Transposon insertion causes cadherin mis-splicing and confers resistance to Bt cotton in pink bollworm from China

Authors: WANG, LING - Hebei Academy Of Agriculture; WANG, JINTAO - Hubei Academy Of Agricultural Sciences; MA, YUEMIN - Central China Normal University; WAN, PENG - Hubei Academy Of Agricultural Sciences; LIU, KAIYU - Central China Normal University; CONG, SHENGBO - Hubei Academy Of Agricultural Sciences; XIAO, YUTAO - Chinese Academy Of Agricultural Sciences; XU, DONG - Hubei Academy Of Agricultural Sciences; WU, KONGMING - Chinese Academy Of Agricultural Sciences; Fabrick, Jeffrey; LI, XIANCHUN - University Of Arizona; TABASHNIK, BRUCE - University Of Arizona

Submitted to: Scientific Reports
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/2/2019
Publication Date: 5/16/2019
Citation: Wang, L., Wang, J., Ma, Y., Wan, P., Liu, K., Cong, S., Xiao, Y., Xu, D., Wu, K., Fabrick, J.A., Li, X., Tabashnik, B.E. 2019. Transposon insertion causes cadherin mis-splicing and confers resistance to Bt cotton in pink bollworm from China. Scientific Reports. 9(1):7479.

Interpretive Summary: Transgenic cotton producing insecticidal proteins from the bacterium Bacillus thuringiensis (Bt) are cultivated extensively worldwide to control important insect pests, including the pink bollworm (Pectinophora gossypiella). However, the onset of resistance can rapidly erode the benefits of Bt crops. To date, pink bollworm resistance to Cry1Ac, one of the Bt proteins produced in Bt cotton involves changes in a cadherin gene from the U.S., India, and China. Here, an ARS scientist at Maricopa, AZ and collaborators isolate a new cadherin mutation from China, caused by insertion of mobile DNA (e.g. transposable elements) that disrupts the normal production of the cadherin protein. This r15 mutation causes production of two different variants, including one that results in production of a truncated, non-functional protein and the other that is missing a key region known to be important for binding the Cry1Ac Bt protein in the insect gut. A strain of pink bollworm harboring only this mutation shows high levels of resistance and could survive on Bt cotton producing Cry1Ac. Experiments reveal that resistance to Cry1Ac in this strain is inherited as a recessive trait and that little or no cross-resistance exists to the second Bt protein produced in Bt cotton. Experiments also reveal that the two r15 cadherin protein isoforms differed in their cellular trafficking to the cell surface in cultured insect cells. These results confirm that some mutations may lead to mis-localization of the cadherin receptor protein and can result in resistance to Bt cotton. However, each receptor protein mutation must be evaluated independently to determine how it affects cellular localization and its interaction with the Bt protein. These data reveal the complexity on how mutations in Bt cadherin receptor proteins can lead to resistance that allow important insect pests to adapt to current management practices. Such information is vital for delaying the onset of resistance and for extending the life of essential Bt technologies.

Technical Abstract: Insertion of mobile DNA can confer advantages to hosts, including resistance to synthetic and biological insecticides. Here we report a 3,370-bp insertion in a cadherin gene (PgCad1) is associated with resistance to Bacillus thuringiensis (Bt) toxin Cry1Ac in the pink bollworm (Pectinophora gossypiella), a devastating global cotton pest. This insertion is a miniature inverted repeat transposable element (MITE) that contains two additional transposons. We found the novel PgCad1 allele (r15) harboring this insertion in a field population in the Yangtze River Valley of China. It produces two mis-spliced transcript variants (r15A and r15B). The r15A transcript lacks exons 28 and 29, yielding a protein missing cadherin repeat 12. The r15B transcript lacks only exon 28, which introduces a premature stop codon and yields a truncated protein. A strain homozygous for the r15 allele had 290-fold resistance to Cry1Ac, little or no cross-resistance to Cry2Ab, and completed its life cycle on Bt cotton producing Cry1Ac. Inheritance of resistance was autosomal, recessive and tightly linked with the r15 allele. For transformed insect cells, susceptibility to Cry1Ac was greater for cells producing the wild-type cadherin than for cells producing the r15 mutant proteins. Furthermore, recombinant cadherin protein occurred on the cell surface in cells transformed with the wild-type or r15A sequences, but not in cells transformed with the r15B sequence. Consistent with previous results, these data show that some mutations affecting extracellular regions of cadherin may confer resistance associated with mis-localization of the receptor protein.