Bt resistance alleles in field populations of pink bollworm from China: Similarities with the United States and decreased frequency from 2012 to 2015

Abstract

BACKGROUND: Although most monitoring of pest resistance to widely cultivated transgenic crops producing insecticidal proteins from Bacillus thuringiensis (Bt) relies on bioassays, DNA screening for alleles associated with resistance has some advantages, particularly for rare, recessively inherited resistance. In China's Yangtze River Valley, where farmers first planted transgenic cotton producing Bt toxin Cry1Ac in 2000, bioassays have been used to monitor the recessive resistance of pink bollworm (Pectinophora gossypiella). Previous bioassay results show a small but significant increase in resistance to Cry1Ac during 2008-2010, followed by a significant decrease in resistance during 2011-2015 associated with extensive planting of second-generation hybrid cotton seeds that boosted the percentage of non-Bt cotton. Here we screened DNA from 19 748 pink bollworm collected during 2012-2015 from the Yangtze River Valley for seven alleles associated with resistance to Cry1Ac. These alleles were previously identified from lab-selected strains; three from the U.S. and four from China. RESULTS: The most common resistance allele was first identified from the U.S. and accounted for over 71% of all resistance alleles detected. Resistance was rare, with the total frequency of the seven resistance alleles showing a significant, 2.3-fold decrease from 0.0105 (95% CI: 0.0084-0.0132) in 2012 to 0.0046 (0.0031-0.0067) in 2015. CONCLUSIONS: The DNA screening data confirm results from bioassays showing pink bollworm resistance to Cry1Ac remained rare in the Yangtze River Valley from 2012-2015. The prevalence in China of the resistance allele identified from the U.S. implies a shared genetic basis of resistance that could facilitate molecular monitoring of resistance.

Shared and Independent Genetic Basis of Resistance to Bt Toxin Cry2Ab in Two Strains of Pink Bollworm

“…Similar to the results showing a prominent role of mis-splicing and diverse mutations disrupting PgABCA2 in pink bollworm resistance to Cry2Ab in both lab-and field-selected resistance to Cry2Ab, mis-splicing and diverse mutations disrupting the cadherin gene PgCad1 are important in lab-and field-selected pink bollworm resistance to Cry1Ac 32,[37][38][39]43,44 . In field populations from China that have remained mostly susceptible to Cry1Ac, the most common PgCad1 resistance allele (r1) was first identified from a lab-selected strain in the United States 32 .…”

“…In field populations from China that have remained mostly susceptible to Cry1Ac, the most common PgCad1 resistance allele (r1) was first identified from a lab-selected strain in the United States 32 . However, unlike the two deletions in PgABCA2 (c1090_1234del and c.3098_3101del) associated with Cry2Ab resistance that are shared between field-selected populations from India and one or both lab-selected strains from the United States, none of the specific PgCad1 mutations in field-selected populations from India were detected in lab-selected strains from the U.S. and China 32,44 . For H. armigera from China, mutations in HaCad and a tetraspanin gene (HaTSPAN1) linked with resistance to Cry1Ac in lab-selected strains also occur in field-selected populations where early warning of resistance has been reported 15,45 .…”

Cadherin repeat 5 mutation associated with Bt resistance in a field-derived strain of pink bollworm

“…In field-selected, Cry1Ac-resistant populations from India, alleles r1-r4 identified from lab-selected strains in the U.S. were not detected in 436 individuals screened by PCR, but sequencing of DNA from eight individuals revealed eight novel, severely disrupted PgCad1 alleles (r5-r12) and 19 transcript isoforms generated by mis-splicing 25 . Whereas the low frequency of mutant PgCad1 alleles in the U.S. and China was associated with sustained susceptibility to Cry1Ac, the high frequency of such alleles in India was associated with practical resistance to Cry1Ac observed there 22,25,39,40 . These results suggest that tracking the frequency of mutant PgCad1 alleles may be useful for monitoring resistance to Cry1Ac in field populations of pink bollworm.…”

“…In DNA screening of field populations of pink bollworm that had sustained susceptibility to Cry1Ac, alleles r1, r2 and r3 were not detected in over 9,000 individuals sampled in the southwestern U.S. from 2000 to 2011, and the combined frequency of seven resistance alleles (r1-r3 and r13-r16) was 0.005 in 19,748 individuals sampled in China's Yangtze River Valley from 2012 to 2015 22,39,40 . The frequency of r14 was 0.0002 22 .…”

“…In DNA screening of field populations of pink bollworm that had sustained susceptibility to Cry1Ac, alleles r1, r2 and r3 were not detected in over 9,000 individuals sampled in the southwestern U.S. from 2000 to 2011, and the combined frequency of seven resistance alleles (r1-r3 and r13-r16) was 0.005 in 19,748 individuals sampled in China's Yangtze River Valley from 2012 to 2015 22,39,40 . The frequency of r14 was 0.0002 22 . In field-selected, Cry1Ac-resistant populations from India, alleles r1-r4 identified from lab-selected strains in the U.S. were not detected in 436 individuals screened by PCR, but sequencing of DNA from eight individuals revealed eight novel, severely disrupted PgCad1 alleles (r5-r12) and 19 transcript isoforms generated by mis-splicing 25 .…”