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Applied and Environmental Microbiology, November 1998, p. 4372-4377, Vol. 64, No. 11
0099-2240/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
Naturally Occurring Deletion Mutants Are Parasitic Genotypes
in a Wild-Type Nucleopolyhedrovirus Population of
Spodoptera exigua
Delia
Muñoz,
Juan
I.
Castillejo, and
Primitivo
Caballero*
Laboratorio de Entomología
Agrícola y Patología de Insectos (LEAPI), Departamento
de Producción Agraria, Universidad Pública de Navarra,
31006 Pamplona, Spain
Received 10 June 1998/Accepted 28 August 1998
A wild-type nucleopolyhedrovirus (NPV) isolate from
Spodoptera exigua from Florida (Se-US2) is a
variant of the SeMNPV type strain since it has a unique
DNA profile but is closely related to other known geographical
isolates of SeMNPV. It consists of several genotypic
variants, of which seven were identified in a Se-US2 virus stock by
a modification of the in vivo cloning method developed by Smith and
Crook (Virology 166:240-244, 1988). The US2A variant was the most
prevalent genotype, and it was designated the prototype Se-US2
variant, while four of the variants (US2B, US2D, US2F, and US2H) were
found at low frequency. US2C and US2E were also very abundant, and
their diagnostic bands were easily observed in wild-type isolate
restriction endonuclease patterns. The analysis of each variant,
compared to the prototype US2A, showed that US2B and US2H presented
minor differences, while US2D and US2F contained slightly larger
insertions or deletions. Variants US2C and US2E contained major
deletions of 21.1 and 14 kb, respectively, mapping at the same
genomic region (between 14.5 and 30.2 map units [m.u.] and
between 12.8 and 23 m.u., respectively). This is the first report
of such deletion mutants in a natural baculovirus population. Variants
US2A, US2B, US2D, US2F, and US2H were isolated as pure genotypes, but
we failed to clone US2C and US2E in vivo. When these two variants
appeared without apparent contamination with any other variant, they
lost their pathogenicity for Spodoptera exigua larvae. A
further biological characterization showed evidence that these two
naturally occurring deletion mutants act as parasitic genotypes in the
virus population. Bioassay data also demonstrated that pure US2A is
significantly more pathogenic against second-instar S. exigua larvae than the wild-type isolate. The need for precise genotypic characterization of a baculovirus prior to its development as
a bioinsecticide is discussed.
*
Corresponding author. Mailing address: Laboratorio de
Entomología Agrícola y Patología de Insectos,
Departamento de Producción Agraria, Universidad Pública de
Navarra, 31006 Pamplona, Spain. Fax: 34-948-169169. Phone:
34-948-169129. E-mail: PCM92{at}upna.es.
Applied and Environmental Microbiology, November 1998, p. 4372-4377, Vol. 64, No. 11
0099-2240/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
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