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A Polydnaviral Gene, CpBV-ELP1, Enhances an Insecticidal Activity of an Alpha-Baculovirus
Polydnavirus (PDV) is a group of double-stranded DNA insect viruses. PDV is mutualistic with some ichneumonid and braconid wasps to parasitize specific lepidopteran hosts. The viral genome is located on the wasp chromosome(s) as a proviral form and replicates only at the female reproductive organ during late pupal stage. The viral particles are accumulated in the oviduct lumen and delivered to the parasitized host along with wasp eggs during parasitization. The viral particles enter target tissues in the parasitized larvae and alter host physiological processes for the wasp development by suppressing immune responses and extending larval period. Cotesia plutellae bracovirus (CpBV) is a PDV symbiotic to C. plutellae parasitizing young larvae of the diamondback moth, Plutella xylostella. The viral particles of CpBV encode 157 open reading frames classified into different gene families. CpBV-PTP family is the largest and comprises of at least 40 gene members. CpBV-BEN, CpBV-ELP, and CpBV-IkB families also share common motifs in each gene group. In addition, two homologous genes of CpBV15α and CpBV15β are encoded in a viral genome segment. To apply these viral genes to enhance an alpha-baculovirus in insecticidal activity, they were recombined with AcNPV under a PDV promoter. As a control, under the same promoter, the recombinant baculovirus expressed an enhanced green fluorescence protein (EGFP). Upon injection or oral feeding tests, three different recombinant baculoviruses (AcNPV-ELP1, AcNPV-CpBV15α, AcNPPV-CpBV15β) enhanced the insecticidal activity compared to a control recombinant (AcNPV-EGFP). However, there was a variation in the up-regulation of the insecticidal activities among the recombinants. AcNPV-ELP1 showed the greatest potency in the insecticidal activity against the beet armyworm, Spodoptera exigua, larvae. AcNPV-ELP1 exhibited a significant variation in insecticidal activity among different larval stages of S. exigua. In the fifth instar, 1.435x107 PIB treatment of AcNPV-ELP1 showed a median lethal time at 112.7 h. ELP1 protein was detected in the hemolymph at 24 h after the viral treatment. Foliar spray of AcNPV-ELP1 was performed in pot assay and resulted in 88% control efficacy against S. exigua, while control efficacies of AcNPV-EGFP and bifenthrin (a pyrethroid insecticide) resulted in 65% and 96%, respectively. These results suggest that a PDV gene, ELP1, may be applied to develop a novel control agent by ameliorating commercial microbial insecticides or by generating transgenic crops.
