A Nicaraguan field isolate (SfNIC) of Spodoptera frugiperda nucleopolyhedrovirus was purified by plaque assay on Sf9 cells. Nine distinct genotypes, A to I, were identified by their restriction endonuclease profiles. Variant SfNIC-B was selected as the standard because its restriction profile corresponded to that of the wild-type isolate. Physical maps were generated for each of the variants. The differences between variants and the SfNIC-B standard were confined to the region between map units 9 and 32.5. This region included PstI-G, PstI-F, PstI-L, PstI-K and EcoRI-L fragments. Eight genotypes presented a deletion in their genome compared with SfNIC-B. Occlusion body-derived virions of SfNIC-C, -D and -G accounted for 41% of plaque-purified clones. These variants were not infectious per os but retained infectivity by injection into S. frugiperda larvae. Median 50% lethal concentration values for the other cloned genotypes were significantly higher than that of the wild type. The variants also differed in their speed of kill. Noninfectious variants SfNIC-C and -D lacked the pif and pif-2 genes. Infectivity was restored to these variants by plasmid rescue with a plasmid comprising both pif and pif-2. Transcription of an SfNIC-G gene was detected by reverse transcription-PCR in insects, but no fatal disease developed. Transcription was not detected in SfNIC-C or -D-inoculated larvae. We conclude that the SfNIC population presents high levels of genetic diversity, localized to a 17-kb region containing pif and pif-2, and that interactions among complete and deleted genotypic variants will likely influence the capacity of this virus to control insect pests.The family Baculoviridae comprises a large group of viruses that are pathogenic to invertebrates and are characterized by enveloped rod-shaped virions with large (85 to 166 kb) circular double-stranded DNA genomes (2). These viruses have been reported from numerous insect species, particularly Lepidoptera (28). Due to their restricted host range (16) and high pathogenicity to a number of insect pest species, they have received considerable attention as microbial insecticides (31).Baculoviruses produce two kinds of particles, one occluded and the other nonoccluded. Usually, both forms of virus are produced in a sequential manner in the same cell. When a susceptible insect consumes foliage contaminated with occlusion bodies (OBs), occlusion derived virions are released from the OB by alkali disruption of the OB protein. The occlusionderived virions infect midgut cells, resulting in a primary infection. Subsequently, nonoccluded budded virus is produced, which buds out of the infected cell to allow cell-to-cell spread of the infection throughout the host. Budded virus particles consist of a nucleocapsid containing a single genome, enveloped by a membrane in which some virus-encoded proteins are anchored. Recent work indicates that each cell within the insect is infected by an average of four to five budded viruses (3). Later, the OB is produced, which stays inside t...

Defective viruses lack genes essential for survival but they can co-infect with complete virus genotypes and use gene products from the complete genotype for their replication and transmission. As such, they are detrimental to the fitness of complete genotypes. Here, we describe a mutualistic interaction between genotypes of an insect baculovirus (nucleopolyhedrovirus of Spodoptera frugiperda (Lepidoptera)) that increases the pathogenicity of the viral population. Mixtures of a complete genotype able to be transmitted orally and a deletion mutant unable to be transmitted orally resulted in a phenotype of increased pathogenicity. Because the infectiousness of mixed genotype infections was greater than that of single genotype infections, we predict that the transmissibility of mixed genotype occlusion bodies will be greater than that of any of their single genotype components. Such interactions will be subject to frequency-dependent selection and will influence the impact of these viruses on insect population dynamics and their efficacy as biological insecticides.

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