| Summary: | Insecticidal proteins from <i>Bacillus thuringiensis</i> (Bt) are widely used to control insect pests, but their efficacy is reduced when pests evolve resistance. We report on a novel allele (<i>r16</i>) of the cadherin gene (<i>PgCad1</i>) in pink bollworm (<i>Pectinophora gossypiella</i>) associated with resistance to Bt toxin Cry1Ac, which is produced by transgenic cotton. The <i>r16</i> allele isolated from a field population in China has 1545 base pairs of a degenerate transposon inserted in exon 20 of <i>PgCad1</i>, which generates a mis-spliced transcript containing a premature stop codon. A strain homozygous for <i>r16</i> had 300-fold resistance to Cry1Ac, 2.6-fold cross-resistance to Cry2Ab, and completed its life cycle on transgenic Bt cotton producing Cry1Ac. Inheritance of Cry1Ac resistance was recessive and tightly linked with <i>r16</i>. Compared with transfected insect cells expressing wild-type <i>PgCad1</i>, cells expressing <i>r16</i> were less susceptible to Cry1Ac. Recombinant cadherin protein was transported to the cell membrane in cells transfected with the wild-type <i>PgCad1</i> allele, but not in cells transfected with <i>r16</i>. Cadherin occurred on brush border membrane vesicles (BBMVs) in the midgut of susceptible larvae, but not resistant larvae. These results imply that the <i>r16</i> allele mediates Cry1Ac resistance in pink bollworm by interfering with the localization of cadherin.
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