important roles in antiviral immunity against LdMNPV.

In vivo transient expression of a viral silencing suppressor, NSs, derived from tomato spotted wilt virus decreases insect RNAi efficiencies

Tomato spotted wilt virus is a single-stranded RNA virus and causes a serious plant disease. Its horizontal transmission depends on some thrips species including Frankliniella occidentalis. Its genome encodes a nonstructural protein, nonstructural (NSs), which acts as a silencing suppressor and plays a crucial role in the pathogenicity by defending antiviral immunity using RNA interference (RNAi) in plant hosts. However, its physiological function as a silencing suppressor was not well clarified in insect vectors. This study assessed any change of RNAi efficiencies in two other insect systems by NSs expression. To this end, the gene was cloned into a eukaryotic expression vector and transiently expressed in two different insect species via in vivo transient expression (IVTE). After feeding the recombinant construct to non-viruliferous F. occidentalis, NSs expression was observed for over 2 days in the thrips. Under this expression of NSs, thrips were rescued from a treatment of a toxic double stranded RNA specific to v-ATPase. Interestingly, the thrips treated with IVTE significantly suppressed the expression of RNAi machinery genes such as SID and Dicer-2. The recombinant vector expressing NSs was injected to a non-vector insect, Spodoptera exigua, larvae. The larvae expressing NSs by the IVTE were highly susceptible to an infection of a RNA virus called iflavirus. These suggest that NSs acts as a silencing suppressor in insects and would be used for a synergist for RNA pathogens to control insect pests.