pif1 expression determines the transmissibility of the Spodoptera frugiperda multiple nucleopolyhedrovirus

Abstract

The Nicaraguan isolate of the Spodoptera frugiperda nucleopolyhedrovirus (SfNIC) survives as a complex mixture of genotypes. Each genotype in individual infections is less pathogenic than the wild-type population. Mixtures of complete genotype B (75%) and defective genotype C (SfNIC-C) (25%) restored the biological activity to that of the wild-type. Previous studies demonstrated that pif1 and/or pif2 genes deletion mutant genotypes were necessary and sufficient to explain the increased potency of the mixtures, but the mechanism is still unknown. In the present study two recombinant viruses were constructed with SfMNPV pif1 gene expression reprogrammed und er the SeMNPV egt, SfNIC-Begt, or p10, SfNIC-Bp10, promoters. pif1 expression was advanced by 20 h in SfNIC-Begt infected larvae and was incresased in intensity at 120 and 144 h in SfNIC-Bp10 infected larvae, which suggests an increased production of this protein in infected cells. No significant differences were observed in final BV titres among the different viruses, however earlier BV production was observed in cells infected with the recombinant viruses. The increased proportion of PIF1 resulted in viruses 5 to 10 fold less pathogenic in terms of lethal concentration metrics, but the same viruses were more virulent, in terms of speed of kill, and consequently total OB production in Sf9 cells or S. frugiperda larvae was significantly reduced in recombinant viruses. Modified expression of pif1 resulted in a 50% reduction in the relative proportion of SfNIC-Begt or SfNIC-Bp10 viruses required to achive wild-type OB potencies in mixtures with SfNIC-C. SfNIC-Begt or SfNIC-Bp10 OBs did not differ in the number of nucleocapsids/ODV or in the number of ODVs/OB from that of pure genotype B. We conclude that both OB pathogenicity and virulence are highly sensitive to the concentration of PIF1.