Variation in the cadherin gene sequence of Cry1Ac susceptible and reistant Helicoverpa armigera (Lepidoptera:Noctuidae) and the identification of mutant alleles in resistant strains.

Abstract

Variations in the cadherin gene sequence between Cry1Ac-resistant BH-R strain showing 227.9-fold resistance and the wild susceptible VA-S strain of Helicoverpa armigera were identified. Amplification of exon 26 from H. armigera cadherin gene identified two mutant alleles, r9 (553 bp) and r10 (717 bp) against the wild S allele (588 bp). Sequence analysis of r9 showed a deletion of 45 bp in the intron region and mutation in the 5′-splice site of exon 26 resulted in an additional amino acid. While in the r10 allele a premature stop codon resulted in the putative translation into two different proteins of 1343 and 335 amino acids relative to a single protein of 1756 amino acids of the susceptible strain. The segregation of these mutant alleles was examined in F2 progenies derived from matings of Cry1Ac-resistant and susceptible individuals and found to be associated with Mendelian principles. Cadherin genotyping showed that resistance to Cry1Ac, survival on Cry1Ac-treated diet and frequency of mutant r allele of cadherin gene were high in the selected strain than the unselected strain and were completely absent from susceptible VA-S strain. DNA-based screening of H. armigera collected in India failed to detect a single r9 or r10 allele in the populations collected from vegetable-growing areas of Delhi. However, very low frequency of mutant alleles was detected in Bt cotton-growing areas of Anand, indicating that these mutations are likely to be rare in the field.