Monitoring insect transposable elements in large double-stranded DNA viruses reveals host-to-virus and virus-to-virus transposition

The mechanisms by which transposable elements (TEs) can be horizontally transferred between animals are unknown, but viruses are possible candidate vectors. Here, we surveyed the presence of host-derived TEs in viral genomes in 35 deep sequencing data sets produced from 11 host–virus systems, encompassing nine arthropod host species (five lepidopterans, two dipterans, and two crustaceans) and six different double-stranded (ds) DNA viruses (four baculoviruses and two iridoviruses). We found evidence of viral-borne TEs in 14 data sets, with frequencies of viral genomes carrying a TE ranging from 0.01% to 26.33% for baculoviruses and from 0.45% to 7.36% for iridoviruses. The analysis of viral populations separated by a single replication cycle revealed that viral-borne TEs originating from an initial host species can be retrieved after viral replication in another host species, sometimes at higher frequencies. Furthermore, we detected a strong increase in the number of integrations in a viral population for a TE absent from the hosts’ genomes, indicating that this TE has undergone intense transposition within the viral population. Finally, we provide evidence that many TEs found integrated in viral genomes (15/41) have been horizontally transferred in insects. Altogether, our results indicate that multiple large dsDNA viruses have the capacity to shuttle TEs in insects and they underline the potential of viruses to act as vectors of horizontal transfer of TEs. Furthermore, the finding that TEs can transpose between viral genomes of a viral species sets viruses as possible new niches in which TEs can persist and evolve.

Bombyx mori nucleopolyhedrovirus protein Bm11 is involved in occlusion body production and occlusion-derived virus embedding

Bombyx mori nucleopolyhedrovirus (BmNPV) orf11 (bm11) is a highly conserved gene with unknown function. It is homologous to AcMNPV orf19. In this study, a bm11 knockout virus was constructed and its role was investigated. Expression analysis indicated that bm11 is a late gene and confocal microscopy analysis demonstrated that Bm11 localizes predominantly in the nuclear ring zone at the late phase of infection. The bm11 deletion did not affect budded virus (BV) production or viral genome replication, but markedly reduced the production of occlusion bodies (OBs) and the embedding of occlusion-derived viruses (ODVs). Bio-assays showed that Bm11 was involved in BmNPV infectivity in vivo by direct injection. In conclusion, our results demonstrated that although Bm11 is not essential for BV production or mature ODV formation, it affects OB production and ODV occlusion.

The complete genome of a baculovirus isolated from an insect of medical interest: Lonomia obliqua (Lepidoptera: Saturniidae)

Lonomia obliqua (Lepidoptera: Saturniidae) is a species of medical importance due to the severity of reactions caused by accidental contact with the caterpillar bristles. Several natural pathogens have been identified in L. obliqua, and among them the baculovirus Lonomia obliqua multiple nucleopolyhedrovirus (LoobMNPV). The complete genome of LoobMNPV was sequenced and shown to have 120,022 bp long with 134 putative open reading frames (ORFs). Phylogenetic analysis of the LoobMNPV genome showed that it belongs to Alphabaculovirus group I (lepidopteran-infective NPV). A total of 12 unique ORFs were identified with no homologs in other sequenced baculovirus genomes. One of these, the predicted protein encoded by loob035, showed significant identity to an eukaryotic transcription terminator factor (TTF2) from the Lepidoptera Danaus plexippus, suggesting an independent acquisition through horizontal gene transfer. Homologs of cathepsin and chitinase genes, which are involved in host integument liquefaction and viral spread, were not found in this genome. As L. obliqua presents a gregarious behavior during the larvae stage the impact of this deletion might be neglectable.

Characterization of Bombyx mori nucleopolyhedrovirus with a knockout of Bm17

Open reading frame 17 (Bm17) gene of Bombyx mori nucleopolyhedrovirus is a highly conserved gene in lepidopteran nucleopolyhedroviruses, but its function remains unknown. In this report, transient-expression and superinfection assays indicated that BM17 localized in the nucleus and cytoplasm of infected BmN cells. To determine the role of Bm17 in baculovirus life cycle, we constructed a Bm17 knockout virus and characterized its properties in cells. Analysis of the production and infection of budded virions, the level of viral DNA replication revealed showed that there was no significant difference among the mutant, the control, and the Bm17 repaired virus strains. These results suggest that BM17 is not essential for virus replication in cultured cells.

Baculovirus-encoded protein BV/ODV-E26 determines tissue tropism and virulence in lepidopteran insects

Lepidopteran nucleopolyhedroviruses (NPVs) show distinct tissue tropism in host insect larvae. However, the molecular mechanism of this tropism is largely unknown. We quantitatively investigated NPV tissue tropism by measuring mRNA levels of viral genes in 16 tissues from Bombyx mori NPV (BmNPV)-infected B. mori larvae and found clear tissue tropism, i.e., BmNPV replicates poorly in the silk glands, midgut, and Malpighian tubule compared with other larval tissues. We next identified the viral genes determining tissue tropism in NPV infection by investigating the phenotypes of larvae infected with 44 BmNPV mutants in which one gene was functionally disrupted by a LacZ cassette insertion. We found that occlusion body (OB) production was markedly enhanced compared with that of the wild type in the middle silk glands (MSGs) of larvae infected with three mutants in which one of three tandemly arrayed genes (Bm7, Bm8, and Bm9) was disrupted. We generated additional mutants in which one or two genes of this gene cluster were partially deleted and showed that Bm8, also known as BV/ODV-E26, was solely required for the suppression of OB production in the MSGs of BmNPV-infected B. mori larvae. Western blotting showed that a LacZ cassette insertion in Bm7 or Bm9 resulted in aberrant expression of Bm8, presumably leading to abnormal OB production in the MSGs. Larval bioassays also revealed that disruption of Bm8 accelerated the death of B. mori larvae. These results suggest that the group I NPV-specific protein BV/ODV-E26 determines tissue tropism and virulence in host lepidopteran insects.

Open reading frame 60 of the Bombyx mori nucleopolyhedrovirus plays a role in budded virus production

Open reading frame 60 (bm60) of the Bombyx mori nucleopolyhedrovirus (BmNPV) is a conserved gene among group I and some group II NPVs. bm60 encodes a late expressed protein that localizes to both the cytoplasm and nucleus of infected cells. This paper describes the characterization of a BmNPV mutant (vbm60-Null) lacking functional bm60. It was observed that the production of budded virus (BV) was reduced by nearly an order of magnitude relative to wt virus in vbm60-Null-infected BmN cells and B. mori larvae. Quantitative real-time PCR assay showed that the viral DNA replication was affected in infected cells due to disruption of bm60. Larval bioassays showed that the speed of kill of vbm60-Null virus was greatly reduced, as it took approximately 28-36 h longer to kill the fifth instar B. mori larvae. These results suggest that BmNPV bm60 is not essential for viral replication, but required for efficient BV production.

Bombyx mori nucleopolyhedrovirus ORF34 is required for efficient transcription of late and very late genes

Homologs of Autographa californica nucleopolyhedrovirus ORF43 (Ac43) are found in all group I and most group II NPVs, but their functions remain unknown. In Bombyx mori NPV (BmNPV)-infected BmN cells, Bm34, a BmNPV homolog of Ac43, is expressed as a late gene and its product is localized in the nucleus. To examine the role of Bm34 during BmNPV infection, we constructed a Bm34 deletion mutant (BmORF34D) and characterized its infectivity in BmN cells and B. mori larvae. BmORF34D produced far fewer occlusion bodies (OBs) in BmN cells as compared with wild-type BmNPV. This reduction is assumed to be mainly due to the transcriptional down-regulation of two genes, very late expression factor (vlf-1) and few polyhedra (fp25K), both of which are required for efficient polyhedrin expression. Larval bioassays also revealed that Bm34 accelerates death of B. mori larvae. These results suggest that Bm34 is required for efficient late and very late gene expression.