Biological and biochemical investigations on five European isolates of Mamestra brassica nuclear polyhedrosis virus

Genomics of alphabaculovirus isolates infecting Mamestra species from North America and Eurasia

Whole genome sequencing and multiplex PCR analysis were used to characterize previously isolated baculovirus isolates from Mamestra populations in Eurasia. Although these viruses have been previously described as Mamestra brassicae nucleopolyhedrovirus (MbNPV/MabrNPV), we demonstrate here that these isolates represent strains of the baculovirus species Alphabaculovirus maconfiguratae (MacoNPV-A) and Alphabaculovirus altermaconfiguratae (MacoNPV-B). The MabrNPV-Bu and -Uk isolates had 96% nucleotide (nt) identity to the type isolate MacoNPV-A 90/2 at the whole genome level and in addition contained a lef-7 homologue which is found in MacoNPV-A but not MacoNPV-B. MabrNPV-Si, -De and -Nl had 96.6, 96.6 and 98.5% nt identity to the type isolate MacoNPV-B 96/2 at the whole genome level, respectively and contained a helicase-2 homologue. Gene content, synteny and K-2-P lef-8, lef-9 and polh analysis also confirmed the presence of both MacoNPV-A and MacoNPV-B isolates in Eurasia. Thus, both these alphabaculovirus species have wide Holarctic distributions in Mamestra host species. MacoNPV-A and MacoNPV-B have wide host ranges and in addition we showed that MacoNPV-B isolates trended to higher infectivity for T. ni larvae.

Evolutionary analysis and biological characterization of a novel alphabaculovirus isolated from Mythimna separata

Baculoviruses are insect-specific pathogens. Novel baculovirus isolates provide new options for the biological control of pests. Therefore, research into the biological characteristics of newly isolated baculoviruses, including accurate classification and nomenclature, is important. In this study, a baculovirus was isolated from Mythimna separata and its complete genome sequence was determined by next-generation sequencing. The double-stranded DNA genome was 153 882 bp in length, encoding 163 open reading frames. The virus was identified as a variant of Mamestra brassicae multiple nucleopolyhedrovirus (MbMNPV) and designated Mamestra brassicae multiple nucleopolyhedrovirus CHN1 (MbMNPV-CHN1) according to ultrastructural analysis, genome comparison and phylogenetic analysis. Phylogenetic inference placed MbMNPV-CHN1 in a clade containing isolates of MacoNPV-A, MacoNPV-B and MbMNPV, which we have designated the Mb-McNPV group. The genomes of isolates in the Mb-McNPV group exhibited a high degree of collinearity with relatively minor differences in the content of annotated open reading frames. The development of codon usage bias in the Mb-McNPV group was affected mainly by natural selection. MbMNPV-CHN1 shows high infectivity against seven species of Lepidoptera. The yield of MbMNPV-CHN1 in the fourth- and fifth-instar M. separata larvae was 6.25×109-1.23×1010 OBs/cadaver. Our data provide insights into the classification, host range and virulence differences among baculoviruses of the Mb-McNPV group, as well as a promising potential new baculoviral insecticide.

Risk Assessment Studies: Detailed Host Range Testing of Wild-Type Cabbage Moth, Mamestra brassicae (Lepidoptera: Noctuidae), Nuclear Polyhedrosis Virus

The host range of a multiply enveloped nuclear polyhedrosis virus (NPV) (Baculoviridae) isolated from the cabbage moth, Mamestra brassicae (Lepidoptera: Noctuidae), was determined by challenging a wide range of insect species with high (10 polyhedral inclusion bodies) and low (10 polyhedral inclusion bodies) doses of the virus. The identity of the progeny virus was confirmed by dot blotting. Analysis of 50% lethal dose was carried out on selected species, and the progeny virus was identified by using restriction enzyme analysis and Southern blotting. Other than the Lepidoptera, none of the species tested was susceptible to M. brassicae NPV. Within the Lepidoptera, M. brassicae NPV was infective to members of four families (Noctuidae, Geometridae, Yponomeutidae, and Nymphalidae). Of 66 lepidopterous species tested, M. brassicae NPV was cross-infective to 32 of them; however, 91% of the susceptible species were in the Noctuidae. The relevance of host range data in risk assessment studies is discussed.

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.

Phenotypic and genotypic characterisation of persistent baculovirus infections in populations of the cabbage moth (Mamestra brassicae) within the British Isles

The genotypic relatedness of persistent baculovirus infections within UK populations of Mamestra brassicae was assessed by sequencing amplified regions from polyhedrin and ie1. Most populations harboured Mamestra brassicae (Mb) nucleopolyhedrosis virus (NPV) which showed very little genotypic variation between populations. However, one population harboured a virus that closely resembled a baculovirus found previously only in Pine Beauty Moth (Panolis flammea) populations in Scotland. Persistent baculoviruses that had emerged spontaneously as lethal, overt infections from two of the insect populations were compared with the type strain of MbNPV and a mixture of P. flammea (Pafl) NPV strains, isolated from a single host, by bioassay in virus-free Spodoptera exigua larvae. Reactivated baculoviruses were as pathogenic as the stock virus and showed phenotypic characteristics closest to the type strain they most resembled genetically. Sequence data from the insect host cytochrome oxidase genes were compared and showed a high degree of sequence conservation between populations and it was not possible to determine whether the persistent baculovirus infections had arisen on many occasions or whether they represented a single initial infection that had spread with the host. However, the presence of two distinct virus genotypes in separate M. brassicae populations suggests multiple colonisations of the host are a possibility.

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

The multiply embedded nucleopolyhedroviruses (MNPV) originally isolated from Mamestra brassicae (German and Dutch isolates) and Heliothis armigera have been studied comparatively to establish their relatedness, both in terms of biological activity and genomic homology. All three viral isolates replicated in M. brassicae, H. armigera, Heliothis zea, and Heliothis virescens, resulting in each case in progeny virus that was essentially similar to the inoculum. Dose-mortality studies carried out on M. brassicae and H. armigera indicate that these viruses do not differ significantly with respect to their virulence to these insects. The same studies also clearly indicate that the susceptibility of M. brassicae and H. armigera larvae to viral infection differs significantly with increasing larval age. The increase in LD(50) values from L1 to L4 is, in fact, over 40,000-fold for M. brassicae, while it is only 1300-fold for H. armigera. The results of the present study also confirm that all three isolates are genetically closely related. Due to their high degree of homology and almost identical biological activity, it is suggested that these isolates should be considered variants of a single virus species.

Conservation of polyhedrin gene promoter function between Autographa californica and Mamestra brassicae nuclear polyhedrosis viruses

The DNA sequence of the polyhedrin gene of the Mamestra brassicae multiple nucleocapsid nuclear polyhedrosis virus (MbMNPV) was determined and compared with the polyhedrin genes of Autographa californica (Ac) and Panolis flammea (Pf) MNPVs. Using this information, a transfer vector was constructed based on the EcoRI I fragment of AcMNPV in which the polyhedrin promoter was replaced by the homologous region extending 481 nucleotides upstream from the MbMNPV polyhedrin coding sequence. The Escherichia coli lacZ gene was also included downstream from the putative MbMNPV promoter. Cotransfection of this transfer vector with wild-type AcMNPV DNA produced stable recombinant viruses expressing the lacZ gene under the control of the MbMNPV polyhedrin promoter. The levels of beta-galactosidase produced by these recombinants in infected cells were 30% lower than the expression level obtained from viruses with the authentic AcMNPV promoter in front of the lacZ gene. The MbMNPV promoter has thus been shown to function efficiently in the genetic environment of AcMNPV. The implications of this finding for the release of genetically manipulated baculovirus insecticides and for the construction of baculovirus multiple expression vectors are discussed.

Cricket Paralysis Virus, a Potential Control Agent for the Olive Fruit Fly, Dacus oleae Gmel

Representatives of several families of insect viruses were tested for growth and pathogenicity in the olive fruit fly, Dacus oleae Gmel. The viruses included nuclear polyhedrosis viruses, an iridovirus, two picornaviruses, and Trichoplusia ni small RNA virus (a member of the Nudaurelia β family), in addition to two naturally occurring viruses of the olive fruit fly. Two viruses, one of the two picornaviruses (cricket paralysis virus [CrPV] and the iridovirus (type 21 from Heliothis armigera ), were found to replicate in adult flies. Flies which were fed on a solution containing CrPV for 1 day demonstrated a high mortality with 50% dying within 5 days and nearly 80% dying within 12 days of being fed. The virus was transmissible from infected to noninfected flies by fecal contamination. The CrPV which replicated in the infected flies was demonstrated to be the same as input virus by infection of Drosophila melanogaster cells and examination of the expressed viral proteins, immunoprecipitation of the virus purified from flies, and electrophoretic analysis of the structural proteins.