Nucleopolyhedrovirus detection and distribution in terrestrial, freshwater, and marine habitats of Appledore Island, Gulf of Maine

Fecal Transmission of Spodoptera frugiperda Multiple Nucleopolyhedrovirus (SfMNPV; Baculoviridae)

The production of viable nucleopolyhedrovirus in the feces of infected lepidopteran larvae represents a poorly understood route for virus transmission prior to host death. In the present study, we examined the presence of viable virus in the feces of fourth-instar Spodoptera frugiperda larvae infected with the Nicaraguan isolate of Spodoptera frugiperda multiple nucleopolyhedrovirus (SfMNPV-NIC). Feces production increased in samples taken at 2 to 6 days post-inoculation but was significantly lower in infected insects compared to controls. Second instars experienced 3.9 to 68.3% of polyhedrosis disease following consumption of feces collected at 2-5 days post-inoculation, which subsequently fell to 29.1% in the 6-day sample. Calibration of the insect bioassay using OB-spiked samples of feces indicated that the concentration of OBs varied between 5.4 × 102 and 4.4 × 105 OBs/100 mg of feces in infected fourth instars. Quantitative PCR analysis of fecal samples indicated the presence of 0 to 7629 copies/mg feces following amplification targeted at the polyhedrin gene. However, no correlation was detected between qPCR estimates of virus concentration and time of sample collection or the quantity of feces collected. The qPCR estimates were positively correlated with the prevalence of lethal infection observed in the insect bioassay, but the correlation was weak and several samples did not amplify. Calibration of the qPCR assay using OB-spiked samples of feces provided estimates that were 5- to 10-fold lower than the insect bioassay, indicating inhibition of the amplification reaction or loss of material during processing. In a greenhouse experiment, 2.5-48.3% of second-instar larvae acquired lethal infection following a 24 h period of feeding on maize plants on which fourth instar larvae had deposited their feces at 3 days and 4 days post-infection, respectively. These findings highlight the potential of OB-contaminated feces as a source of biologically significant quantities of inoculum for virus transmission prior to the death of infected insects and represent an additional contribution to the biological control of lepidopteran pests by these pathogens.

A qPCR Assay for the Quantification of Selected Genotypic Variants of Spodoptera frugiperda Multiple Nucleopolyhedrovirus (Baculoviridae)

Alphabaculoviruses are lethal dsDNA viruses of Lepidoptera that have high genetic diversity and are transmitted in aggregates within proteinaceous occlusion bodies. This mode of transmission has implications for their efficacy as biological insecticides. A Nicaraguan isolate of Spodoptera frugiperda multiple nucleopolyhedrovirus (SfMNPV-NIC) comprising nine genotypic variants has been the subject of considerable study due to the influence of variant interactions on the insecticidal properties of mixed-variant occlusion bodies. As part of a systematic study on the replication and transmission of variant mixtures, a tool for the accurate quantification of a selection of genotypic variants was developed based on the quantitative PCR technique (qPCR). First, primer pairs were designed around a region of high variability in four variants named SfNic-A, SfNic-B, SfNic-C and SfNic-E to produce amplicons of 103-150 bp. Then, using cloned purified amplicons as standards, amplification was demonstrated over a dynamic range of 108-101 copies of each target. The assay was efficient (mean ± SD: 98.5 ± 0.8%), reproducible, as shown by low inter- and intra-assay coefficients of variation (<5%), and specific to the target variants (99.7-100% specificity across variants). The quantification method was validated on mixtures of genotype-specific amplicons and demonstrated accurate quantification. Finally, mixtures of the four variants were quantified based on mixtures of budded virions and mixtures of DNA extracted from occlusion-derived virions. In both cases, mixed-variant preparations compared favorably to total viral genome numbers by quantification of the polyhedrin (polh) gene that is present in all variants. This technique should prove invaluable in elucidating the influence of variant diversity on the transmission and insecticidal characteristics of this pathogen.

Occurrence and seasonal dynamics of RNA viral genotypes in three contrasting temperate lakes

Decades of research have demonstrated the crucial importance of viruses in freshwater ecosystems. However, few studies have focused on the seasonal dynamics and potential hosts of RNA viruses. We surveyed microbial-sized (i.e. 5-0.2 μm) mixed community plankton transcriptomes for RNA viral genomes and investigated their distribution between microbial and macrobial plankton over a seasonal cycle across three temperate lakes by quantitative reverse transcriptase PCR (qRT-PCR). A total of 30 contigs bearing similarity to RNA viral genomes were recovered from a global assembly of 30 plankton RNA libraries. Of these, only 13 were found in >2 libraries and recruited >100 reads (of 9.13 x 107 total reads), representing several picornaviruses, two tobamoviruses and a reovirus. We quantified the abundance of four picornaviruses and the reovirus monthly from August 2014 to May 2015. Patterns of viral abundance in the >5 μm size fraction and representation in microbial-sized community RNA libraries over time suggest that one picornavirus genotype (TS24835) and the reovirus (TS148892) may infect small (<5 μm) eukaryotic microorganisms, while two other picornaviruses (TS24641 and TS4340) may infect larger (>5 μm) eukaryotic microorganisms or metazoa. Our data also suggest that picornavirus TS152062 may originate from an allochthonous host. All five viral genotypes were present in at least one size fraction across all 3 lakes during the year, suggesting that RNA viruses may easily disperse between adjacent aquatic habitats. Our data therefore demonstrate that RNA viruses are widespread in temperate lacustrine ecosystems, and may provide evidence of viral infection in larger eukaryotes (including metazoa) inhabiting the lakes.

Molecular detection and pathogenicity of a nucleopolyhedrovirus isolated from looper caterpillar (Hyposidra talaca), a tea pest

Hyposidra talaca is a major defoliating pest of tea plants in north-eastern part of India. In this study, we look for variations (if any) in the attacking virus. Viral samples were collected from different regions of the northern part of West Bengal in India and were analyzed by PCR technique to study the variations across the region. The partial segment of the HytaNPV polyhedrin gene was cloned and sequenced. A 527 bp nucleotide sequence containing highly conserved region from polyhedrin gene of HytaNPV was observed. The blast homology search for studied polyhedrin gene showed 98% sequence identity with the sequence of previous reported NPV of H. talaca, H. infixaria and Buzura suppressaria. Pathogenicity study against second instar H. talaca indicated that the LC₅₀ values ranged from 4.61 × 10⁵ to 7.57 × 10⁵ polyhedral occlusion bodies per ml (POBs/ml) and the LT₅₀ values ranged from 4.2 to 6.66 days. Sequencing result reveals that the same HytaNPV strain dominates across this area and the pathogenicity indicates its potential as an alternative to chemical insecticides to control H. talaca.