Comparative effects of "defective" genotypes in NPV populations: their roles in SfMNPV vs SeMNPV

Abstract

The Spodoptera exigua multiple nucleopolyhedrovirus (SeMNPV) isolate from Florida, US (SeUS2) is a complex mixture of genotypes. The genotypic structure of this isolate is very similar to that of the S. frugiperda MNPV isolated from Nicaragua (SfNIC), which is composed of at least nine genotypes. Both isolates present defective genotypes with similar deletions that lack peroral infectivity. The SfNIC defective genotype (SfNIC-C) plays a functional role in the transmissibility of the viral population since its presence increased the pathogenicity of OBs in co-occluded mixtures of complete and defective genotypes. The aim of this study was to determine the functional role of the defective genotype SeUS2-C, that is not infective per os, in the SeUS2 isolate. For this, different experimental populations were constructed that included the complete genotype SeUS2-A and the deletion mutant genotype SeUS2-C at different proportions in OB mixtures or co-occluded genotypic mixtures. The SeUS2-C was the most abundant isolated genotype in cell culture accounting for ~75% of plaques. However, the proportion of SeUS2-C in the viral population as determined by semiquantitative PCR was ~25%. SeUS2-A OBs were as pathogenic as wild-type OBs. As predicted, the potency of the mixtures of SeUS2-A OBs and SeUS2-C OBs was dependent on the proportion of SeUS2-A present in the mixture. In contrast, the co-occluded mixtures that included the defective genotype SeUS2-C at the proportions of 10 and 25%, respectively, were as pathogenic asSeUS2-A. However, the presence of a higher proportion of SeUS2-C in the mixtures reduced the biological activity of OBs. We conclude that the defective genotype SeUS2-C appears to persist as a parasitic genotype in the SeUS2 population. This contrasts with the role of defective genotypes in the SfMNPV population that potentiate OB potency.