Biological and biochemical relationships between the nucleopolyhedroviruses of Mamestra brassicae and Heliothis armigera

Coocclusion of Helicoverpa armigera Single Nucleopolyhedrovirus (HearSNPV) and Helicoverpa armigera Multiple Nucleopolyhedrovirus (HearMNPV): Pathogenicity and Stability in Homologous and Heterologous Hosts

Helicoverpa armigera single nucleopolyhedrovirus (HearSNPV) is a virulent pathogen of lepidopterans in the genera Heliothis and Helicoverpa, whereas Helicoverpa armigera multiple nucleopolyhedrovirus (HearSNPV) is a different virus species with a broader host range. This study aimed to examine the consequences of coocclusion of HearSNPV and HearMNPV on the pathogenicity, stability and host range of mixed-virus occlusion bodies (OBs). HearSNPV OBs were approximately 6-fold more pathogenic than HearMNPV OBs, showed faster killing by approximately 13 h, and were approximately 45% more productive in terms of OB production per larva. For coocclusion, H. armigera larvae were first inoculated with HearMNPV OBs and subsequently inoculated with HearSNPV OBs at intervals of 0–72 h after the initial inoculation. When the interval between inoculations was 12–24 h, OBs collected from virus-killed insects were found to comprise 41–57% of HearSNPV genomes, but the prevalence of HearSNPV genomes was greatly reduced (3–4%) at later time points. Quantitative PCR (qPCR) analysis revealed the presence of HearSNPV genomes in a small fraction of multinucleocapsid ODVs representing 0.47–0.88% of the genomes quantified in ODV samples, indicating that both viruses had replicated in coinfected host cells. End-point dilution assays on ODVs from cooccluded mixed-virus OBs confirmed the presence of both viruses in 41.9–55.6% of wells that were predicted to have been infected by a single ODV. A control experiment indicated that this result was unlikely to be due to the adhesion of HearSNPV ODVs to HearMNPV ODVs or accidental contamination during ODV band extraction. Therefore, the disparity between the qPCR and end-point dilution estimates of the prevalence of mixed-virus ODVs likely reflected virus-specific differences in replication efficiency in cell culture and the higher infectivity of pseudotyped ODVs that were produced in coinfected parental cells. Bioassays on H. armigera, Spodoptera frugiperda and Mamestra brassicae larvae revealed that mixed-virus OBs were capable of infecting heterologous hosts, but relative potency values largely reflected the proportion of HearMNPV present in each mixed-virus preparation. The cooccluded mixtures were unstable in serial passage; HearSNPV rapidly dominated during passage in H. armigera whereas HearMNPV rapidly dominated during passage in the heterologous hosts. We conclude that mixed-virus coocclusion technology may be useful for producing precise mixtures of viruses with host range properties suitable for the control of complexes of lepidopteran pests in particular crops, although this requires validation by field testing.

Genetic Variation and Biological Activity of Two Closely Related Alphabaculoviruses during Serial Passage in Permissive and Semi-Permissive Heterologous Hosts

Phylogenetic analyses suggest that Mamestra brassicae multiple nucleopolyhedrovirus (MbMNPV) and Helicoverpa armigera multiple nucleopolyhedrovirus (HearMNPV) may be strains of the same virus species. Most of the studies comparing their biological activities have been performed in their homologous hosts. A comparison of host range and stability in alternative hosts was performed. The host range of these viruses was compared using high concentrations of inoculum to inoculate second instars of six species of Lepidoptera. One semi-permissive host (Spodoptera littoralis) and one permissive host (S. exigua) were then selected and used to perform six serial passages involving a concentration corresponding to the ~25% lethal concentration for both viruses. Restriction endonuclease analysis showed fragment length polymorphisms in every host-virus system studied. In S. littoralis, serial passage of MbMNPV resulted in decreased pathogenicity and an increase in speed-of-kill, whereas no significant changes were detected for HearMNPV with respect to the initial inoculum. In contrast, both viruses showed a similar trend in S. exigua. These results highlight the low genetic diversity and a high phenotypic stability of HearMNPV with respect to the original inoculum after six successive passages in both insect hosts. This study concludes that host-baculovirus interactions during serial passage are complex and the process of adaptation to a novel semi-permissive host is far from predictable.

Lacanobia oleracea nucleopolyhedrovirus (LaolNPV): A new European species of alphabaculovirus with a narrow host range

During an insect sampling program in alfalfa crops near Montpellier, France in 2011, Lacanobia oleracea larvae were collected that died due to nucleopolyhedrovirus infection (LaolNPV). This virus was subjected to molecular and biological characterization. The virus was a multiple nucleocapsid NPV that showed similar restriction profiles to Mamestra configurata NPV-A (MacoNPV-A) but with significant differences. Polypeptide analysis demonstrated similar proteins in occlusion bodies and occlusion derived virions, to those observed in NPVs from Mamestra spp. Terminal sequencing revealed that the genome organization shared similarity with that of MacoNPV-A. The most homologous virus was MacoNPV-A 90/2 isolate (95.63% identity and 96.47% similarity), followed by MacoNPV-A 90/4 strain (95.37% and 96.26%), MacoNPV-B (89.21% and 93.53%) and M. brassicae MNPV (89.42% and 93.74%). Phylogenetic analysis performed with lef-8, lef-9, polh and a concatenated set of genes showed that LaolNPV and the Mamestra spp. NPVs clustered together with HaMNPV, but with a closer genetic distance to MacoNPV-A strains. The Kimura 2-parameter (K-2-P) distances of the complete genes were greater than 0.05 between LaolNPV and the MbMNPV/MacoNPV-B/HaMNPV complex, which indicates that LaolNPV is a distinct species. K-2-P distances were in the range 0.015–0.050 for comparisons of LaolNPV with MacoNPV-A strains, such that additional biological characteristics should be evaluated to determine species status. While MacoNPV-A was pathogenic to seven lepidopteran species tested, LaolNPV was only pathogenic to Chrysodeixis chalcites. Given these findings, Lacanobia oleracea nucleopolyhedrovirus should be considered as a new species in the Alphabaculovirus genus.

Selection of a nucleopolyhedrovirus isolate from Helicoverpa armigera as the basis for a biological insecticide

BACKGROUND

The cotton bollworm, Helicoverpa armigera, is an insect that causes damage in a wide range of crops in Spain. Seven isolates of H. armigera single nucleopolyhedrovirus (HearSNPV) from the Iberian Peninsula were subjected to molecular and biological characterization and compared with a Chinese genotype (HearSNPV-G4).

RESULTS

The estimated sizes of the Iberian genomes varied between 116.2 and 132.4 kb, compared to 131.4 kb of the HearSNPV-G4 reference genome. Phylogenetic analysis based on the lef-8, lef-9 and polh genes revealed that the Iberian strains were more closely related to one another than to other HearSNPV isolates. Occlusion body (OB) concentration-mortality responses (LC50 values) did not differ significantly among Iberian isolates when tested against a Helicoverpa armigera colony from Oxford (UK). Despite being the fastest killing isolate, HearSNPV-SP1 was as productive as isolates with lower virulence, with an average yield of 3.1 × 10(9) OBs larva(-1) . OBs of HearSNPV-SP1 and HearSNPV-G4 were similarly pathogenic against a recently established colony from southern Spain, although HearSNPV-SP1 was faster killing than HearSNPV-G4 against a range of instars.

CONCLUSION

The insecticidal properties of HearSNPV-SP1 mean that this strain is likely to prove useful as the basis for a biological insecticide for control of Helicoverpa armigera in Spain.

Complete genomic sequences and comparative analysis of Mamestra brassicae nucleopolyhedrovirus isolated in Korea

Mamestra brassicae nucleopolyhedrovirus-K1 (MabrNPV-K1) was isolated from naturally infected M. brassicae (Lepidoptera: Noctuidae) larvae in Korea. The full genome sequences of MabrNPV-K1 were determined, analysed and compared to those of other baculoviruses. The MabrNPV-K1 genome consisted of 152,710 bp and had an overall G + C content of 39.9%. Computer-assisted analysis predicted 158 open reading frames (ORFs) of 150 nucleotides or greater that showed minimal overlap. Two inhibitor of apoptosis (iap) and six baculovirus repeated ORFs were interspersed in the MabrNPV-K1 genome. The unique MabrNPV-K1 ORF133 was identified in the MabrNPV-K1 genome that was not previously reported in baculoviruses. The gene content and arrangement in MabrNPV-K1 had the highest similarity with those of Helicoverpa armigera MNPV (HearMNPV) and Mamestra configurata NPV-B (MacoNPV-B), and their shared homologous genes were 99% collinear. The MabrNPV-K1 genome contained four homologous repeat regions (hr1, hr2, hr3 and hr4) that accounted for 3.3% of the genome. The genomic positions of the four MabrNPV-K1 hr regions were conserved among those of HearMNPV and MacoNPV-B. The gene parity plot, percent identity of the gene homologues and a phylogenetic analysis suggested that these three viruses are closely related not only to each other but also to the same virus strains rather than different virus species.

Interspecific interaction between Spodoptera exigua multiple nucleopolyhedrovirus and Microplitis bicoloratus (Hymenoptera: Braconidae: Microgastrina) in Spodoptera exigua (Lepidoptera: Noctuidae) larvae

Baculoviruses and parasitoids are important biological control factors of insects in integrated pest management. Microplitis bicoloratus Chen (Hymenoptera: Braconidae: Microgastrina) is a solitary endoparasitoid of beet armyworm, Spodoptera exigua (Hübner) (Lepidoptera: Noctuidae) larvae. M. bicoloratus parasitized first to third instars of S. exigua and most effectively parasitized second instars. The survival rate of parasitoids emerging from S. exigua multiple nucleopolyhedrovirus (SeMNPV)-infected hosts decreased with increasing SeMNPV doses at second to fourth instars. In addition, the mortality rate of the host was > 80% when coinfected with SeMNPV and M. bicoloratus, regardless of virus doses or the timing of virus treatment. Occlusion body production was significantly reduced in parasitized hosts. A comparison of host weights showed that hosts coinfected with SeMNPV and M bicoloratus were significantly lighter than those infected with SeMNPV alone, suggesting that the decrease in virus yield resulted from a reduction in larval growth. The viral genome copy number in parasitized host was significantly lower than that in nonparasitized host at 48 and 72 h postinfection. These results suggest that SeMNPV and M. bicoloratus are compatible as S. exigua control agents.

In vivo characterization of a group II nucleopolyhedrovirus isolated from Mamestra brassicae (Lepidoptera: Noctuidae) in Japan

A Japanese isolate of Mamestra brassicae nucleopolyhedrovirus (MabrNPV) was identified phylogenetically as a group II nucleopolyhedrovirus (NPV) that is related closely to other NPVs isolated from Mamestra spp. based on nucleotide sequence data of its polh, egt and lef-3 genes. The multiplication of MabrNPV in M. brassicae larvae was characterized following inoculation at various doses and in combination with the fluorescent brightener Tinopal by measuring temporal changes in the concentrations of its viral DNA using real-time quantitative PCR. The growth curves of budded-virus replication were analysed by fitting the data of viral DNA concentration in the host haemolymph to a modified Gompertz model. When fifth-instar larvae were inoculated with an LD95 equivalent dose of MabrNPV and Tinopal, the time lag between the onset of primary and secondary infection was estimated to be 25 h. Another 65 h was required to reach a plateau titre equivalent to a level of 10(9) virions ml(-1) in the haemolymph. All larvae died during the sixth instar following this inoculation regime. In contrast, following inoculation with a 1000-fold higher dose of MabrNPV and Tinopal, the time lag between the onset of primary and secondary infection was only 20 h. Subsequently, the same plateau titre was reached after a further 20 h. Following this inoculation regime, most larvae died during the fifth instar. Quantification of viral DNA by real-time quantitative PCR and application of the Gompertz model are valuable for the characterization of baculovirus replication in vivo.