Nucleopolyhedrovirus interactions with their insect hosts

Regulatory Mechanisms, Protein Expression and Biological Activity of Photolyase Gene from Spodoptera littoralis Granulovirus Genome

One of the most important factor that affects the efficient using of baculoviruses as a biopesticide is their sensitivity to UV irradiation. In this study, a photolyase gene (phr) of 1.4 kbp DNA fragment was cloned and characterized from Spodoptera littoralis granulovirus, an Egyptian isolate (SpliGV-EG1). A sequence of 466 amino acid were deduced when the gene was completely sequenced with a predicted molecular mass of ~ 55 kDa. Transcriptional regulation analyses revealed that phr transcripts were detected early at 6-h post-infection (hpi) and remained detectable until 72 hpi, suggesting their transcriptional regulation from a putative early promoter motif. An approximately ~ 55 kDa protein fragment was expressed from phr-induced bacterial culture and detected by SDS-PAGE and western blotting. In addition, direct exposure to UV irradiation resulted in a twofold decrease in SpliGV-EG1 occlusion bodies activation compared with Spodoptera littoralis nucleopolyhedrovirus (SpliNPV) occlusion bodies which decreased with about 129-fold after exposure to UV irradiation based on median lethal concentration value (LC50). The obtained results suggested that the presence of photolyase gene possibly alters the inactivation of SpliGV-EG1-occluded bodies by UV irradiation. These results support the role and application of the photolyase protein to improve the damaged DNA repair mechanism as well as resistance of SpliGV to UV light inactivation.

Separate seasons of infection and reproduction can lead to multi-year population cycles

Many host-pathogen systems are characterized by a temporal order of disease transmission and host reproduction. For example, this can be due to pathogens infecting certain life cycle stages of insect hosts; transmission occurring during the aggregation of migratory birds; or plant diseases spreading between planting seasons. We develop a simple discrete-time epidemic model with density-dependent transmission and disease affecting host fecundity and survival. The model shows sustained multi-annual cycles in host population abundance and disease prevalence, both in the presence and absence of density dependence in host reproduction, for large horizontal transmissibility, imperfect vertical transmission, high virulence, and high reproductive capability. The multi-annual cycles emerge as invariant curves in a Neimark-Sacker bifurcation. They are caused by a carry-over effect, because the reproductive fitness of an individual can be reduced by virulent effects due to infection in an earlier season. As the infection process is density-dependent but shows an effect only in a later season, this produces delayed density dependence typical for second-order oscillations. The temporal separation between the infection and reproduction season is crucial in driving the cycles; if these processes occur simultaneously as in differential equation models, there are no sustained oscillations. Our model highlights the destabilizing effects of inter-seasonal feedbacks and is one of the simplest epidemic models that can generate population cycles.

A Nymphalid-Infecting Group I Alphabaculovirus Isolated from the Major Passion Fruit Caterpillar Pest Dione juno juno (Lepidoptera: Nymphalidae)

Baculoviruses are capable of infecting a wide diversity of insect pests. In the 1990s, the Dione juno nucleopolyhedrovirus (DijuNPV) was isolated from larvae of the major passionfruit defoliator pest Dione juno juno (Nymphalidae) and described at ultrastructural and pathological levels. In this study, the complete genome sequence of DijuNPV was determined and analyzed. The circular genome presents 122,075 bp with a G + C content of 50.9%. DijuNPV is the first alphabaculovirus completely sequenced that was isolated from a nymphalid host and may represent a divergent species. It appeared closely related to Orgyia pseudotsugata multiple nucleopolyhedrovirus (OpMNPV) and other Choristoneura-isolated group I alphabaculoviruses. We annotated 153 open reading frames (ORFs), including a set of 38 core genes, 26 ORFs identified as present in lepidopteran baculoviruses, 17 ORFs unique in baculovirus, and several auxiliary genes (e.g., bro, cathepsin, chitinase, iap-1, iap-2, and thymidylate kinase). The thymidylate kinase (tmk) gene was present fused to a dUTPase (dut) gene in other baculovirus genomes. DijuNPV likely lost the dut portion together with the iap-3 homolog. Overall, the genome sequencing of novel alphabaculoviruses enables a wide understanding of baculovirus evolution.

Efficacy of an alphabaculovirus-based biological insecticide for control of Chrysodeixis chalcites (Lepidoptera: Noctuidae) on tomato and banana crops

BACKGROUND

Chrysodeixis chalcites (Esper) is a major pest of tomato in Mediterranean countries and attacks banana in the Canary Islands (Spain). The efficacy of Chrysodeixis chalcites single nucleopolyhedrovirus (ChchSNPV-TF1) was evaluated in plant growth chambers and greenhouse trials performed on tomato and banana plants respectively. Treatments were applied using a compressed air sprayer.

RESULTS

Mean (± SE) lethal infection varied from 77 ± 10% to 94 ± 3% in second-instar larvae fed for 2 days on tomato plants treated with 2 × 10(6) to 5 × 10(7) virus occlusion bodies (OBs) L(-1) , increasing to ∼100% infection after 7 days. Mortality of larvae collected from banana at different intervals post-application varied from 54 ± 10% to 96 ± 4% in treatments involving 1 × 10(8) -1 × 10(9) OBs L(-1) , whereas indoxacarb (Steward 30% WG) and Bacillus thuringiensis var. kurstaki (Biobit 16% WP) treatments produced between 22 ± 6% and 32 ± 5% pest mortality. All treatments significantly reduced plant defoliation compared with untreated controls. Application of 1 × 10(9) OBs L(-1) was 3-4-fold more effective than chemical or B. thuringiensis treatments. Larvae acquired lethal infection more rapidly when feeding on tomato than banana plants, but this difference disappeared following >60 min of feeding.

CONCLUSION

This information should prove useful in the registration of ChchSNPV-TF1 as a bioinsecticide in the Canary Islands and Europe.

The role of environmental, virological and vector interactions in dictating biological transmission of arthropod-borne viruses by mosquitoes

Arthropod-borne viruses (arboviruses) are transmitted between vertebrate hosts and arthropod vectors. An inherently complex interaction among virus, vector, and the environment determines successful transmission of the virus. Once believed to be "flying syringes," recent advances in the field have demonstrated that mosquito genetics, microbiota, salivary components, and mosquito innate immune responses all play important roles in modulating arbovirus transmissibility. The literature on the interaction among virus, mosquito, and environment has expanded dramatically in the preceding decade and the utilization of next-generation sequencing and transgenic vector methodologies assuredly will increase the pace of knowledge acquisition in this field. This chapter outlines the interplay among the three factors in both direct physical and biochemical manners as well as indirectly through superinfection barriers and altered induction of innate immune responses in mosquito vectors. The culmination of the aforementioned interactions and the arms race between the mosquito innate immune response and the capacity of arboviruses to antagonize such a response ultimately results in the subjugation of mosquito cells for viral replication and subsequent transmission.

The relationship between parasite fitness and host condition in an insect--virus system

Research in host-parasite evolutionary ecology has demonstrated that environmental variation plays a large role in mediating the outcome of parasite infection. For example, crowding or low food availability can reduce host condition and make them more vulnerable to parasite infection. This observation that poor-condition hosts often suffer more from parasite infection compared to healthy hosts has led to the assumption that parasite productivity is higher in poor-condition hosts. However, the ubiquity of this negative relationship between host condition and parasite fitness is unknown. Moreover, examining the effect of environmental variation on parasite fitness has been largely overlooked in the host-parasite literature. Here we investigate the relationship between parasite fitness and host condition by using a laboratory experiment with the cabbage looper Trichoplusia ni and its viral pathogen, AcMNPV, and by surveying published host-parasite literature. Our experiments demonstrated that virus productivity was positively correlated with host food availability and the literature survey revealed both positive and negative relationships between host condition and parasite fitness. Together these data demonstrate that contrary to previous assumptions, parasite fitness can be positively or negatively correlated with host fitness. We discuss the significance of these findings for host-parasite population biology.

Gut Transcription in Helicoverpa zea is Dynamically Altered in Response to Baculovirus Infection

The Helicoverpa zea transcriptome was analyzed 24 h after H. zea larvae fed on artificial diet laced with Helicoverpa zea single nucleopolyhedrovirus (HzSNPV). Significant differential regulation of 1,139 putative genes (p < 0.05 T-test with Benjamini and Hochberg False Discovery Rate) was detected in the gut epithelial tissue; where 63% of these genes were down-regulated and 37% of genes were up-regulated compared to the mock-infected control. Genes that play important roles in digestive physiology were noted as being generally down-regulated. Among these were aminopeptidases, trypsin-like serine proteases, lipases, esterases and serine proteases. Genes related to the immune response reacted in a complex nature having peptidoglycan binding and viral antigen recognition proteins and antiviral pathway systems down-regulated, whereas antimicrobial peptides and prophenoloxidase were up-regulated. In general, detoxification genes, specifically cytochrome P450 and glutathione S-transferase were down-regulated as a result of infection. This report offers the first comparative transcriptomic study of H. zea compared to HzSNPV infected H. zea and provides further groundwork that will lead to a larger understanding of transcriptional perturbations associated with viral infection and the host response to the viral insult in what is likely the most heavily infected tissue in the insect.

Toward system-level understanding of baculovirus-host cell interactions: from molecular fundamental studies to large-scale proteomics approaches

Baculoviruses are insect viruses extensively exploited as eukaryotic protein expression vectors. Molecular biology studies have provided exciting discoveries on virus-host interactions, but the application of omic high-throughput techniques on the baculovirus-insect cell system has been hampered by the lack of host genome sequencing. While a broader, systems-level analysis of biological responses to infection is urgently needed, recent advances on proteomic studies have yielded new insights on the impact of infection on the host cell. These works are reviewed and critically assessed in the light of current biological knowledge of the molecular biology of baculoviruses and insect cells.

The role of peritrophic membrane in the resistance of Anticarsia gemmatalis larvae (Lepidoptera: Noctuidae) during the infection by its nucleopolyhedrovirus (AgMNPV)

The aim of this study was to analyze morphologically the peritrophic membrane (PM) of Anticarsia gemmatalis larvae resistant (RL) and non-resistant (susceptible) (SL) to the A. gemmatalis multicapsid nucleopolyhedrovirus (AgMNPV), in the presence of viral infection. Also, in this investigation the results between SL and RL were compared to improve the understanding of the resistance mechanisms to the virus. The PM of SL of A. gemmatalis was less efficient as a barrier against the viral infection since it was found to be more fragile than the PM of RL. The lower chitin content as seen from weaker fluorescent staining in SL as well as the abundance of non-solubilized vesicular materials in the ectoperitrophic space, would cause the malformation of this membrane, facilitating the passage of the virus toward the epithelium of the midgut. On the other hand, in RL, the intensity of WGA (wheat germ agglutinin)-conjugated FITC (fluorescein) reaction of the PM was greater than in SL, making this insect more resistant to infection. We can conclude that the effectiveness of the PM in protecting against pathogens is dependent on the integrity of the epithelial cells of the midgut and of the structural preservation of the PM, being directly implicated in the resistance of A. gemmatalis larvae to AgMNPV.

Comparative transient expression assay analysis of hycu-hr6- and IE1-dependent regulation of baculovirus gp64 early promoters in three insect cell lines

We previously demonstrated that the Hyphantria cunea multicapsid nucleopolyhedrovirus (HycuMNPV) gp64 gene (hycu-gp64) is uniquely localized on the viral genome with a large homologous region of 1582bp, hycu-hr6, immediately upstream of the hycu-gp64 gene. In the present study, we compared the regulation of gp64 early promoters from HycuMNPV, Autographa californica multicapsid NPV (AcMNPV) and Bombyx mori NPV (BmNPV) by cis-acting hycu-hr6 and trans-acting IE1s in three cell lines (Spodoptera frugiperda Sf9, Bombyx mori BM-N and Spilosoma imparilis SpIm). A transient expression assay with plasmids harboring a reporter luciferase gene demonstrated that the gp64 early promoters are positively regulated by hycu-hr6, independent of virus and cell types. In contrast, gp64 early promoters were regulated positively or negatively by trans-acting IE1s, in a cell- and virus-type dependent manner, indicating that cellular factors, as well as viral factors, are responsible for IE1-dependent regulation of gp64 early promoters. However, hycu-gp64 early promoter activity was consistently suppressed by HycuMNPV IE1 (Hycu-IE1), irrespective of the cell lines used. Analysis of the hycu-gp64 early promoter region revealed two novel sequence elements that were involved in Hycu-IE1-dependent negative regulation of the hycu-gp64 early promoter. These two novel regulatory sequence elements could compensate for each other but could not be substituted with AcMNPV IE1 binding motif (Ac-IBM). These results suggest that IE1 regulates gp64 early promoters to produce the proper amount of GP64 protein, depending upon NPV-insect cell systems.

A single amino acid substitution modulates low-pH-triggered membrane fusion of GP64 protein in Autographa californica and Bombyx mori nucleopolyhedroviruses

We have previously shown that budded viruses of Bombyx mori nucleopolyhedrovirus (BmNPV) enter the cell cytoplasm but do not migrate into the nuclei of non-permissive Sf9 cells that support a high titer of Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV) multiplication. Here we show, using the syncytium formation assay, that low-pH-triggered membrane fusion of BmNPV GP64 protein (Bm-GP64) is significantly lower than that of AcMNPV GP64 protein (Ac-GP64). Mutational analyses of GP64 proteins revealed that a single amino acid substitution between Ac-GP64 H155 and Bm-GP64 Y153 can have significant positive or negative effects on membrane fusion activity. Studies using bacmid-based GP64 recombinant AcMNPV harboring point-mutated ac-gp64 and bm-gp64 genes showed that Ac-GP64 H155Y and Bm-GP64 Y153H substitutions decreased and increased, respectively, the multiplication and cell-to-cell spread of progeny viruses. These results indicate that Ac-GP64 H155 facilitates the low-pH-triggered membrane fusion reaction between virus envelopes and endosomal membranes.

Changes in gene expression in the permissive larval host lightbrown apple moth (Epiphyas postvittana, Tortricidae) in response to EppoNPV (Baculoviridae) infection

Host cell and virus gene expression were measured five days after per os inoculation of 3rd instar lightbrown apple moth (LBAM) larvae with the Epiphyas postvittana nucleopolyhedrovirus (EppoNPV). Microarray analysis identified 84 insect genes that were up-regulated and 18 genes that were down-regulated in virus-infected larvae compared with uninfected larvae. From the 134 viral open reading frames represented on the microarray, 81 genes showed strong expression. Of the 38 functionally identifiable regulated insect genes, 23 coded for proteins that have roles in one of five processes; regulation of transcription and translation, induction of apoptosis, and maintenance of both juvenility and actin cytoskeletal integrity. Of the 34 functionally identifiable viral genes that were most strongly expressed, 12 had functions associated with these five processes, as did a further seven viral genes which were expressed at slightly lower levels. A survey of the LBAM-expressed sequence tag library identified further genes involved in these processes. In total, 135 insect genes and 38 viral genes were analysed by quantitative polymerase chain reaction. Twenty-one insect genes were strongly up-regulated and 31 genes strongly down-regulated. All 38 viral genes examined were highly expressed. These data suggest that induction of apoptosis and regulation of juvenility are the major 'battlegrounds' between virus and insect, with the majority of changes observed representing viral control of insect gene expression. Transcription and translational effects seem to be exerted largely through modulation of mRNA and protein degradation. Examples of attempts by the insect to repel the infection via changes in gene expression within these same processes were, however, also noted. The data also showed the extent to which viral transcription dominated in the infected insects at five days post inoculation.

Transgenesis approaches for functional analysis of peptidergic cells in the silkworm Bombyx mori

The domestic silkworm, Bombyx mori represents an insect model of great scientific and economic importance. Besides the establishment of a stable germline transformation using the PiggyBac vector, technically feasible methods for in vivo gene delivery and transient gene expression were developed using viral based vectors, especially Sindbis viruses and baculoviruses. The recombinant baculovirus, Autographa californica multiple nucleopolyhedrovirus (AcMNPV), commonly used for large-scale protein production in permissive cell lines or insects, has been used for foreign gene transfer into specific peptidergic cells of B. mori in vivo. Since targeted gene expression is essential for functional analysis of neuropeptide genes and their receptors, the baculovirus-mediated gene transfer can serve as a reliable approach in reverse genetic studies in the silkworm. We review various strategies employing the baculovirus vector system for transient expression of molecular markers and transcription factors in specific peptidergic cells to investigate their roles in B. mori. We also use this system for functional analysis of neuropeptide signaling in the ecdysis behavioral sequence. Our data indicate that the AcMNPV vector is suitable for efficient delivery of foreign genes and their expression directed into specific peptidergic neurons and endocrine cells of B. mori larvae and pupae. However, some modifications of the vector and steps for optimization are necessary to minimize negative effects of viral infection on the host development. The transient gene expression using the AcMNPV and other virus vectors are promising tools for analysis of molecular mechanisms underlying various neuroendocrine processes during development of B. mori.

Helicoverpa armigera (Lepidoptera: Noctuidae) larvae that survive sublethal doses of nucleopolyhedrovirus exhibit high metabolic rates

To determine the effect of sublethal doses of Helicoverpa armigera single nucleocapsid nucleopolyhedrovirus (HearSNPV) on the metabolic rate of H. armigera, the respiration rates of third instar H. armigera larvae inoculated with sublethal doses of HearSNPV were evaluated. Respiration rates, measured as the rate of CO(2) production (VCO(2)), were recorded daily using closed-system respirometry. By 4 days post-inoculation (dpi), the metabolic rates of LD(25) or LD(75) survivors were significantly higher than that of uninoculated controls. When dose data were pooled, the VCO(2) values of larvae that survived inoculation (0.0288mlh(-1)), the uninoculated controls (0.0250mlh(-1)), and the larvae that did not survive inoculation (0.0199mlh(-1)) differed significantly from one another. At 4dpi, the VCO(2) of the uninoculated controls were significantly lower than the VCO(2) of inoculation survivors, but significantly higher than the VCO(2) of inoculation non-survivors. Inoculation survivors may have had high metabolic rates due to a combination of viral replication, organ damage, and an energy-intensive induced cellular immune response. The high 4dpi metabolic rate of inoculation survivors may reflect an effective immune response and may be seen as the metabolic signature of larvae that are in the process of surviving inoculation with HearSNPV.

Mixtures of complete and pif1- and pif2-deficient genotypes are required for increased potency of an insect nucleopolyhedrovirus

The insecticidal potency of a nucleopolyhedrovirus population (SfNIC) that infects Spodoptera frugiperda (Lepidoptera) is greater than the potency of any of the component genotypes alone. Occlusion bodies (OBs) produced in mixed infections comprising the complete genotype and a deletion genotype are as pathogenic as the natural population of genotypes from the field. To test whether this increased potency was due to the deletion or to some other characteristic of the deletion variant genome, we used the SfNIC-B genome to construct a recombinant virus (SfNIC-B Delta 16K) with the same 16.4-kb deletion as that observed in SfNIC-C and another recombinant (SfNIC-B Delta pifs) with a deletion encompassing two adjacent genes (pif1 and pif2) that are essential for transmission per os. Mixtures comprising SfNIC-B and SfNIC-B Delta 16K in OB ratios that varied between 10:90 and 90:10 were injected into insects, and the progeny OBs were fed to larvae in an insecticidal potency assay. A densitometric analysis of PCR products indicated that SfNIC-B was generally more abundant than expected in mixtures based on the proportions of OBs used to produce the inocula. Mixtures derived from OB ratios of 10, 25, or 50% of SfNIC-B Delta 16K and the corresponding SfNIC-B proportions showed a significant increase in potency compared to SfNIC-B alone. The results of potency assays with mixtures comprising various proportions of SfNIC-B plus SfNIC-B Delta pifs were almost identical to the results observed with SfNIC-B Delta 16K, indicating that deletion of the pif gene region was responsible for the increased potency observed in mixtures of SfNIC-B and each deletion recombinant virus. Subsequently, mixtures produced from OB ratios involving 10 or 90% of SfNIC-B Delta 16K with the corresponding proportions of SfNIC-B were subjected to four rounds of per os transmission in larvae. The composition of each experimental mixture rapidly converged to a common equilibrium with a genotypic composition of approximately 85% SfNIC-B plus approximately 15% SfNIC-B Delta 16K. Nearly identical results were observed in peroral-passage experiments involving mixtures of SfNIC-B plus SfNIC-B Delta pifs. We conclude that (i) the deletion of the pif1 and pif2 region is necessary and sufficient to explain the increased potency observed in mixtures of complete and deletion genotypes and (ii) viral populations with decreased ratios of pif1- and pif2-deficient genotypes in the virus population increase the potency of genotypic mixtures and are likely to positively influence the transmission of this pathogen.

Can the protein costs of bacterial resistance be offset by altered feeding behaviour?

1. Mounting an immune response is likely to be costly in terms of energy and nutrients, and so it is predicted that dietary intake should change in response to infection to offset these costs. The present study focuses on the interactions between a specialist grass-feeding caterpillar species, the African armyworm Spodoptera exempta, and an opportunist bacterium, Bacillus subtilis. 2. The main aims of the study were (i) to establish the macronutrient costs to the insect host of surviving a systemic bacterial infection, (ii) to determine the relative importance of dietary protein and carbohydrate to immune system functions, and (iii) to determine whether there is an adaptive change in the host's normal feeding behaviour in response to bacterial challenge, such that the nutritional costs of resisting infection are offset. 3. We show that the survival of bacterially infected larvae increased with increasing dietary protein-to-carbohydrate (P:C) ratio, suggesting a protein cost associated with bacterial resistance. As dietary protein levels increased, there was an increase in antibacterial activity, phenoloxidase (PO) activity and protein levels in the haemolymph, providing a potential source for this protein cost. However, there was also evidence for a physiological trade-off between antibacterial activity and phenoloxidase activity, as larvae whose antibacterial activity levels were elevated in response to immune activation had reduced PO activity. 4. When given a choice between two diets varying in their P:C ratios, larvae injected with a sub-lethal dose of bacteria increased their protein intake relative to control larvae whilst maintaining similar carbohydrate intake levels. These results are consistent with the notion that S. exempta larvae alter their feeding behaviour in response to bacterial infection in a manner that is likely to enhance the levels of protein available for producing the immune system components and other factors required to resist bacterial infections ('self-medication').

Plant-mediated alteration of the peritrophic matrix and baculovirus infection in lepidopteran larvae

The peritrophic matrix (PM) lines the midgut of most insects, providing protection to the midgut epithelial cells while permitting passage of nutrients and water. Herein, we provide evidence that plant-mediated alteration of the PM contributes to the well-documented inhibition of fatal infection by Autographa californica multiple nucleopolyhedrovirus (AcMNPV) of Heliothis virescens F. larvae fed cotton foliage. We examined the impact of the PM on pathogenesis using a viral construct expressing a reporter gene (AcMNPV-hsp70/lacZ) orally inoculated into larvae with either intact PMs or PMs disrupted by Trichoplusia ni granulovirus occlusion bodies containing enhancin, known to degrade insect intestinal mucin. Larvae possessing disrupted PMs displayed infection foci (lacZ signaling) earlier than those with intact PMs. We then examined PMs from larvae fed artificial diet or plant foliage using electron microscopy; foliage-fed larvae had significantly thicker PMs than diet-fed larvae. Moreover, mean PM width was inversely related to both the proportion of larvae with lacZ signaling at 18h post-inoculation and the final percentage mortality from virus. Thus, feeding on foliage altered PM structure, and these foliage-mediated changes reduced baculoviral efficacy. These data indicate that the PM is an important factor determining the success of an ingested pathogen in foliage-fed lepidopteran larvae.

Impact of a Turkish isolate and a plaque-purified variant of SpliNPV-B on larval stage development of Spodoptera littoralis (Lepidoptera: Noctuidae) Boisd

Baculoviruses can alter the development and physiological status of their insect hosts. In the present study, two Spodoptera littoralis nucleopolyhedroviruses (SpliNPV-TR1 and SpliNPV-M2) were examined in terms of their effect on the stage development of S. littoralis at two doses/concentrations by inoculation of neonates or third instars. Both isolates had an acute pathological effect on neonates. However, larval development was prolonged (6-36 h) by either infection when neonates were infected. When third-instar larvae were inoculated, no mortality occurred until insects moulted to later stages. When the isolates were compared, more mortality occurred in the fifth instar with SpliNPV-M2 than with SpliNPV-TR1, whereas less mortality occurred in the sixth instar with SpliNPV-M2 than with SpliNPV-TR1. However, none of the isolates demonstrated a dose effect. Larvae died at 108-198 h post-inoculation at all infections, and none of them pupated. Larval development was retarded (0-126 h) for each stage when third-instar larvae were inoculated. Consequently, SpliNPV-M2 would be a more effective control agent than SpliNPV-TR1 for control of S. littoralis since it kills larvae at earlier developmental stages.

Impact of a basement membrane-degrading protease on dissemination and secondary infection of Autographa californica multiple nucleopolyhedrovirus in Heliothis virescens (Fabricus)

ScathL is a cathepsin L-like cysteine protease from the flesh fly, Sarcophaga peregrina, that digests components of the basement membrane (BM) during insect metamorphosis. A recombinant baculovirus that expresses ScathL (AcMLF9.ScathL) kills larvae of the tobacco budworm, Heliothis virescens, significantly faster than the wild-type virus and triggers melanization and tissue fragmentation in infected larvae shortly before death. As BMs are a potential barrier to the spread of baculovirus secondary infection to other tissues in the host, this study tested the hypothesis that the rapid death of insects infected with AcMLF9.ScathL was caused by accelerated secondary infection resulting from the degradation of host BMs by ScathL. Viruses expressing catalytically active or inactive ScathL were used to examine the effects of ScathL activity on budded virus release into the haemocoel during infection, the production of polyhedra in infected larvae and the rate of infection of the gut, trachea, haemocytes, fat body and Malpighian tubules. It was concluded that the enhanced insecticidal efficacy of the recombinant baculovirus that expresses ScathL does not result from altered tissue tropism or accelerated systemic infection. Implications for the role of the BM as a barrier to baculovirus dissemination within the host insect are discussed.

Virus-derived genes for insect-resistant transgenic plants

Insect viruses have evolved to counter physiological barriers to infection presented by the host insect. For the Lepidoptera (butterflies and moths), these barriers include (1) the peritrophic membrane (PM) lining the gut, which presents a physical barrier to virus infection of the midgut epithelial cells, (2) the basement membrane (BM) that overlies the gut thereby restricting secondary infection of other tissues, and (3) the immune system of the host insect. Hence, insect viruses provide a resource for genes that disrupt host physiology in a specific manner, and these genes in turn serve as a resource both for the study of physiological processes, and for disruption of these processes for pest management purposes. There are several examples of the application of genes used by an insect virus to overcome the PM barrier for production of insect-resistant transgenic plants. There are other examples of intrahemocoelic effectors, such as BM-degrading proteases that can only be used with an appropriate system for delivery of the agent from the gut into the hemocoel (body cavity) of the insect pest. In this chapter, we describe (1) baculovirus- and entomopoxvirus-derived genes that alter the physiology of the host insect, (2) use of these and homologous genes for production of insect-resistant transgenic plants, (3) other viral genes that have potential for use in development of insect-resistant transgenic plants, and (4) the use of plant lectins for delivery of intrahemocoelic toxins from transgenic plants. Plant expression of polydnavirus-derived genes is described by Gill et al. (this volume, pp. 393-426).

Dynamics of deletion genotypes in an experimental insect virus population

Defective viruses, that are deficient in certain essential genes, are maintained in the population by trans-complementation, exploiting the gene products of complete genotypes in co-infected cells. This process becomes prevalent only when cells are frequently infected by several virus particles, and only then will the fitness of defective viruses be subjected to frequency-dependent selection. Deletion variants that are not infectious per os are present in a multicapsid nucleopolyhedrovirus (SfMNPV, Baculoviridae) that infects the fall army worm, Spodoptera frugiperda. These variants enhance the pathogenicity and, therefore, the likelihood of transmission of the virus when co-infecting cells with complete genotypes, resulting in occlusion bodies (OBs) that may contain both genotypes co-occluded. Mixtures of complete (B) and defective (C) variants in ratios of 90% B+10% C, 50% B+50% C and 10% B+90% C were used to inoculate by injection S. frugiperda larvae. Viral OBs extracted from diseased insects were subjected to four or five successive rounds of per os infection. Following successive passages, genotype frequencies in all three experimental populations converged to a single equilibrium frequency comprising approximately 20% of deletion genotype C and approximately 80% of complete genotype B. This mirrors the relative proportions of deletion (22%) and complete (78%) genotypes observed in the wild-type SfMNPV population. The pathogenicity of experimental populations at the final passage was not significantly different from that of the wild-type isolate. In contrast, OBs of all genotype mixtures were significantly more pathogenic than OBs of genotype B alone. A population genetics model, in which virus populations were assigned linear frequency-dependent transmissibility values, was in remarkably close agreement to empirical data. Clearly, non-infectious deletion variants can profoundly affect the likelihood of transmission and the genetic structure and stability of virus populations.

Pathogenesis of Rift Valley fever virus in mosquitoes--tracheal conduits & the basal lamina as an extra-cellular barrier

Knowledge of the fate of an arbovirus in a mosquito is fundamental to understanding the mosquito's competence to transmit the virus. When a competent mosquito ingests viremic vertebrate blood, virus infects midgut epithelial cells and replicates, then disseminates to other tissues, including salivary glands and/or ovaries. The virus is then transmitted to the next vertebrate host horizontally via bite and/or vertically to the mosquito's offspring. Not all mosquitoes that ingest virus become infected or, if infected, transmit virus. Several "barriers" to arbovirus passage, and ultimately transmission, have been identified in incompetent or partially competent mosquitoes, including, among others, gut escape barriers and salivary gland infection barriers. The extra-cellular basal lamina around the midgut epithelium and the basal lamina that surrounds the salivary glands may act as such barriers. Midgut basal lamina pore sizes are significantly smaller than arboviruses and ultrastructural evidence suggests that midgut tracheae and tracheoles may provide a means for viruses to circumvent this barrier. Further, immunocytochemical evidence indicates the existence of a salivary gland infection barrier in Anopheles stephensi. The basal lamina may prevent access to mosquito cell surface virus receptors and help explain why anopheline mosquitoes are relatively incompetent arbovirus transmitters when compared to culicines.

Abortive replication of Bombyx mori nucleopolyhedrovirus in Sf9 and High Five cells: defective nuclear transport of the virions

Despite close genetic relationship, Bombyx mori nucleopolyhedrovirus (BmNPV) and Autographa californica multicapsid NPV (AcMNPV) display a distinct host range property. Here, BmNPV replication was examined in Sf9 and High Five cells that were nonproductive for BmNPV infection but supported high titers of AcMNPV replication. Recombinant BmNPV, vBm/gfp/lac, containing bm-ie1 promoter-driven egfp showed that few Sf9 and High Five cells infected with vBm/gfp/lac expressed EGFP, while large proportion of EGFP-expressing cells was observed when transfected with vBm/gfp/lac DNA. Immunocytochemical analysis showed that BmNPV was not imported into the nucleus of these two cell lines, while recombinant BmNPV, vBmDelta64/ac-gp64 possessing AcMNPV gp64 was imported into the nucleus, yielding progeny virions in High Five cells, but not Sf9 cells. These results indicate that the defective nuclear import of infected virions due to insufficient BmNPV GP64 function is involved in the restricted BmNPV replication in Sf9 and High Five cells.

Small RNA Viruses of Insects: Expression in Plants and RNA Silencing

Interest in insect small RNA viruses (SRVs) has grown slowly but steadily. A number of new viruses have been analyzed at the sequence level, adding to our knowledge of their diversity at the level of both individual virus species and families. In particular, a number of possible new virus families have emerged. This research has largely been driven by interest in their potential for pest control, as well as in their importance as the causal agents of disease in beneficial arthropods. At the same time, research into known viruses has made valuable contributions to our understanding of an emerging new field of central importance to molecular biology-the existence of RNA-based gene silencing, developmental control, and adaptive immune systems in eukaryotes. Subject to RNA-based adaptive immune responses in their hosts, viruses have evolved a variety of genes encoding proteins capable of suppressing the immune response. Such genes were first identified in plant viruses, but the first examples known from animal viruses were identified in insect RNA viruses. This chapter will address the diversity of insect SRVs, and attempts to harness their simplicity in the engineering of transgenic plants expressing viruses for resistance to insect pests. We also describe RNA interference and antiviral pathways identified in plants and animals, how they have led viruses to evolve genes capable of suppressing such adaptive immunity, and the problems presented by these pathways for the strategy of expressing viruses in transgenic plants. Approaches for countering these problems are also discussed.

Functional importance of deletion mutant genotypes in an insect nucleopolyhedrovirus population

A Nicaraguan isolate of a nucleopolyhedrovirus (SfNIC) that attacks the fall armyworm, Spodoptera frugiperda, survives as a mixture of nine genotypes (SfNIC A to I) that all present genomic deletions, except variant B (complete genotype). Sequencing of cloned restriction fragments revealed that genotypic variants lack between 5 and 16 of the open reading frames present in a contiguous sequence of 18 kb of the SfNIC genome. The absence of oral infectivity of SfNIC-C and -D variants is related to the deletion of the pif and/or pif-2 gene, while that of SfNIC-G remains unexplained. The presence of open reading frame 10, homolog of Se030, also appeared to influence pathogenicity in certain variants. Previous studies demonstrated a significant positive interaction between genotypes B and C. We compared the median lethal concentration of single genotypes (A, B, C, D, and F) and co-occluded genotype mixtures (B+A, B+D, B+F, A+C, and F+C in a 3:1 ratio). Mixtures B+A and B+D showed increased pathogenicity, although only B+D restored the activity of the mixture to that of the natural population. Mixtures of two deletion variants (A+C and F+C) did not show interactions in pathogenicity. We conclude that minority genotypes have an important influence on the overall pathogenicity of the population. These results clearly demonstrate the value of retaining genotypic diversity in virus-based bioinsecticides.

The role of apoptosis in defense against baculovirus infection in insects

The baculoviruses make up a large, diverse family of DNA viruses that have evolved a number of fascinating mechanisms to manipulate their insect hosts. One of these is the ability to regulate apoptosis during infection by expressing proteins that can inhibit caspase activation and/or activity, including the caspase inhibitor P35 and its relatives, and the inhibitor of apoptosis (IAP) proteins. Experimental manipulations of the expression of these antiapoptotic genes, either by genetic deletions or by RNAi, have shed light on the effectiveness of apoptosis in combating baculovirus infection. The results of these experiments indicate that apoptosis can be an extremely powerful response to baculovirus infection, reducing viral replication, infectivity, and the ability of the virus to spread within the insect host even if a successful infection is established. Apoptosis is especially effective when it is combined with other innate antiviral defenses, which are largely unexplored in insects to date.

Should Models of Disease Dynamics in Herbivorous Insects Include the Effects of Variability in Host‐Plant Foliage Quality?

Interactions between insects and their baculovirus pathogens are often described using simple disease models. Baculoviruses, however, are transmitted when insects consume virus-contaminated foliage, and foliage variability, whether within or between host-plant species, can affect viral infectiousness. Insect-baculovirus interactions may thus be embedded in a tritrophic interaction with the insect's host plant, but disease models include only the host and the pathogen. We tested these models by measuring the transmission of a baculovirus of gypsy moths (Lymantria dispar) on red oak (Quercus rubra) and white oak (Quercus alba) in the field in six experiments over four years. In all experiments, there were only weak effects of host-tree species, and in only one did the best-fitting model include tree species effects. These weak effects of foliage variability on transmission were not due to a lack of foliage variability on viral infectiousness, because when larvae were force-fed virus-contaminated foliage, infection rates were higher on white oak. Our results suggest that feeding behavior plays an important role in baculovirus transmission and that models can usefully describe baculovirus dynamics even without including foliage variability. Our work provides a clear example of how two-species models are sometimes sufficient to describe what appear to be tritrophic interactions.

Expression and subcellular targeting of canine parvovirus capsid proteins in baculovirus-transduced NLFK cells

A mammalian baculovirus delivery system was developed to study targeting in Norden Laboratories feline kidney (NLFK) cells of the capsid proteins of canine parvovirus (CPV), VP1 and VP2, or corresponding counterparts fused to EGFP. VP1 and VP2, when expressed alone, both had equal nuclear and cytoplasmic distribution. However, assembled form of VP2 had a predominantly cytoplasmic localization. When VP1 and VP2 were simultaneously present in cells, their nuclear localization increased. Thus, confocal immunofluorescence analysis of cells transduced with the different baculovirus constructs or combinations thereof in the absence or presence of infecting CPV revealed that the VP1 protein is a prerequisite for efficient targeting of VP2 to the nucleus. The baculovirus vectors were functional and the genes of interest efficiently introduced to this CPV susceptible mammalian cell line. Thus, we show evidence that the system could be utilized to study targeting of the CPV capsid proteins.

The deletion of the pif gene improves the biosafety of the baculovirus-based technologies

Our goal was to improve the biosafety of baculovirus-based technologies by deleting the pif (per os infectivity factor) gene from baculovirus expression vectors. Such a deletion would block transmission in nature without disturbing protein production. A pif deletion mutant of Autographa californica multiplecapsid nucleopolyhedrovirus (AcMNPV) was constructed and its infectivity to two host species was tested by oral or intrahemocoelic inoculation. Virus replication after oral inoculation was monitored using PCR. Oral inoculations with a mixture of the wild type and the pif deletion viruses were carried out. The pif deletion blocked oral infection but it did not hamper infectivity in cell culture. The blockage took place early after inoculation and could not be overcome by mixed inoculations with the wild type. The cat gene was inserted under the control of the polyhedrin promoter in the deletion mutant and the wild type CAT yield was measured in Spodoptera frugiperda insect cells (Sf9) infected with either recombinant. The pif deletion did not hamper CAT production. This deletion significantly improved CAT yields early in the infection. Hence, expression vectors lacking pif may produce higher quality protein. The pif deletion is a simple measure that dramatically reduces the chances of virus spread or gene transfer in nature.

Systemic and in vitro infection process of Bombyx mori nucleopolyhedrovirus

To analyse the systemic progression of infection by Bombyx mori nucleopolyhedrovirus (BmNPV) through oral ingestion by the silkworm larvae, a recombinant virus (vBmp10GFP) expressing the green fluorescent protein (GFP) under the control of the very late, viral p10 promoter (which still forms the polyhedral occlusion bodies) was constructed. Infection of B. mori derived BmN cells with the recombinant virus resulted in the expression of GFP from 12 h post infection (hpi), with maximal accumulation of the expressed protein by 60 hpi. B. mori larvae that ingested the polyhedra containing vBmp10GFP showed localized expression of GFP in the midgut epithelial cells within 24 hpi, indicating virus replication. The primary spread of the virus infection occurred through the tracheae. Viral multiplication was subsequently detected in nearly all the larval tissues including the neurons and regions of silk-glands that were in contact with the tracheae. Infection in fat bodies was widespread by 48 hpi, by which time the haemocytes also showed infection. In vitro infection of isolated organs/tissues from B. mori with the budded virions (BV) of vBmp10GFP also showed viral multiplication in the cells that were associated with the tracheae, confirming the role of tracheae in spreading the infection.

The larval midgut of Anticarsia gemmatalis (Hübner) (Lepidoptera: Noctuidae): light and electron microscopy studies of the epithelial cells

The morphology of the midgut epithelium cells of Anticarsia gemmatalis (Hübner) larvae is described by light and transmission electron microscopy. The midgut of A. gemmatalis is the largest portion of the digestive tract, with three distinct regions: proximal, media and distal. Its wall is formed by pseudostratified columnar epithelial tissue having four cell types: columnar, goblet, regenerative, and endocrine cells. The columnar cells are numerous and long, with the apical portion showing many lengthy microvilli and the basal portion invaginations forming a basal labyrinth. The goblet cells have a large goblet-shaped central cavity delimited by cytoplasmic projections filled with mitochondria. The regenerative cells present electron-dense cytoplasm and few organelles. The endocrine cells are characterized by electron-dense secretory granules, usually concentrated in the cytoplasm basal region.

Autographa californica M nucleopolyhedrovirus early GP64 synthesis mitigates developmental resistance in orally infected noctuid hosts

The unusual early synthesis of the Autographa californica M nucleopolyhedrovirus (AcMNPV) budded virus (BV) structural protein GP64 is an important virulence factor during oral infection of Heliothis virescens larvae. Considering the breadth of the AcMNPV host range, the importance of early GP64 synthesis in orally infected permissive hosts (Trichoplusia ni and Spodoptera exigua) from subfamilies other than that of H. virescens was assessed. An AcMNPV recombinant, having wild-type early and late GP64 synthesis, was compared with one in which only late GP64 synthesis occurred. Early GP64 synthesis was found to have more of an effect on virulence in orally inoculated T. ni than S. exigua and that virulence was dependent on two factors: the ability of the host to slough occlusion-derived virus (ODV)-infected midgut cells and the rapidity with which BV was transmitted to the tracheal cells. In both host species, insects inoculated orally with the control virus transmitted BV to tracheal cells hours before those inoculated with the gp64 temporal mutant. Moreover, with early GP64 synthesis, the lag between the onset of viral gene expression in midgut and tracheal cells was only 3-4 h, supporting the conclusion that in these insects, the first systemic infections arose from ODV-derived nucleocapsids repackaged as BV. These results provide further empirical proof that early GP64 synthesis is a component of a unique and selectively advantageous baculovirus infection strategy for exploiting larval lepidopterans by counteracting developmental resistance.

Evidence for arbovirus dissemination conduits from the mosquito (Diptera: Culicidae) midgut

The mechanism by which arboviruses bypass the basal lamina of mosquito midgut cells and enter the body cavity has been unclear. Experiments using Venezuelan equine encephalitis viral replicon particles, which express the green fluorescent protein gene in cells, indicate the operation of tissue conduits, possibly involving tracheae and visceral muscles, that facilitate virus movement through the basal lamina. Ultrastructural studies of the midgut reveal evidence for possible complete penetration of the basal lamina by tracheal cells and regions of modified basal lamina associated with visceral muscle. The modified basal lamina closely resembles proventricular matrix material known to allow virus passage.

Transcription and promoter analysis of pif, an essential but low-expressed baculovirus gene

The pif gene (per os infectivity factor) of Spodoptera littoralis nucleopolyhedrovirus (SpliNPV) encodes a structural protein essential for oral infection. This protein is expressed in very low quantities. In this study, transcription and promoter analysis of SpliNPV pif were carried out to understand more fully the regulation of pif gene expression. Transcription in the pif gene region was examined using RT-PCR, Northern blot, primer extension, ribonuclease protection and 3' RACE. The pif gene was encoded by a late bicistronic messenger, which was characterized. This 1.9 kb messenger was present in very small amounts. In addition, this messenger was part of a set of six late mRNAs overlapping the pif sequence. A functional complementation assay was used to analyse the pif promoter. This assay allowed the detection of amounts of PIF which were sufficient for the production of orally infectious virions. The 13 bp region upstream from the initial ATG of pif was required and sufficient for the production of orally infectious virions. This promoter region was much shorter than the studied baculovirus promoters. A late promoter motif (TTAAG) is situated at the 5' end of this region. This motif was shown to be the promoter core by using single mutations of the motif in the complementation assay. These results suggest that the low expression of the pif gene is regulated chiefly at the transcriptional level.

Defective or effective? Mutualistic interactions between virus genotypes

Defective viruses lack genes essential for survival but they can co-infect with complete virus genotypes and use gene products from the complete genotype for their replication and transmission. As such, they are detrimental to the fitness of complete genotypes. Here, we describe a mutualistic interaction between genotypes of an insect baculovirus (nucleopolyhedrovirus of Spodoptera frugiperda (Lepidoptera)) that increases the pathogenicity of the viral population. Mixtures of a complete genotype able to be transmitted orally and a deletion mutant unable to be transmitted orally resulted in a phenotype of increased pathogenicity. Because the infectiousness of mixed genotype infections was greater than that of single genotype infections, we predict that the transmissibility of mixed genotype occlusion bodies will be greater than that of any of their single genotype components. Such interactions will be subject to frequency-dependent selection and will influence the impact of these viruses on insect population dynamics and their efficacy as biological insecticides.

Early pathogenesis of Autographa californica multiple nucleopolyhedrovirus and Helicoverpa zea single nucleopolyhedrovirus in Heliothis virescens: a comparison of the 'M' and 'S' strategies for establishing fatal infection

Nucleopolyhedroviruses (NPVs) (Baculoviridae) produce fatal infections in larval lepidopteran insects. NPVs are designated SNPVs or MNPVs based on whether the occlusion-derived virus (ODV) that initiates primary midgut infections contains single (S) or multiple (M) nucleocapsids. The principal consequence of this ODV packaging is that primary target cells infected with the M phenotype receive multiple nucleocapsids, whereas those infected by the S phenotype receive only one. To determine the biological significance of this difference in the initial infection strategy, a comparison of the primary and secondary infection patterns of the recombinants Helicoverpa zea SNPV (HzSNPV-hsp70/lacZ) and Autographa californica MNPV (AcMNPV-hsp70/lacZ) in orally inoculated larvae of Heliothis virescens was carried out. At dosages yielding similar final mortalities ( approximately 85 %), primary midgut infections by HzSNPV-hsp70/lacZ (indicated by lacZ expression) were observed 6 h earlier and in greater numbers than those generated by AcMNPV-hsp70/lacZ. Infection of secondary target cells in the tracheal epidermis, however, occurred at the same time and at the same rate for both NPVs. A 2 h delay was observed between the onset of primary and secondary AcMNPV-hsp70/lacZ infection, supporting the hypothesis that early tracheal infections were initiated by ODV nucleocapsids repackaged as budded virus. In contrast, an 8 h delay was observed with HzSNPV-hsp70/lacZ, suggesting that systemic infections were established only after virus replication in primary targets. Significant numbers of both MNPV- and SNPV-infected primary target cells were sloughed from the midgut beginning as early as 16 h post-infection. Midgut cell sloughing may be an important host-mediated selection pressure influencing the evolution of NPV morphology and gene regulation, shaping, in part, baculovirus infection strategies.

Differentially expressed midgut transcripts in Culicoides sonorensis (Diptera: ceratopogonidae) following Orbivirus (reoviridae) oral feeding

Understanding the vector insect's gene expression response to a virus infection may aid design of control measures for arbovirus diseases. Culicoides sonorensis is a vector of several agriculturally important pathogens, such as epizootic haemorrhagic disease virus (EHDV) that causes disease in ruminants. Two approaches, differential display and suppression subtractive hybridization, were used to identify 400+ Culicoides transcripts that were more abundant in midguts 1 day following an oral meal containing EHDV. Of these, quantitative PCR confirmed seven to be more abundant in virus-fed midguts than controls. One such transcript encodes a putative RNA editase, CsRED1, induced by dsRNA. Transcripts encoding putative receptors involved in cell differentiation included CsLAR, a protein tyrosine phosphatase, and CsFZ2, homologous to the wingless receptor in D. melanogaster. Transcripts encoding putative translation machinery components included CseIF3, CseIF5A and CsRPS6. Overall, the cDNA fragments identified in this study increased in the midgut at one day postfeeding; by 2 days postfeeding, increases in transcript levels shifted from the midgut to the remainder of the infected midge.

Lack of involvement of haemocytes in the establishment and spread of infection in Spodoptera frugiperda larvae infected with the baculovirus Autographa californica M nucleopolyhedrovirus by intrahaemocoelic injection

It is thought that insect haemocytes, or blood cells, play an important role in baculovirus pathogenesis by amplifying and helping to spread the infection within the insect. Here, infection is described of the lepidopteran noctuid species Spodoptera frugiperda with the baculovirus Autographa californica M nucleopolyhedrovirus (AcMNPV). Late instar S. frugiperda larvae were infected by intrahaemocoelic injection using a recombinant of AcMNPV expressing the enhanced green fluorescent protein gene to visualize infected cells. Approximately 1000-fold higher doses of injected virus were required to initiate infection in S. frugiperda larvae than in another permissive noctuid species, Trichoplusia ni. Infected S. frugiperda larvae survived twice as long as T. ni larvae and exhibited a slower build-up of virus in the haemolymph. In S. frugiperda, infection of fat body and epithelium was observed prior to significant infection of haemocytes, even though the virus was delivered by intrahaemocoelic injection. Expression of eGFP was first detected 12-18 h post-injection within the fat body and, by 24 h, infection had spread to the tracheal and body wall epithelium. In contrast, only 5% of haemocytes were infected at 24 h and the proportion of infected haemocytes increased slowly to only around 50% at 5 days post-infection, when most larval death occurred. Thus, in S. frugiperda, haemocytes do not appear to have a primary role in AcMNPV pathogenesis. This relative lack of infection of haemocytes may in part explain why S. frugiperda larvae are more resistant to AcMNPV infection than T. ni larvae.

Infection of its lepidopteran host by the Helicoverpa armigera stunt virus (Tetraviridae)

Techniques of microscopy and histopathology were employed to study the positive-sense, single-stranded RNA virus, the Helicoverpa armigera stunt virus (HaSV; omegatetravirus, Tetraviridae) infecting its caterpillar host. Infection of the virus per os during the first three instars of larval development is virulent and leads to rapid stunting and mortality. In contrast, no detectable symptoms occur in later larval development, signifying a high degree of developmental resistance. A quantitative study of cell populations in the host midgut during this time showed that increased cell numbers during development alone could not account for the increase in resistance. HaSV infection was restricted to the midgut and three of its four cell types. In younger larvae, the virus initiated its infection in closely situated foci that appeared to expand to link with others to cover larger areas of the midgut. The midgut cells of the infected larvae responded with an increased rate of sloughing to an extent rendering the midgut incapable of maintenance or recovery of normal function. In contrast, infection of older larvae by HaSV did not lead to overt pathology although foci of HaSV infection were detected in their midguts. However, the foci were more sparsely situated, failed to expand, and eventually disappeared, presumably due to cell sloughing. These observations indicate that cell sloughing is an immune response existing throughout larval development but midguts of older larvae have an additional mechanism to account for the increased resistance. This second mechanism results in midgut cells becoming more refractory to infection and, combined with cell sloughing, allows the midguts of older larvae to recover more readily from HaSV infection. These two mechanisms are similar to those seen with host responses to baculoviruses, which display developmental resistance to a lesser degree against more general infections. HaSV remaining in the midgut appears to amplify the degree of developmental resistance.

Autographa californica M nucleopolyhedrovirus ProV-CATH is activated during infected cell death

V-CATH, a cathepsin L-like cysteine protease encoded by the baculovirus Autographa californica M nucleopolyhedrovirus, has been shown to play an essential role in host liquefaction. Similar to cellular cathepsin L, V-CATH is synthesized as an inactive proenzyme and is activated by cleavage of the propeptide. Previous studies indicated that removal of the propeptide was rapid, occurring as soon as the protein could be detected by Western blot, 22 h postinfection. We found, however, that these results reflected artifactual processing of the proenzyme. When the protease inhibitor E-64 was used to prevent this aberration, we found that proV-CATH accumulated in infected cells and activation did not begin until the onset of cell death, at approximately 80 h postinfection. Western blot analysis of fractions of live and dead cells isolated by fluorescence-activated cell sorting revealed that mature V-CATH was found only in dead cells. The regulation of activation of proV-CATH, therefore, was quite different from that of cellular cathepsins. Acridine orange staining revealed that lysosome integrity was lost in dead cells, an occurrence that could lead to the activation of proV-CATH by lysosomal proteases.

Transcriptional mapping of two genes encoding baculovirus envelope-associated proteins

Genes encoding two representatives of the LD130 family of baculovirus envelope-associated proteins were transcriptionally mapped. These included ld130, which encodes a low pH-induced envelope fusion protein of the Lymantria dispar multinucleocapsid nucleopolyhedrovirus, and op21, which is related to ld130 but is encoded by Orgyia pseudotsugata MNPV and appears to lack an envelope fusion activity. The size and temporal expression of mRNA of both genes were examined by Northern blot analysis of RNA extracted from infected cells at selected timepoints. In addition, 5' rapid amplification of cDNA ends (RACE) in combination with DNA sequence analysis was used to map the start sites of mRNA. Ld130 predominantly utilized its early promoter at 24 h post-infection but by 72 h post-infection ld130 expression was almost exclusively from its late promoter. In contrast, op21 was expressed predominantly from its early promoter throughout the timecourse, even though a consensus late promoter sequence was present within 100 bp of the translation start codon. A significant fraction of late transcripts that mapped to op21 were spliced transcripts originating in the op18 gene region. The 3' termini of the transcripts were also mapped using 3' RACE.

Persistence of an occlusion-negative recombinant nucleopolyhedrovirus in Trichoplusia ni indicates high multiplicity of cellular infection

We use data from the serial passage of co-occluded recombinant Autographa californica nuclear polyhedrosis virus (AcMNPV) to estimate the viral multiplicity of infection of cells within infected insects. Co-occlusion, the incorporation of wild-type and mutant virus genomes in the same occlusion body, has been proposed as a strategy to deliver genetically modified viruses as insecticides in a way that contains their spread in the environment. It may also serve as a means whereby naturally occurring mutant forms of NPVs can be maintained in a stable polymorphism. Here, a recombinant strain of AcMNPV was constructed with a deletion of its polyhedrin gene, rendering it incapable of producing occlusion bodies (i.e., occlusion negative). This was co-occluded with wild-type AcMNPV and used to infect fifth-instar Trichoplusia ni larvae. The fate of both genotypes was monitored over several rounds of insect infection. Levels of the occlusion-negative virus genome declined slowly over successive rounds of infection. We applied these data to a model of NPV population genetics to derive an estimate of 4.3 +/- 0.3 viral genomes per occlusion body-producing cell.