Characterization of resistance to insecticidal Vip3A and Cry1A Bt proteins in Helicoverpa zea

Transgenic corn and cotton that produce Cry and Vip3Aa insecticidal proteins from the bacterium, Bacillus thuringiensis, have protected against lepidopteran pests in the United States since 1996. However, field populations of bollworm (Helicoverpa zea) have developed widespread resistance to the Cry proteins in transgenic Bt crops, thus Vip3Aa proteins are currently the only insecticidal proteins in Bt crops that remain effective against H. zea. Consequently, Bt crops that produce Vip3Aa proteins will become increasingly adopted by growers for the management of H. zea, and selection for resistance will accordingly increase. Therefore, it is vital to characterize the resistance mechanisms and alleles that occur in the field. This will contribute to the improvement of transgenic traits and resistance monitoring efforts to ensure the sustained efficacy of insecticidal plant-incorporated protectants. Cry-resistant (Cry-RR) and Vip-resistant (Vip-RR) H. zea strains harboring resistance alleles isolated from field-collected individuals in Texas were molecularly characterized. The Vip-RR strain displayed reduced binding of Vip3A to midgut receptors relative to susceptible insects, but the Cry-RR strain showed no signs of altered Cry1Ac-binding. Cry-resistant insects showed signs of enhanced proteolytic activity relative to susceptible insects, but no discernable differences in Cry1A processing kinetics was detected among any of the insects tested. Interestingly, genomic and transcriptomic analysis revealed amplification of a serine protease cluster in Cry-RR strain and field-collected insects that survived on Cry-expressing corn. Bulked segregant analysis revealed candidate genes in genomic regions that were consistently, highly associated with resistance in the Cry-RR and Vip-RR strains. Further investigation will confirm which candidate genes are involved in resistance. Doing so could result in the first linkage of a biochemical mechanism with a Cry or Vip3Aa resistance allele derived from field populations of H. zea. Additionally, it could be the first case in which a Vip3Aa receptor was implicated in resistance.