Parasitism and survival rate of Diadegma fenestrale (Hymenoptera: Ichneumonidae) and DfIV gene expression patterns in two lepidopteran hosts

Genome, host genome integration, and gene expression in Diadegma fenestrale ichnovirus from the perspective of coevolutionary hosts

Polydnaviruses (PDVs) exhibit species-specific mutualistic relationships with endoparasitoid wasps. PDVs can be categorized into bracoviruses and ichnoviruses, which have independent evolutionary origins. In our previous study, we identified an ichnovirus of the endoparasitoid Diadegma fenestrale and named it DfIV. Here, DfIV virions from the ovarian calyx of gravid female wasps were characterized. DfIV virion particles were ellipsoidal (246.5 nm × 109.0 nm) with a double-layered envelope. Next-generation sequencing of the DfIV genome revealed 62 non-overlapping circular DNA segments (A1-A5, B1-B9, C1-C15, D1-D23, E1-E7, and F1-F3); the aggregate genome size was approximately 240 kb, and the GC content (43%) was similar to that of other IVs (41%-43%). A total of 123 open reading frames were predicted and included typical IV gene families such as repeat element protein (41 members), cysteine motif (10 members), vankyrin (9 members), polar residue-rich protein (7 members), vinnexin (6 members), and N gene (3 members). Neuromodulin N (2 members) was found to be unique to DfIV, along with 45 hypothetical genes. Among the 62 segments, 54 showed high (76%-98%) sequence similarities to the genome of Diadegma semiclausum ichnovirus (DsIV). Three segments, namely, D22, E3, and F2, contained lepidopteran host genome integration motifs with homologous regions of about 36-46 bp between them (Diadegma fenestrale ichnovirus, DfIV and lepidopteran host, Plutella xylostella). Most of the DfIV genes were expressed in the hymenopteran host and some in the lepidopteran host (P. xylostella), parasitized by D. fenestrale. Five segments (A4, C3, C15, D5, and E4) were differentially expressed at different developmental stages of the parasitized P. xylostella, and two segments (C15 and D14) were highly expressed in the ovaries of D. fenestrale. Comparative analysis between DfIV and DsIV revealed that the genomes differed in the number of segments, composition of sequences, and internal sequence homologies.

Identification of Two Diamondback Moth Parasitoids, Diadegma fenestrale and Diadegma semiclausum, Using LAMP for Application in Biological Control

The diamondback moth, Plutella xylostella L., is a lepidopteran pest that damages various vegetable plants belonging to the genus Brassica worldwide. Various biological controls, such as parasitoid wasps, have been used to control this pest. Among these, Diadegma semiclausum and Diadegmafenestrale are widely used globally. In field-based biological control research, the investigation of the population dynamics of parasitoids and the rate of parasitism within the pest population is very important. However, achieving profundity in research is difficult when morphologically similar species coexist in the field. The morphological characteristics of D. semiclausum and D. fenestrale are very similar, and they both parasitize P. xylostella larvae. Therefore, to accurately identify these species, in this study, we developed a molecular diagnostic method by using loop-mediated isothermal amplification (LAMP). The mitochondrial genome of D. fenestrale and partial nucleotide sequences, including the ITS region of D. semiclausum, were analyzed for use as species diagnosis markers. The results showed that the homology of D. fenestrale to D. semiclausum was 94%, due to the excessively low homology of the D loop, but the actual homology was higher than 94%, particularly in the coding region. D. fenestrale species-specific primers for LAMP were designed based on the region encoding COX3, and the optimal diagnostic reaction condition for the four primers (F3, B3, FIP, and BIP) was 63 °C for 35 min. A species-specific primer capable of classifying D. semiclausum was developed based on the ITS2 region, and the optimal reaction condition for diagnosis was 63 °C for 40 min. Under optimal conditions for both species, upon addition of the loop primer LB, the reaction efficiency increased, and the reaction time was shortened by more than 5 min. The diagnostic limit concentration was up to 10 pg under both optimal conditions; therefore, it was possible to detect even very low concentrations. For both species, diagnosis was possible by using LAMP assay with a DNA-releasing technique, without a DNA extraction process, and by incubating a tissue sample or the homogenized whole body at 95 °C for 5 min. In the case of D. fenestrale, it was possible to diagnose the parasitoid in P. xylostella larvae. Therefore, the developed LAMP diagnostic method can be used in a variety of ways to determine whether P. xylostella has been parasitized in the process of field research and mass breeding, and to accurately distinguish the species that are parasitic to P. xylostella larvae. This LAMP-based diagnostic method can be applied to identify various parasitoids that are used for the biological control of P. xylostella.

Polydnavirus Innexins Disrupt Host Cellular Encapsulation and Larval Maturation

Polydnaviruses are dsDNA viruses associated with endoparasitoid wasps. Delivery of the virus during parasitization of a caterpillar and subsequent virus gene expression is required for production of an amenable environment for parasitoid offspring development. Consequently, understanding of Polydnavirus gene function provides insight into mechanisms of host susceptibility and parasitoid wasp host range. Polydnavirus genes predominantly are arranged in multimember gene families, one of which is the vinnexins, which are virus homologues of insect gap junction genes, the innexins. Previous studies of Campoletis sonorensis Ichnovirus Vinnexins using various heterologous systems have suggested the four encoded members may provide different functionality in the infected caterpillar host. Here, we expressed two of the members, vnxG and vnxQ2, using recombinant baculoviruses in susceptible host, the caterpillar Heliothis virescens. Following intrahemocoelic injections, we observed that >90% of hemocytes (blood cells) were infected, producing recombinant protein. Larvae infected with a vinnexin-recombinant baculovirus exhibited significantly reduced molting rates relative to larvae infected with a control recombinant baculovirus and mock-infected larvae. Similarly, larvae infected with vinnexin-recombinant baculoviruses were less likely to survive relative to controls and showed reduced ability to encapsulate chromatography beads in an immune assay. In most assays, the VnxG protein was associated with more severe pathology than VnxQ2. Our findings support a role for Vinnexins in CsIV and more broadly Ichnovirus pathology in infected lepidopteran hosts, particularly in disrupting multicellular developmental and immune physiology.

Intracellular dynamics of polydnavirus innexin homologues

Polydnaviruses associated with ichneumonid parasitoid wasps (Ichnoviruses) encode large numbers of genes, often in multigene families. The Ichnovirus Vinnexin gene family, which is expressed in parasitized lepidopteran larvae, encodes homologues of Innexins, the structural components of insect gap junctions. Here, we have examined intracellular behaviours of the Campoletis sonorensis Ichnovirus (CsIV) Vinnexins, alone and in combination with a host Innexin orthologue, Innexin2 (Inx2). QRT-PCR verified that transcription of CsIV vinnexins occurs contemporaneously with inx2, implying co-occurrence of Vinnexin and Inx2 proteins. Confocal microscopy demonstrated that epitope-tagged VinnexinG (VnxG) and VinnexinQ2 (VnxQ2) exhibit similar subcellular localization as Spodoptera frugiperda Inx2 (Sf-Inx2). Surface biotinylation assays verified that all three proteins localize to the cell surface, and cytochalasin B and nocodazole that they rely on actin and microtubule cytoskeletal networks for localization. Immunomicroscopy following co-transfection of constructs indicates extensive co-localization of Vinnexins with each other and Sf-Inx2, and live-cell imaging of mCherry-labelled Inx2 supports that Vinnexins may affect Sf-Inx2 distribution in a Vinnexin-specific fashion. Our findings support that the Vinnexins may disrupt host cell physiology in a protein-specific manner through altering gap junctional intercellular channel communication, as well as indirectly by affecting multicellular junction characteristics.