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Journal of Virology, May 2009, p. 5127-5136, Vol. 83, No. 10
0022-538X/09/$08.00+0     doi:10.1128/JVI.02020-08
Copyright © 2009, American Society for Microbiology. All Rights Reserved.

Mixtures of Complete and pif1- and pif2-Deficient Genotypes Are Required for Increased Potency of an Insect Nucleopolyhedrovirus{triangledown}

Gabriel Clavijo,1,2 Trevor Williams,3 Oihane Simón,1,2 Delia Muñoz,1,2 Martine Cerutti,4 Miguel López-Ferber,5 and Primitivo Caballero1,2*

Laboratorio de Entomología Agrícola y Patología de Insectos, Departamento de Producción Agraria, Universidad Pública de Navarra, Pamplona, Spain,1 Instituto de Agrobiotecnología, CSIC, Gobierno de Navarra, Mutilva Baja, Spain,2 Instituto de Ecología A.C., Xalapa, Veracruz, Mexico,3 CNRS GDR 2590, Centre de Pharmacologie et Biotechnologie pour la Santé, Laboratoire Baculovirus et Thérapie, Saint Christol-Les-Alés, France,4 Laboratoire de Gènie de l'Environnement Industriel, Ecole de Mines d'Alès, Alès, France5

Received 25 September 2008/ Accepted 17 February 2009

The insecticidal potency of a nucleopolyhedrovirus population (SfNIC) that infects Spodoptera frugiperda (Lepidoptera) is greater than the potency of any of the component genotypes alone. Occlusion bodies (OBs) produced in mixed infections comprising the complete genotype and a deletion genotype are as pathogenic as the natural population of genotypes from the field. To test whether this increased potency was due to the deletion or to some other characteristic of the deletion variant genome, we used the SfNIC-B genome to construct a recombinant virus (SfNIC-B{Delta}16K) with the same 16.4-kb deletion as that observed in SfNIC-C and another recombinant (SfNIC-B{Delta}pifs) with a deletion encompassing two adjacent genes (pif1 and pif2) that are essential for transmission per os. Mixtures comprising SfNIC-B and SfNIC-B{Delta}16K in OB ratios that varied between 10:90 and 90:10 were injected into insects, and the progeny OBs were fed to larvae in an insecticidal potency assay. A densitometric analysis of PCR products indicated that SfNIC-B was generally more abundant than expected in mixtures based on the proportions of OBs used to produce the inocula. Mixtures derived from OB ratios of 10, 25, or 50% of SfNIC-B{Delta}16K and the corresponding SfNIC-B proportions showed a significant increase in potency compared to SfNIC-B alone. The results of potency assays with mixtures comprising various proportions of SfNIC-B plus SfNIC-B{Delta}pifs were almost identical to the results observed with SfNIC-B{Delta}16K, indicating that deletion of the pif gene region was responsible for the increased potency observed in mixtures of SfNIC-B and each deletion recombinant virus. Subsequently, mixtures produced from OB ratios involving 10 or 90% of SfNIC-B{Delta}16K with the corresponding proportions of SfNIC-B were subjected to four rounds of per os transmission in larvae. The composition of each experimental mixture rapidly converged to a common equilibrium with a genotypic composition of ~85% SfNIC-B plus ~15% SfNIC-B{Delta}16K. Nearly identical results were observed in peroral-passage experiments involving mixtures of SfNIC-B plus SfNIC-B{Delta}pifs. We conclude that (i) the deletion of the pif1 and pif2 region is necessary and sufficient to explain the increased potency observed in mixtures of complete and deletion genotypes and (ii) viral populations with decreased ratios of pif1- and pif2-deficient genotypes in the virus population increase the potency of genotypic mixtures and are likely to positively influence the transmission of this pathogen.

* Corresponding author. Mailing address: Departamento de Producción Agraria, Universidad Pública de Navarra, 31006 Pamplona, Spain. Phone: 34 948 169 129. Fax: 34 948 169 732. E-mail: pcm92{at}unavarra.es

{triangledown} Published ahead of print on 4 March 2009.

Journal of Virology, May 2009, p. 5127-5136, Vol. 83, No. 10
0022-538X/09/$08.00+0     doi:10.1128/JVI.02020-08
Copyright © 2009, American Society for Microbiology. All Rights Reserved.





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