Assessment of the application of baculoviruses for control of Lepidoptera

Genetic structure of a Spodoptera frugiperda nucleopolyhedrovirus population: high prevalence of deletion genotypes

A Nicaraguan field isolate (SfNIC) of Spodoptera frugiperda nucleopolyhedrovirus was purified by plaque assay on Sf9 cells. Nine distinct genotypes, A to I, were identified by their restriction endonuclease profiles. Variant SfNIC-B was selected as the standard because its restriction profile corresponded to that of the wild-type isolate. Physical maps were generated for each of the variants. The differences between variants and the SfNIC-B standard were confined to the region between map units 9 and 32.5. This region included PstI-G, PstI-F, PstI-L, PstI-K and EcoRI-L fragments. Eight genotypes presented a deletion in their genome compared with SfNIC-B. Occlusion body-derived virions of SfNIC-C, -D and -G accounted for 41% of plaque-purified clones. These variants were not infectious per os but retained infectivity by injection into S. frugiperda larvae. Median 50% lethal concentration values for the other cloned genotypes were significantly higher than that of the wild type. The variants also differed in their speed of kill. Noninfectious variants SfNIC-C and -D lacked the pif and pif-2 genes. Infectivity was restored to these variants by plasmid rescue with a plasmid comprising both pif and pif-2. Transcription of an SfNIC-G gene was detected by reverse transcription-PCR in insects, but no fatal disease developed. Transcription was not detected in SfNIC-C or -D-inoculated larvae. We conclude that the SfNIC population presents high levels of genetic diversity, localized to a 17-kb region containing pif and pif-2, and that interactions among complete and deleted genotypic variants will likely influence the capacity of this virus to control insect pests.

Virus entry or the primary infection cycle are not the principal determinants of host specificity of Spodoptera spp. nucleopolyhedroviruses

The multicapsid nucleopolyhedroviruses (NPVs) of Spodoptera exigua (SeMNPV), Spodoptera frugiperda (SfMNPV), and Spodoptera littoralis (SpliNPV) are genetically similar (78 % similarity) but differ in their degree of host specificity. Infection by each of the three NPVs in these three Spodoptera host species was determined by oral inoculation of larvae with occlusion bodies (OBs) or intrahaemocoelic injection with occlusion derived virions (ODVs). RT-PCR analysis of total RNA from inoculated insects, targeted at immediate early (ie-0), early (egt, DNA polymerase), late (chitinase) and very late genes (polyhedrin), indicated that each of the NPVs initiated an infection in all three host species tested. SpliMNPV produced a fatal NPV disease in both heterologous hosts, S. frugiperda and S. exigua, by oral inoculation or injection. SfMNPV was lethal to heterologous hosts, S. exigua and S. littoralis, but infected larvae did not melt and disintegrate, and progeny OBs were not observed. SeMNPV was able to replicate in heterologous hosts and all genes required for replication were present in the genome, as the virus primary infection cycle was observed. However, gene expression was significantly lower in heterologous hosts. SeMNPV pathogenesis in S. frugiperda and S. littoralis was blocked at the haemocoel transmission stage and very nearly cleared. SeMNPV mixtures with SpliMNPV or SfMNPV did not extend the host range of SeMNPV; in all cases, only the homologous virus was observed to proliferate. It is concluded that entry and the primary virus infection cycle are not the only, or the major determinants, for SeMNPV infection of heterologous Spodoptera species.

Anticarsia gemmatalis multicapsid nucleopolyhedrovirus v-trex gene encodes a functional 3' to 5' exonuclease

The viral three-prime repair exonuclease (v-trex) gene of the Anticarsia gemmatalis multicapsid nucleopolyhedrovirus (AgMNPV) is the first baculovirus gene to be described with significant homology to a 3' exonuclease. v-trex is an early gene that is expressed by AgMNPV from 3 h post-infection. In the present study, the AgMNPV v-trex ORF was cloned into the baculovirus Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV) under the control of a polyhedrin promoter. The resulting virus produced an abundant, soluble protein that migrated with an apparent size of 23.7 kDa. The 3' to 5' exonuclease activity associated with this v-trex-expressing recombinant AcMNPV was 2000-fold above that of wild-type AcMNPV. This exonuclease activity was inhibited by EDTA and was activated in the presence of Mg2+ and, to a lesser extent, Mn2+. From these results, the AgMNPV v-trex gene is concluded to encode an independently active 3' to 5' exonuclease.

Characterization and phylogenetic analysis of the chitinase gene from the Helicoverpa armigera single nucleocapsid nucleopolyhedrovirus

A putative chitinase gene was identified within the fragment EcoRI-K of the Helicoverpa armigera single-nucleocapsid nucleopolyhedrovirus (HearNPV, also called HaSNPV) genome. The open reading frame (ORF) contains 1713 nucleotides (nt) and encodes a protein of 570 amino acids (aa) with a predicted molecular weight of 63.6 kDa. Transcription started at about 18 h post infection (p.i.) and the protein was first detected at 20 h p.i. The times of transcription and expression are characteristic of a late baculovirus gene. 5' and 3' RACE indicated that transcription was initiated from the adenine residue located at -246 nt upstream from the ATG start site and the poly (A) tail was added at 267 nt downstream from the stop codon. This is the first report on the molecular characterization of a chitinase from a single nucleocapsid NPV. The phylogeny of baculoviral chitinase genes were extensively examined in comparison with chitinases derived from bacteria, fungi, nematode, actinomycetes, viruses, insects and mammals. Neighbor-joining and most parsimony analyses showed that the baculoviral chitinases were clustered exclusively within gamma-proteobacteria. Our results strongly suggest that baculoviruses acquired their chitinase genes from bacteria.

Morphological characterization of Anticarsia gemmatalis M nucleopolyhedrovirus infection in haemocytes from its natural larval host, the velvet bean caterpillar Anticarsia gemmatalis (Hübner) (Lepidoptera: Noctuidae)

For a better understanding of virus x host interactions, transmission electron microscopy was used to characterize the intrahaemocoelic infection of Anticarsia gemmatalis larval haemocytes by A. gemmatalis M nucleopolyhedrovirus (AgMNPV). At 12 h post-infection (h p.i.), we observed nuclear hypertrophy, budded virus assembling, and protrusion towards the cytoplasm, virion envelopment, and accumulation of fibrillar aggregates in the cytoplasm. Around 24 h p.i., fibrillar aggregates also appeared inside nuclei of infected cells. By 48 h p.i., virogenic stroma and polyhedra were visualised in nuclei and at 72 h p.i., widespread infection in haemocytes was observed. Cell remnants and free polyhedra were phagocytosed by granular haemocyte 1 and plasmatocytes. Entire cells were phagocytosed only by plasmatocytes. Necrosis of infected cells was quite common, suggesting a putative cytotoxic response. Granular haemocyte 1 presented a more exuberant protrusion of budded viruses in comparison to other haemocytes. All types of haemocytes were shown to be infected, and the intense virus replication in some of these cells reveals the importance of haemolymph for AgMNPV spread in its natural host, a critical factor for permissiveness.

Australian funnel-web spiders: master insecticide chemists

Arthropods are the most diverse animal group on the planet. Their ability to inhabit a vast array of ecological niches has inevitably brought them into conflict with humans. Although only a small minority are classified as pest species, they nevertheless destroy about a quarter of the world's annual crop production and transmit an impressive array of pathogens of human and veterinary public health importance. Arthropod pests have been controlled almost exclusively with chemical insecticides since the introduction of DDT in the 1940s. However, the evolution of resistance to many insecticides, coupled with increased awareness of the potential environmental and human and animal health impacts of these chemicals, has stimulated the search for new insecticidal compounds, novel molecular targets, and alternative control methods. Spider venoms are complex chemical cocktails that have evolved to kill or paralyze arthropod prey, and they represent a largely untapped reservoir of insecticidal compounds. This review focuses on several families of invertebrate-specific peptide neurotoxins that were isolated from the venom of Australian funnel-web spiders. These peptides are promising insecticide leads because of their selectivity for invertebrates and activity on previously unvalidated targets. These toxins should facilitate the development of novel target-based screens for new insecticide leads, while their mapped pharmacophores will provide templates for rational design of mimetics that act at these target sites. Furthermore, genes encoding these toxins can be used to improve the efficacy of insect-specific viruses.

Invertebrate immune systems - not homogeneous, not simple, not well understood

The approximate 30 extant invertebrate phyla have diversified along separate evolutionary trajectories for hundreds of millions of years. Although recent work understandably has emphasized the commonalities of innate defenses, there is also ample evidence, as from completed genome studies, to suggest that even members of the same invertebrate order have taken significantly different approaches to internal defense. These data suggest that novel immune capabilities will be found among the different phyla. Many invertebrates have intimate associations with symbionts that may play more of a role in internal defense than generally appreciated. Some invertebrates that are either long lived or have colonial body plans may diversify components of their defense systems via somatic mutation. Somatic diversification following pathogen exposure, as seen in plants, has been investigated little in invertebrates. Recent molecular studies of sponges, cnidarians, shrimp, mollusks, sea urchins, tunicates, and lancelets have found surprisingly diversified immune molecules, and a model is presented that supports the adaptive value of diversified non-self recognition molecules in invertebrates. Interactions between invertebrates and viruses also remain poorly understood. As we are in the midst of alarming losses of coral reefs, increased pathogen challenge to invertebrate aquaculture, and rampant invertebrate-transmitted parasites of humans and domestic animals, we need a better understanding of invertebrate immunology.

High-frequency homologous recombination between baculoviruses involves DNA replication

We determined the frequency of DNA recombination between Bombyx mori nucleopolyhedroviruses (BmNPVs) and between BmNPV and the closely related Autographa californica NPV (AcMNPV) in BmN cells, Sf-21 cells, and larvae of Heliothis virescens. The BmN cells were coinfected with two BmNPVs, one with a mutation at the polyhedrin gene (polh) locus and a second carrying a lacZ gene marker cassette. Eleven different BmNPV mutants carrying the lacZ gene marker at various distances (1.4 to 61.7 kb) from polh were used for the coinfections. The Sf-21 cells and larvae of H. virescens were coinfected with wild-type AcMNPV and 1 of the 11 lacZ-marked BmNPV mutants. In BmN cells, high-frequency recombination was detected as early as 15 h postcoinfection but not at 12 h postcoinfection. At 18 h postcoinfection, the mean frequency of recombination ranged between 20.0 and 35.4% when the polh and lacZ marker genes were separated by at least 9.7 kb. When these marker genes were separated by only 1.4 kb, the mean frequency of recombination was 2.7%. In BmN cells, the mean recombination frequency between two BmNPVs increased only marginally when the multiplicity of infection of each virus was increased 10-fold. In Sf-21 cells and the larvae of H. virescens, the recombination frequency between BmNPV and AcMNPV was </=1.0%. AcMNPV DNA replication occurred normally after the coinfection of Sf-21 cells. BmNPV DNA replication, however, was not detected, indicating that normal DNA replication by both viruses is required for high-frequency recombination.

Foraging behavior and prey interactions by a guild of predators on various lifestages of Bemisia tabaci

The sweetpotato whitefly, Bemisia tabaci (Gennadius) is fed on by a wide variety of generalist predators, but there is little information on these predator-prey interactions. A laboratory investigation was conducted to quantify the foraging behavior of the adults of five common whitefly predators presented with a surfeit of whitefly eggs, nymphs, and adults. The beetles, Hippodamia convergens Guérin-Méneville and Collops vittatus (Say) fed mostly on whitefly eggs, but readily and rapidly preyed on all of the whitefly lifestages. The true bugs, Geocoris punctipes (Say) and Orius tristicolor (Say) preyed almost exclusively on adult whiteflies, while Lygus hesperus Knight preyed almost exclusively on nymphs. The true bugs had much longer prey handling times than the beetles and spent much more of their time feeding (35–42%) than the beetles (6–7%). These results indicate that generalist predators vary significantly in their interaction with this host, and that foraging behavior should be considered during development of a predator-based biological control program for B. tabaci.

Abbreviation:ELISA

enzyme-linked immunosorbent assay

The gp64 locus of Anticarsia gemmatalis multicapsid nucleopolyhedrovirus contains a 3′ repair exonuclease homologue and lacks v-cath and ChiA genes

Anticarsia gemmatalis multicapsid nucleopolyhedrovirus (AgMNPV) is one of the most successful biological insecticides. In this study, we cloned and sequenced a 12.5 kbp BamHI-D restriction endonuclease fragment of the AgMNPV isolate 2D genome that includes the gp64 gene. We compared this highly conserved region with that of other baculoviruses. AgMNPV contained two genes, p22.2 and v-trex, in common with Choristoneura fumiferana MNPV (CfMNPV) that were not present in other baculoviruses. The v-trex gene has homology to a eukaryotic 3' repair exonuclease and appears to have been acquired from an invertebrate host. The v-trex gene product has the potential to be involved in virus recombination or UV-light tolerance. Multigene phylogenetic analysis suggested that AgMNPV is most closely related to Orgyia pseudotsugata MNPV (OpMNPV). AgMNPV differed from other group I NPVs in that ChiA and v-cath gene homologues were missing from the region downstream of the gp64 gene. Proteinase assays and genetic probes suggest the v-cath gene is absent from AgMNPV.

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.

Sequence analysis of a 5.1 kbp region of the Spodoptera frugiperda multicapsid nucleopolyhedrovirus genome that comprises a functional ecdysteroid UDP-glucosyltransferase (egt) gene

The ecdysteroid UDP-glucosyltransferase gene from the Spodoptera frugiperda multicapsid nucleopolyhedrovirus (SfMNPV) was identified using degenerate primers whose sequence were derived from conserved regions of the EGT proteins encoded by other baculoviruses. Analysis of the gene sequence revealed the presence of an open reading frame (ORF) with potential to encode a polypeptide of 525 amino acids. Promoter sequences typical of baculovirus genes were found in the 5' region of this ORF. A polyadenylation signal was identified downstream the translation stop codon. A transient expression assay showed that the product of this ORF was able to conjugate glucose from UDP-glucose with ecdysone confirming that the gene identified was indeed the SfMNPV egt gene. The SfMNPV egt gene and the sequences of other baculovirus egt genes were used to infer a phylogenetic tree. The nucleotide sequence of the entire BamHI fragment that contains the SfMNPV egt gene was determined. Search of the available sequence databases suggested that, besides the egt gene, this region contains 5 ORFs similar to the baculovirus genes gp37 (fusolin), to ptp2 and to ORFs 28, 29, and 30 of Spodoptera exigua multicapsid nucleopolyhedrovirus. Both the phylogenetic analysis of the egt genes and the gene order of the region that flanks the egt gene indicated that SfMNPV is closely related to the baculoviruses that infects S. exigua and Mamestra configurata.

Comparative susceptibilities of insect cell lines to infection by the occlusion-body derived phenotype of baculoviruses

Twelve insect cell lines from six species were tested for susceptibility to baculovirus infection by occlusion-derived virus (ODV) phenotype through the use of a typical endpoint assay procedure. ODV from three nucleopolyhedroviruses were prepared by alkali treatment (sodium carbonate) of occlusion bodies (OBs) and the virus preparations were titered on various cell lines. More than a four-log difference was realized for each of theses viruses between the various cell lines. The TN368 line from Trichoplusia ni was only marginally susceptible to ODV from each virus, showing only 3-6 infectious units (IU) per million OBs while the gypsy moth line, LdEp was most susceptible, realizing more than 100,000 IU/million OBs. The other lines tested showed various levels of susceptibility between these two extremes and also varied between the three viruses tested. In additional tests, the ODV were treated with trypsin prior to application to the cells. With most cell lines, this treatment increased the infectivity of each virus by 2-10-fold. Exceptions to this trend included the gypsy moth LdEp line, on which the trypsinized ODV from two of the viruses were slightly less infectious than each virus without trypsin, and the TN-368 line, on which the trypsinized ODV was 5,000-75,000 times more infectious. The variable results of trypsinized virus on the different lines are probably due to the levels of endogenous protease activity in the various lines, but the mode of action of the trypsin has not been elucidated. Ultimately, the variable response of cell lines to ODV of different viruses, and the variable effects of trypsin on the ODV may lead to an improved understanding of the infection process of this virus phenotype as well as factors relating to baculovirus host range.

Comparative analysis of the genomes of Rachiplusia ou and Autographa californica multiple nucleopolyhedroviruses

The Rachiplusia ou multiple nucleopolyhedrovirus (RoMNPV) is a variant of Autographa californica MNPV (AcMNPV) but is significantly more virulent against several major agricultural pests. The genome sequence of the R1 strain of RoMNPV was determined and compared to that of AcMNPV strain C6. The RoMNPV genome is approximately 131.5 kbp with a G+C content of 39.1 %. The homologous repeat regions (hrs) described for AcMNPV-C6 are present in RoMNPV-R1 but the hrs of RoMNPV have fewer palindromic repeats. The RoMNPV-R1 nucleotide sequence is almost completely collinear with the sequence of AcMNPV-C6 and contains homologues of 150 of the 155 ORFs described for AcMNPV-C6. Deletions, insertions and substitutions have resulted in the loss of homologues for AcMNPV ORFs ac2 (bro), ac3 (ctl), ac97, ac121 and ac140 from the RoMNPV genome. The average amino acid sequence identity between RoMNPV and AcMNPV ORFs is 96.1 % and there are differences in promoter motif composition for 23 of these ORFs. Maximum-likelihood analysis of selection pressures on AcMNPV and RoMNPV ORFs indicate that ORFs ro18/ac20-ac21 (arif-1) and ro135/ac143 (odv-e18) have undergone positive selection.

Baculoviruses as Vectors for Gene Therapy against Human Prostate Cancer

Current curative strategies for prostate cancer are restricted to the primary tumour, and the effect of treatments to control metastatic disease is not sustained. Therefore, the application of gene therapy to prostate cancer is an attractive alternative. Baculoviruses are highly restricted insect viruses, which can enter, but not replicate in mammalian cells. Baculoviruses can incorporate large amounts of extra genetic material, and will express transgenes in mammalian cells when under the control of a mammalian or strong viral promoter. Successful gene delivery has been achieved both in vitro and in vivo and into both dividing and nondividing cells, which is important since prostate cancers divide relatively slowly. In addition, the envelope protein gp64 is sufficiently mutable to allow targeted transduction of particular cell types. In this review, the advantages of using baculoviruses for prostate cancer gene therapy are explored, and the mechanisms of viral entry and transgene expression are described.

Molecular cloning and sequence analysis of the Anticarsia gemmatalis multicapsid nuclear polyhedrosis virus GP64 glycoprotein

The gp64 locus of Anticarsia gemmatalis multicapsid nucleopolyhedrovirus isolate Santa Fe (AgMNPV-SF) was characterised molecularly in our laboratory. To this end, we have located and cloned a AgMNPV-SF genomic DNA fragment containing the gp64 gene and sequenced the complete gp64 locus. Nucleotide sequence analysis indicated that the AgMNPV gp64 gene consists of a 1500 nucleotide open reading frame (ORF), encoding a protein of 499 amino acids. Of the seven gp64 homologues identified to date, the AgMNPV gp64 ORF shared most sequence similarity with the gp64 gene of Orgyia pseudotsugata MNPV. The GP64 from AgMNPV is the smallest baculoviral envelope glycoprotein found to date, differing in 10 or more residues from the other group I nucleopolyhedroviruses. The biological activity of AgMNPV GP64 protein was assessed by cell fusion assays in UFL-AG-286 cells using the obtained recombinant plasmids. In the upstream and downstream regions, relative to the gp64 ORF, we found different conserved transcriptional and post-transcriptional regulatory elements, respectively.

Physical and genetic map of Epinotia aporema granulovirus genome

The bean shoot borer, Epinotia aporema (Lep. Tortricidae), is an economically important pest of legume crops in South America. Recently, a granulovirus (EpapGV) was isolated from E. aporema larvae, and evaluated as a potential biological control agent. In order to generate a restriction map and to investigate the gene organisation of EpapGV genome, DNA isolated from occlusion bodies as well as a set of cloned genomic fragments were analysed using combinations of restriction endonucleases and Southern blot analyses that lead to a first version of the physical map. It was subsequently confirmed and refined by sequencing the termini of the cloned fragments and assessing their contiguity by comparing the sequences with databases to identify putative ORFs spanning neighbour fragments. This was also aided by PCR amplifications with primers that pointed outwards of the cloned viral DNA. The granulin gene was positioned on the physical map, cloned and sequenced. Its 747-nucleotide-long ORF encodes a predicted protein of 29 kDa and the core of the baculovirus very late promoter ATAAG was found 29 nucleotides upstream the initiation codon. In addition, 27 putative ORFs were located on the map and used to explore the genome organisation by GeneParityPlot against the fully sequenced granulovirus genomes. These data, taken together with the phylogenetic tree generated by alignment of the major occlusion proteins, indicate that EpapGV is closely related to CpGV, but has a distinct gene organisation.

Bollworm responses to release of genetically modified Helicoverpa armigera nucleopolyhedroviruses in cotton

Helicoverpa armigera single nucleocapsid nucleopolyhedrovirus (HaSNPV) has been developed as a commercial biopesticide to control the cotton bollworm, H. armigera, in China. The major limitation to a broader application of this virus has been the relative long time to incapacitate the target insect. Two HaSNPV recombinants with improved insecticidal properties were released in bollworm-infested cotton. One recombinant (HaCXW1) lacked the ecdysteroid UDP-glucosyltransferase (egt) gene and in another recombinant (HaCXW2), an insect-selective scorpion toxin (AaIT) gene replaced the egt gene. In a cotton field situation H. armigera larvae treated with either HaCXW1 or HaCXW2 were killed faster than larvae in HaSNPV-wt treated plots. Second instar H. armigera larvae, which were collected from HaCXW1 and HaCXW2 treated plots and further reared on artificial diet, showed reduced ST(50) values of 15.3 and 26.3%, respectively, as compared to larvae collected from HaSNPV-wt treated plots. The reduction in consumed leaf area of field collected larvae infected with HaCXW1 and HaCXW2 was approximated 50 and 63%, respectively, as compared to HaSNPV-wt infected larvae at 108 h after treatment. These results suggest that in a cotton field situation the recombinants will be more effective control agents of the cotton bollworm than wild-type HaSNPV.

Characterization of the p10 gene region of Anticarsia gemmatalis nucleopolyhedrovirus

The Anticarsia gemmatalis nucleopolyhedrovirus (AgMNPV) p10 gene region was cloned, sequenced and the putative p10 gene expression characterized by Northern-blot analysis. Sequence analysis of the p10 gene region indicated the presence of two complete open reading frames (ORFs) of 713 and 281 nucleotides, which codes for polypeptides of 273 and 93 amino acids, with homology to the P26 and P10 proteins of baculoviruses, respectively. Two additional partial ORFs, coding for partial polypeptides of 110 and 146 amino acids, showed homology to the p22.2 gene of Choristoneura fumiferana nucleopolyhedrovirus (CfMNPV) and p74 genes of different baculoviruses, respectively. A small ORF of 224 nucleotides coding for a protein of 74 amino acids showed homology to the 3'-end of the early p94 gene of AcMNPV. A putative baculovirus very late promoter motif TAAG was identified in the 5'-non-translated region (5'-UTR) at position-54 upstream of the start codon. The consensus polyadenylation sequence AATAAA is present 146nt downstream of the termination codon and the p10 ORF is flanked by the p26 and p74 ORFs. Homology comparisons showed that the P10 protein of AgMNPV is most closely related (82% amino acid sequence identity) to the P10 from the Orgyia pseudotsugata nucleopolyhedrovirus (OpMNPV). Transcriptional analysis of the AgMNPV p10 gene showed that p10-specific transcripts could be detected late in infection.

Production of viral progeny in insect cells undergoing apoptosis induced by a mutant Anticarsia gemmatalis nucleopolyhedrovirus

The Anticarsia gemmatalis nucleopolyhedrovirus (AgMNPV) is the most successful viral biopesticide in use worldwide. We have demonstrated that despite widespread apoptosis and no protein synthesis at 48 h p.i., UFL-AG-286 cells infected with a mutant of AgMNPV (vApAg), produced significant amounts of budded virus (BVs) and viral DNA late in infection. However, a different susceptible cell line (BTI-Tn5B 1-4) showed no signs of apoptosis and produced 3.5 times more budded virus when infected with vApAg. A comparison of DNA from AgMNPV and vApAg digested with the same restriction enzymes showed differences in the restriction pattern, indicating that the vApAg phenotype might be due to a mutation in a gene or genes responsible for directly or indirectly inhibiting apoptosis in UFL-AG-286 cells.

Vertical transmission of sublethal granulovirus infection in the Indian meal moth, Plodia interpunctella

Knowledge of the mechanisms of pathogen persistence in relation to fluctuations in host density is crucial to our understanding of disease dynamics. In the case of insect baculoviruses, which are typically transmitted horizontally via a lifestage that can persist outside the host, a key issue that remains to be elucidated is whether the virus can also be transmitted vertically as a sublethal infection. We show that RNA transcripts for the Plodia interpunctella GV granulin gene are present in a high proportion of P. interpunctella insects that survive virus challenge. Granulin is a late-expressed gene that is only transcribed after viral genome replication, its presence thus strongly indicates that viral genome replication has occurred. Almost all insects surviving the virus challenge tested positive for viral RNA in the larval and pupal stage. However, this proportion declined in the emerging adults. Granulin mRNA was also detected in both the ovaries and testes, which may represent a putative mechanism by which reduced fecundity in sublethally affected hosts might be manifested. RNA transcripts were also detected in 60-80% of second-generation larvae that were derived from mating surviving adults, but there was no difference between the sexes, with both males and females capable of transmitting a sublethal infection to their offspring. The data indicate that low-level persistent infection, with at least limited gene expression, can occur in P. interpunctella following survival of a granulovirus challenge. We believe that this is the first demonstration of a persistent, sublethal infection by a baculovirus to be initiated by a sublethal virus dose. We hypothesize that the 'latent' baculovirus infections frequently referred to in the literature may also be low level persistent, sublethal infections resulting from survival from initial baculovirus exposure.

Identification and characterization of a baculovirus from Lonomia obliqua (Lepidoptera: Saturniidae)

A baculovirus has been isolated from larvae of Lonomia obliqua, a Saturniidae of medical importance due to a potent toxin found in their spines. Electron Microscopy analysis of the occlusion body obtained from diseased larvae showed polyhedra of approximately 1 microm in diameter containing multiple nucleocapsids per envelope. This baculovirus was thus named Lonomia obliqua multicapsid nucleopolyhedrovirus (LoobMNPV). Restriction endonuclease profiles of viral DNA digested with three restriction enzymes were obtained and the genome size was estimated to be 95.52 +/- 2.3 kbp. The polyhedrin gene of LoobMNPV was identified and its DNA sequence was determined. Phylogenetic analysis of the polyhedrin gene showed that the LoobMNPV polyhedrin belongs to group I NPV and that it is closely related to the polyhedrin of the NPV of Amsacta albistriga.

Quantification of soil-to-plant transport of recombinant nucleopolyhedrovirus: effects of soil type and moisture, air currents, and precipitation

Significantly more occlusion bodies (OB) of DuPont viral construct HzSNPV-LqhIT2, expressing a scorpion toxin, were transported by artificial rainfall to cotton plants from sandy soil (70:15:15 sand-silt-clay) than from silt (15:70:15) and significantly more from silt than from clay (15:15:70). The amounts transported by 5 versus 50 mm of precipitation were the same, and transport was zero when there was no precipitation. In treatments that included precipitation, the mean number of viable OB transported to entire, 25- to 35-cm-tall cotton plants ranged from 56 (clay soil, 5 mm of rain) to 226 (sandy soil, 50 mm of rain) OB/plant. In a second experiment, viral transport increased with increasing wind velocity (0, 16, and 31 km/h) and was greater in dry (-1.0 bar of matric potential) than in moist (-0.5 bar) soil. Wind transport was greater for virus in a clay soil than in silt or sand. Only 3.3 x 10(-7) (clay soil, 5 mm rain) to 1.3 x 10(-6) (sandy soil, 50 mm rain) of the OB in surrounding soil in experiment 1 or 1.1 x 10(-7) (-0.5 bar sandy soil, 16-km/h wind) to 1.3 x 10(-6) (-1.0 bar clay soil, 31-km/h wind) in experiment 2 were transported by rainfall or wind to cotton plants. This reduces the risk of environmental release of a recombinant nucleopolyhedrovirus (NPV), because only a very small proportion of recombinant virus in the soil reservoir is transported to vegetation, where it can be ingested by and replicate in new host insects.

Aedes aegypti transducing densovirus pathogenesis and expression in Aedes aegypti and Anopheles gambiae larvae

Aedes aegypti densovirus (AeDNV) is a small DNA virus that has been developed into an expression and transducing vector for mosquitoes [Afanasiev et al. (1994) Exp Parasitol 79: 322-339; Afanasiev et al. (1999) Virology 257: 62-72; Carlson et al. (2000) Insect Transgenesis: Methods and Applications (Handler, A.M. & James, A.A., eds), pp. 139-159. CRC Press, Boca Raton]. Virions carrying a recombinant genome expressing the GFP gene were used to characterize the pathogenesis of the virus in 255 individual Aedes aegypti larvae. The anal papillae of the larvae were the primary site of infection confirming previous observations (Afanasiev etal., 1999; Allen-Muira et al. (1999) Virology 257: 54-61). GFP expression was observed in most cases to spread from the anal papillae to cells of the fat body, and subsequently to many other tissues including muscle fibers and nerves. Infected anal papillae were also observed to shrink, or melanize and subsequently fall off in a virus dependent manner. Three to four day-old larvae were less susceptible to viral infection and, if infected, were more likely to survive into adulthood, with 14% of them still expressing GFP as adults. Higher salt concentrations of 0.10-0.15 M inhibited viral infection. Anopheles gambiae larvae also showed infection of the anal papillae (17%) but subsequent viral dissemination did not occur. The persistence of the reporter gene expression into adulthood of Aedes aegypti indicates that transduction of mosquito larvae with recombinant AeDNV may be a means of introducing a gene of interest into a mosquito population for transient expression.

Potential of Agrotis ipsilon nucleopolyhedrovirus for suppression of the black cutworm (Lepidoptera: Noctuidae) and effect of an optical brightener on virus efficacy

Studies were performed in the laboratory, greenhouse and field to assess the potential of Agrotis ipsilon multicapsid nucleopolyhedrovirus (AgipMNPV) and a viral enhancing agent, M2R, for suppression of Agrotis ipsilon (Hufnagel). In laboratory droplet feeding bioassays, AgipMNPV was shown to be highly active against third-instar A. ipsilon. The optical brightener M2R significantly reduced LD50 estimates by approximately 160-fold, but had no direct effect on survival time estimates. In greenhouse trials, spray and bait formulations of AgipMNPV significantly reduced feeding damage to corn seedlings caused by third-instar A. ipsilon. In two sets of replicated field trials, bait formulations of AgipMNPV significantly reduced feeding damage to corn seedlings by third-instar A. ipsilon. However, there were no beneficial effects attributable to the inclusion of M2R in AgipMNPV formulations under greenhouse or field conditions. It seems likely that in an appropriately designed pest management program AgipMNPV could be used to suppress field populations of early and mid-instar A. ipsilon.

Recombinant baculoviruses for insect control

Baculoviruses are double-stranded DNA viruses which are highly selective for several insect groups. They are valuable natural control agents, but their utility in many agricultural applications has been limited by their slow speed of kill and narrow host specificity. Baculoviruses have been genetically modified to express foreign genes under powerful promoters in order to accelerate their speed of kill. In our and other laboratories, the expression of genes coding for insect juvenile hormone esterases and various peptide neurotoxins has resulted in recombinant baculoviruses with promise as biological insecticides. These viruses are efficacious in the laboratory, greenhouse and field and dramatically reduce damage caused by insect feeding. The recombinant viruses synergize and are synergized by classical pesticides such as pyrethroids. Since they are highly selective for pest insects, they can be used without disrupting biological control. Because the recombinant virus produces fewer progeny in infected larvae than the wild-type virus, they are rapidly out-competed in the ecosystem. The viruses can be used effectively with crops expressing endotoxins of Bacillus thuringiensis. They can be produced industrially but also by village industries, indicating that they have the potential to deliver sustainable pest control in developing countries. It remains to be seen, however, whether the current generation of recombinant baculoviruses will be competitive with the new generation of synthetic chemical pesticides. Current research clearly indicates, though, that the use of biological vectors of genes for insect control will find a place in agriculture. Baculoviruses will also prove valuable in testing the potential utility of proteins and peptides for insect control.

The genome sequence and evolution of baculoviruses

Comparative analysis of the complete genome sequences of 13 baculoviruses revealed a core set of 30 genes, 20 of which have known functions. Phylogenetic analyses of these 30 genes yielded a tree with 4 major groups: the genus Granulovirus (GVs), the group I and II lepidopteran nucleopolyhedroviruses (NPVs), and the dipteran NPV, CuniNPV. These major divisions within the family Baculoviridae were also supported by phylogenies based on gene content and gene order. Gene content mapping has revealed the patterns of gene acquisitions and losses that have taken place during baculovirus evolution, and it has highlighted the fluid nature of baculovirus genomes. The identification of shared protein phylogenetic profiles provided evidence for two putative DNA repair systems and for viral proteins specific for infection of lymantrid hosts. Examination of gene order conservation revealed a core gene cluster of four genes, helicase, lef-5, ac96, and 38K(ac98), whose relative positions are conserved in all baculovirus genomes.

Characterization of a granulovirus isolated from Epinotia aporema Wals. (Lepidoptera: Tortricidae) larvae

A granulovirus (GV) isolated from Epinotia aporema (Lepidoptera: Tortricidae)-a major soybean pest-was studied in terms of its main morphological, biochemical, and biological properties. The ovoidal occlusion bodies were 466 by 296 nm in size, and their most prominent protein had an apparent molecular mass of 29 kDa. Its amino-terminal sequence was remarkably homologous to that of the granulins of other GVs. The DNA genome size was estimated to be 120 kbp. The high specificity and pathogenicity of this newly described granulovirus (EpapGV) indicate that it is indeed a good candidate for the biological control of this pest.

Generation of a recombinant Anticarsia gemmatalis multicapsid nucleopolyhedrovirus expressing a foreign gene under the control of a very late promoter

A recombinant Anticarsia gemmatalis multicapsid nucleopolyhedrovirus (AgMNPV) expressing beta-galactosidase under the control of the polyhedrin promoter was generated in our laboratory. To this end, we cloned the AgMNPV-2D genomic DNA fragment containing the polh gene and subcloned and sequenced the polyhedrin gene and its flanking regions. Based on this sequence information, sets of primers were designed to amplify the flanking regions by PCR, including appropriate restriction sites. The transfer vector (pAgPHZ) was constructed by the consecutive cloning of these PCR fragments flanking the Escherichia coli LacZ gene, in place of the polh gene. pAgPHZ was used for cotransfection of UFL-AG-286 insect cells with AgMNPV-2D DNA and the required recombinant, generated by homologous recombination with the polh locus, was identified by its polh(-)/LacZ+ plaque phenotype. Its genome structure was confirmed by PCR, restriction digestion and Southern blot analyses. The kinetics and levels of expression of beta-galactosidase in UFL-AG-286 cells infected with the recombinant were tested by SDS-PAGE and enzymatic activity assays.

Characterization of the ecdysteroid UDP-glucosyltransferase (egt) gene of Anticarsia gemmatalis nucleopolyhedrovirus

The Anticarsia gemmatalis nucelopolyhedrovirus (AgMNPV) egt gene was cloned, sequenced and its expression characterized by RT-PCR and western blot analysis. Sequence analysis of the gene indicated the presence of an open reading frame (ORF) of 1482 nucleotides, which codes for a polypeptide of 494 amino acids. ATATA box and a conserved regulatory sequence (CATT) found in other baculovirus early genes were present in the promoter region of the egt gene. A poly-A consensus sequence was present in the 3' untranslated region (3'-UTR) of the gene. Homology comparisons showed that the EGT protein of AgMNPV is most closely related (95.9% amino acid sequence identity) to the EGT from the Choristoneura fumiferana DEF nucleopolyhedrovirus (CfDEF). Transcriptional analysis of the AgMNPV egt gene showed that egt-specific transcripts can be detected both early and late in infection. The EGT protein was detected, by western blot analysis, in the intra- (from 12 to 48 h post-infection) and extra-cellular (from 12 to 96 h post-infection) fractions of infected insect cells. The AgMNPV Bgl II-F fragment, which has homology to the AcMNPV ie-1 gene, was cloned and used to cotransfect SF21 cells with the cloned AgMNPV egt gene. EGT activity was observed, suggesting that AgMNPV ie-1 can transactivate egt expression.

The sequence of the Helicoverpa armigera single nucleocapsid nucleopolyhedrovirus genome

The nucleotide sequence of the Helicoverpa armigera single-nucleocapsid nucleopolyhedrovirus (HaSNPV) DNA genome was determined and analysed. The circular genome encompasses 131,403 bp, has a G+C content of 39.1 mol% and contains five homologous regions with a unique pattern of repeats. Computer-assisted analysis revealed 135 putative ORFs of 150 nt or larger; 100 ORFs have homologues in Autographa californica multicapsid NPV (AcMNPV) and a further 15 ORFs have homologues in other baculoviruses such as Lymantria dispar MNPV (LdMNPV), Spodoptera exigua MNPV (SeMNPV) and Xestia c-nigrum granulovirus (XcGV). Twenty ORFs are unique to HaSNPV without homologues in GenBank. Among the six previously sequenced baculoviruses, AcMNPV, Bombyx mori NPV (BmNPV), Orgyia pseudotsugata MNPV (OpMNPV), SeMNPV, LdMNPV and XcGV, 65 ORFs are conserved and hence are considered as core baculovirus genes. The mean overall amino acid identity of HaSNPV ORFs was the highest with SeMNPV and LdMNPV homologues. Other than three 'baculovirus repeat ORFs' (bro) and two 'inhibitor of apoptosis' (iap) genes, no duplicated ORFs were found. A putative ORF showing similarity to poly(ADP-ribose) glycohydrolases (parg) was newly identified. The HaSNPV genome lacks a homologue of the major budded virus (BV) glycoprotein gene, gp64, of AcMNPV, BmNPV and OpMNPV. Instead, a homologue of SeMNPV ORF8, encoding the major BV envelope protein, has been identified. GeneParityPlot analysis suggests that HaSNPV, SeMNPV and LdMNPV (group II) have structural genomic features in common and are distinct from the group I NPVs and from the granuloviruses. Cluster alignment between group I and group II baculoviruses suggests that they have a common ancestor.

Genetic engineering of Helicoverpa armigera single-nucleocapsid nucleopolyhedrovirus as an improved pesticide

The Helicoverpa armigera single-nucleocapsid nucleopolyhedrovirus (HearNPV) has been registered and is commercially produced in China as a biopesticide to control the bollworm in cotton. However, the virus has a relatively slow speed of action. To improve its efficacy, recombinant HearNPVs were generated by deleting the ecdysteroid UDP-glucosyltransferase (egt) gene (HaCXW1 and HaLM2) or by inserting the insect-specific toxin gene AaIT in the egt locus (HaCXW2) of HearNPV using conventional recombination strategies in insect cell culture. The various recombinants remained genetically stable when cultured in HzAM1 insect cells. Bioassay data showed a significant reduction in the time required for all HearNPV recombinants to kill second instar H. armigera larvae. The LT(50) of the egt deletion recombinants HaCXW1 and HaLM2 was about 27% faster than that of wild-type HearNPV. The largest reduction in LT(50) was achieved by inserting the gene for the insect-specific neurotoxin, AaIT, in the egt locus, giving a reduction in LT(50) of 32% compared to wild-type HearNPV. The ability to genetically improve the properties of HearNPV as a biopesticide provides a further opportunity to develop this virus into a commercially viable product to control the bollworm in China.

Characterization of a nucleopolyhedrovirus from the black cutworm, agrotis ipsilon (Lepidoptera: noctuidae)

The black cutworm, Agrotis ipsilon (Hufnagel) (Lepidoptera: Noctuidae), is a serious localized pest of vegetable and field crops. We have characterized a newly discovered baculovirus, the Agrotis ipsilon multicapsid nucleopolyhedrovirus (AgipMNPV), that was isolated from A. ipsilon in Illinois. Restriction enzyme fragment profiles of AgipMNPV DNA were distinct from those of previously described nucleopolyhedroviruses. Electron microscopy of AgipMNPV-infected tissues indicated that nucleocapsids of this virus are multiply enveloped. A. ipsilon was highly susceptible to infection by AgipMNPV and significantly more susceptible to infection by AgipMNPV than by Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV). Host range studies showed that Heliothis virescens and Helicoverpa zea were moderately susceptible to infection; Pseudaletia unipuncta and Spodoptera frugiperda were only partially susceptible, and Anticarsia gemmatalis, Spodoptera exigua, Trichoplusia ni, and Ostrinia nubilalis were not susceptible to infection by AgipMNPV. Because of its high virulence, AgipMNPV has potential as an alternative to chemical insecticides for control of A. ipsilon. Copyright 1999 Academic Press.