Selection and molecular characterization of Bacillus thuringiensis strains efficient against soybean looper (Chrysodeixis includens) and Spodoptera species

ABSTRACT Soybean looper (Chrysodeixis includens) and Spodoptera group are important defoliators insects responsible for significant yield losses in soybean, cotton and maize in Brazil. Bioinsecticides and transgenic plants expressing insecticidal proteins from Bacillus thuringiensis are among the most used sustainable control methods for pest control in agriculture, especially for several lepidopteran species. To provide new components for insect control, this study aimed to select and characterize B. thuringiensis strains toxic to C. includens, S. cosmioides, S. eridania, and S. frugiperda. We performed initial bioassays with fifty B. thuringiensis strains for C. includens and selected four strains - 1608A, 726, 773, and 775E - that caused high larval mortality (100%) to be tested against Spodoptera species. These strains harbored cry insecticidal genes, megaplasmids, absence of β-exotoxin and showed two major proteins about 65 and 130 kDa in SDS-page attributed to Cry protein classes toxic to lepidopteran species. The 1608A and 775E strains also showed high toxicity to Spodoptera species that demonstrate great potential for B. thuringiensis-based bioproduct development and as sources of new insecticidal genes for transgenic crops for multiple pest control relevant in the Brazilian agricultural system.

Aislamiento y caracterización molecular, ribotipificación, de cepas nativas de Bacillus thuringiensis aisladas en suelos y larvas muertas de Hylesia metabus en la región nororiental de Venezuela / Isolation and molecular characterization, ribotyping, of Bacillus thuringiensis native strains isolated from soils and Hylesia metabus death larvae at northeast region of Venezuela

Bacillus thuringiensis es una bacteria esporogénica grampositiva del grupo B. cereus, caracterizada por la producción de δ-endotoxinas codificadas por una familia de genes cry, mayoritariamente plasmídicos con actividad patógena específica contra larvas de insectos. Esta propiedad ha sido usada en la formulación de productos comerciales para control de insectos plaga como Hylesia metabus “palometa peluda” que afecta a los pobladores de la región nororiental de Venezuela. En el Centro Venezolano de Colecciones de Microorganismos se ha iniciado la construcción de una colección de cepas nativas de B. thuringiensis, a partir de muestras de suelos y larvas muertas de H. metabus colectadas en zonas afectadas, que pudiera ser utilizada en la elaboración local de bioinsecticidas para control de ésta y otras plagas. El empleo de condiciones selectivas de crecimiento, pruebas bioquímicas complementarias y presencia de inclusiones parasporales, seguido de la caracterización molecular empleando la técnica de ribotipificación automatizada, permitió la identificación de 11 cepas nativas de B. thuringiensis. La variedad de ribotipos encontrados refleja la heterogeneidad genética de B. thuringiensis en la muestra examinada.

Bacillus thuringiensis is a Gram positive spore-forming bacteria of the B. cereus group, that produces δ-endotoxins, encoded mainly by cry genes, with pathogenic specific activity against some insect larvae. This property has been used in products applied to the control of Hylesia metabus, a pest known as “palometa peluda” that affects living population in the northeast region of Venezuela. In the Venezuelan Center for Culture Collections we have initiated the construction of a collection of B. thuringiensis native strains isolated from soils and H. metabus death larvae that could be applied in pest control. The use of selective growth conditions, biochemical tests and detection of parasporal inclusions, followed by ribotyping methods, allowed the identification of 11 B. thuringiensis native strains. Variety of ribotypes shows the genetic heterogeneity of B. thuringiensis identified.