Genetic and biological variation among nucleopolyhedrovirus isolates from the fall armyworm, Spodoptera frugiperda (Lepidoptera: Noctuidae)

A PCR-based method was used to identify and distinguish among 40 uncharacterized nucleopolyhedrovirus (NPV) isolates from larvae of the moth Spodoptera frugiperda that were part of an insect virus collection. Phylogenetic analysis was carried out with sequences amplified from two strongly conserved loci (polh and lef-8) from the 40 isolates in the collection and from eight previously studied S. frugiperda NPV (SfMNPV) isolates. To further distinguish these isolates, analysis was also carried out with sequences from two less-conserved loci, hr4 and hr5. Phylogenetic inference from the sequence data could distinguish among several of the individual isolates and between different groups of isolates from Georgia (USA) and Colombia, South America. A stronger degree of bootstrap support for the phylogenetic trees was obtained with the hr4 and hr5 homologous repeat sequences. Sequencing and phylogenetic analysis detected a relatively high degree of larva-to-larva sequence divergence occurring among isolates of SfMNPV collected from the same field in Missouri, USA. Restriction endonuclease analysis of viral DNA from larvae infected with five isolates from Georgia, Missouri, Louisiana, Florida (USA), and Colombia allowed for further comparison with other previously reported isolates of SfMNPV. Bioassays with these five geographically distinct isolates detected minor differences in virulence. This study highlights the use of PCR to rapidly distinguish and characterize large numbers of historical baculovirus isolates from the same host using minimal quantities of material, and the use of sequences from homologous repeat regions to distinguish closely related isolates of the same NPV species.