Host range and gene contents of Bacillus thuringiensis strains toxic towards Spodoptera exigua
2000
Porcar, Manuel | Martínez, Clara | Caballero, Primitivo
Thirty‐five strains of the entomopathogenic bacterium Bacillus thuringiensis active on Spodoptera exigua, were characterized by means of serological identification and determination of cry gene contents by PCR. The insecticidal activity of these 35 strains was further confirmed against S. exigua and tested against two other species of the same genus: S. littoralis and S. frugiperda. The results indicate that serovars aizawai, thuringiensis, and kurstaki were the most frequent within S. exigua ‐active strains and that serovar aizawai had the highest number of strains exhibiting toxicity against the three species bioassayed. The presence in cry genes as determined by PCR suggests a non random distribution of some cry genes among serovars. Genes cry1C, cry1D, and cry1E, which are known to code for proteins toxic against Spodoptera species, were very common within S. exigua‐active strains, specially in those belonging to serovar aizawai. However, some strains harbouring one or more of these genes were not toxic to S. littoralis or S. frugiperda; and some strains lacking all of the Spodoptera‐active genes were found to be toxic to all three species. This suggests differences in the expression levels among strains bearing toxic genes and the involvement of other genes toxic to Spodoptera species. Since strains sharing the same cry genes exhibited different host ranges, the results indicate the need to perform toxicity bioassays in addition to other tests (serological identification and PCR) in order to determine the insecticidal activity of B. thuringiensis strains.
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