Whole genome sequence and comparative genomic sequence analysis of Helicoverpa armigera nucleopolyhedrovirus (HearNPV-L1) isolated from India

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.

Genome sequence analysis of a Helicoverpa armigera single nucleopolyhedrovirus (HearNPV-TR) isolated from Heliothis peltigera in Turkey

The entire genome of Helicoverpa armigera single nucleopolyhedrovirus (HearNPV-TR) was sequenced, and compared to genomes of other existing isolates. HearNPV-TR genome is 130.691 base pairs with a 38.9% G+C content and has 137 open reading frames (ORFs) of ≥ 150 nucleotides. Five homologous repeated sequences (hrs) and two baculovirus repeated ORFs (bro-a and bro-b) were identified. Phylogenetic analysis showed that HearNPV-TR is closer to HaSNPV-C1, HaSNPV-G4, HaSNPV-AU and HasNPV. However, there are significant differences in hr3, hr5 regions and in bro-a gene. Pairwise Kimura-2 parameter analysis of 38 core genes sequences of HearNPV-TR and other Helicoverpa NPVs showed that the genetic distances for these sequences were below 0.015 substitutions/site. Genomic differences as revealed by restriction profiles indicated that hr3, hr5 regions and bro-a gene may play a role in the virulence of HearNPV-TR.

Single nucleotide polymorphism (SNP) frequencies and distribution reveal complex genetic composition of seven novel natural isolates of Cydia pomonella granulovirus

The co-evolution between baculoviruses and their insect hosts results in selection of virus populations. To explore this phenomenon at the molecular level, seven natural isolates of Cydia pomonella granulovirus (CpGV) collected from orchards in northwest China were studied using Illumina next generation sequencing (NGS). A total of 540 genome positions with single nucleotide polymorphisms (SNPs) were detected in comparison with known CpGV isolates. New members of previously defined phylogenetic genome groups A, D and E of CpGV, as well as two novel phylogenetic lines, termed genome group F and G, were identified. Combining SNP frequency distribution with the prevalence of genome group-specific SNPs, revealed that six isolates of CpGV were mixtures of different ratios of at least two genotypes, whereas only one isolate, CpGV-WW, was genetically highly homogeneous. This study significantly extends our current understanding of the genetic diversity of CpGV and opens new lines of application of this virus.