Comparative genomic sequence analysis of novel Helicoverpa armigera nucleopolyhedrovirus (NPV) isolated from Kenya and three other previously sequenced Helicoverpa spp. NPVs

A newly cloned Helicoverpa armigera nucleopolyhedrovirus (HearNPV) from Kenya, HearNPV-NNg1, has a higher insecticidal activity than HearNPV-G4, which also exhibits lower insecticidal activity than HearNPV-C1. In the search for genes and/or nucleotide sequences that might be involved in the observed virulence differences among Helicoverpa spp. NPVs, the entire genome of NNg1 was sequenced and compared with previously sequenced genomes of G4, C1 and Helicoverpa zea single-nucleocapsid NPV (Hz). The NNg1 genome was 132,425 bp in length, with a total of 143 putative open reading frames (ORFs), and shared high levels of overall amino acid and nucleotide sequence identities with G4, C1 and Hz. Three NNg1 ORFs, ORF5, ORF100 and ORF124, which were shared with C1, were absent in G4 and Hz, while NNg1 and C1 were missing a homologue of G4/Hz ORF5. Another three ORFs, ORF60 (bro-b), ORF119 and ORF120, and one direct repeat sequence (dr) were unique to NNg1. Relative to the overall nucleotide sequence identity, lower sequence identities were observed between NNg1 hrs and the homologous hrs in the other three Helicoverpa spp. NPVs, despite containing the same number of hrs located at essentially the same positions on the genomes. Differences were also observed between NNg1 and each of the other three Helicoverpa spp. NPVs in the diversity of bro genes encoded on the genomes. These results indicate several putative genes and nucleotide sequences that may be responsible for the virulence differences observed among Helicoverpa spp., yet the specific genes and/or nucleotide sequences responsible have not been identified.

Spodoptera frugiperda resistance to oral infection by Autographa californica multiple nucleopolyhedrovirus linked to aberrant occlusion-derived virus binding in the midgut

Spodoptera frugiperda larvae are highly resistant to oral infection by Autographa californica multiple nucleopolyhedrovirus (AcMNPV) (LD(50), approximately 9200 occlusions), but extremely susceptible to budded virus within the haemocoel (LD(50), <1 p.f.u.). The inability of AcMNPV occlusion-derived virus (ODV) to establish primary infections readily within midgut cells accounts for a major proportion of oral resistance. To determine whether inappropriate binding of AcMNPV ODV to S. frugiperda midgut cells contributes to lack of oral infectivity, the binding and fusion properties of AcMNPV ODV were compared with those of the ODV of a new isolate of Spodoptera frugiperda multiple nucleopolyhedrovirus (SfMNPV) obtained from a field-collected larva (oral LD(50), 12 occlusions). By using a fluorescence-dequenching assay conducted in vivo, it was found that AcMNPV ODV bound to the midgut epithelia of S. frugiperda larvae at approximately 15 % of the level of SfMNPV ODV, but that, once bound, the efficiencies of fusion for the two ODVs were similar: 60 % for AcMNPV and 53 % for SfMNPV. Whilst the difference in binding efficiencies was significant, it could not account entirely for the observed differences in infectivity. Competition experiments, however, revealed that, in S. frugiperda larvae, SfMNPV ODV bound to a midgut cell receptor that was not bound by AcMNPV ODV, indicating that ODV interaction with a specific receptor(s) was necessary for productive infection of midgut columnar epithelial cells. Fusion in the absence of this ligand-receptor interaction did not result in productive infections.

Pathogenesis of Autographa californica M nucleopolyhedrovirus in fifth instar Spodoptera frugiperda

We have characterized infection and pathogenesis of an Autographa californica M nucleopolyhedrovirus recombinant, AcMNPV-hsp70/lacZ, carrying the lacZ reporter gene, in penultimate (fifth) instar Spodoptera frugiperda. Bioassays revealed that while <0.1 p.f.u. of budded virus was required to generate an LD(50) by intrahaemocoelic injection, approximately 6000 occlusions were required orally to achieve the same mortality in newly moulted fifth instar (5(0)) larvae. In pathogenesis experiments, 78 % of the 5(0) larvae inoculated orally with 6000 occlusions of AcMNPV-hsp70/lacZ were LacZ-positive at 8 h post-inoculation (p.i.) and 50 % had LacZ signals in tracheal cells indicating that in these larvae infection had been transmitted from the midgut to secondary target cells. At 24 h p.i., maximum numbers of midgut and midgut-associated tracheal foci were observed (mean of 35 foci per infected larva), and 88 % of the larvae were LacZ-positive. The extremely low foci-per-occlusion ratio (0.006) indicated that successful infection of midgut cells was the primary barrier to fatal infection. A second barrier involved the loss of infected tracheal cells associated with the midgut. At 24 h p.i., 88 % of the inoculated larvae had a systemic infection, but in bioassays only 51 % succumbed to polyhedrosis disease. The absence of melanized tracheal cells in the insects throughout the time-course suggested that the larvae that cleared their infections (38 %) did so by a mechanism other than a classical immune response.

Characterization of a single-nucleocapsid nucleopolyhedrovirus of Thysanoplusia orichalcea L. (Lepidoptera: Noctuidae) from Indonesia

A single-nucleocapsid nucleopolyhedrovirus (NPV) isolated from Thysanoplusia orichalcea L. (Lepidoptera:Noctuidae) (ThorNPV) in Indonesia has tetrahedral occlusion bodies (OBs) with a width of 1. 22 microm (range = 0.803-1.931 microm). The length of the virion with an envelope averaged 0.29 and 0.23 microm without an envelope. ThorNPV was propagated in Pseudoplusia includens (Walker) and its authenticity was confirmed by sequence analysis of the polyhedrin gene of the ThorNPV produced in T. orichalcea and P. includens. Polyhedrin amino acid sequence analysis revealed that ThorNPV belongs to Group II of baculoviruses and is closely related to Trichoplusia ni single nucleocapsid NPV, sharing 97.6% sequence identity. Infectivity of ThorNPV against third instar P. includens was low, with a LD(50) value of 65,636 OBs/larva. Electron microscopy of infected tissues showed many polyhedra without virions embedded, which might explain the low virulence against P. includens. Differences in virion occlusion rates between individual cells in the same tissue suggested that the inoculum consisted of at least two variants that differed in the gene(s) controlling virion occlusion. In a host range test using the LD(50) value to P. includens against Spodoptera exigua, S. frugiperda, S. eridania, Anticarsia gemmatalis, Helicoverpa zea, Trichoplusia ni, and P. includens, P. includens was the only species infected. The virus infected primarily the fat body, tracheal epithelium, and hypodermis. The genomic size of the ThorNPV is 135 kb.

DNA restriction polymorphism in wild isolates of Spodoptera frugiperda nuclear polyhedrosis virus

Restriction endonuclease analysis was used to examine variation in DNA of 22 wild isolates of Spodoptera frugiperda nuclear polyhedrosis virus (SfNPV). Eleven of the 15 isolated from Louisiana were distinguishable based on restriction fragment profiles from the enzymes BamHI, HindIII, and EcoRI. There was significant genetic variation in SfNPV isolates within single agricultural fields. Nucleotide sequence divergence values, based on restriction fragment profiles, indicated that genetic variation among isolates foreign to Louisiana (Ohio, Ecuador, Mexico, Georgia, Colombia, and Venezuela) was greater than that among the Louisiana isolates. However, certain foreign isolates were similar to or identical with Louisiana isolates. Genetic variation of the viral DNA was not influenced by the insect's host plan species.