Identification of molecular mechanisms of resistance to transgenic maize producing the Cry1Fa protein in different Spodoptera frugiperda populations

The use of transgenic maize event TC1507 producing the Cry1Fa insecticidal protein from the bacterium Bacillus thuringiensis has been very useful to control the target lepidopteran insect pest Spodoptera frugiperda. The extensive use of this transgenic maize event resulted in high selection pressure for development of Cry1Fa resistance in field insects in multiple locations, including Brazil, Puerto Rico and the southeastern USA. Resistance developed in Puerto Rico was characterized in the 456LS3 strain originated from that island as monogenic, autosomal recessive and associated with reduced expression of a membrane bound alkaline phosphatase (SfmALP2). Currently, we focused on identifying the allele responsible for resistance in the 456LS3 and in alternative Cry1Fa-resistant strains from Florida (FL39) and North Carolina (NCBt13).

Linkage tests between resistance to TC1507 and reduced levels of SfmALP2 in 456LS3 supported absence of a genetic linkage. We identified an allele of an ATP-binding cassette subfamily C2 (SfABCC2) gene resulting in a premature stop codon and truncated protein. We tested linkage of this allele (SfABCC2mut) with resistance to TC1507 and found it cosegregated with resistance to Cry1Fa maize. The SfABCC2mut allele was also detected in field populations of S. frugiperda from Puerto Rico. Additional SfABCC2 mutant alleles were identified from S. frugiperda surviving on TC1507 plants in Puerto Rico.

Characterization of a TC1507- resistant strain of S. frugiperda originated from Florida (FL39), suggested a shared resistance locus with 456LS3. We found lack of linkage between reduced SfmALP2 levels and resistance to TC1507 in FL39, and detected another variant of the SfABCC2 gene (SfABCC2mut3) that showed strong linkage with Cry1Fa maize resistance.

An additional mutant allele of SfABCC2 resulting in a truncated protein was detected in the NCBt13 resistant strain from North Carolina.

Our findings support linkage of different alleles of a single gene (SfABCC2) with resistance to Cry1Fa maize that evolved in diverse locations in North America. This information and the genotyping methods developed in this work are relevant to monitor and understand dispersal of resistance, and to guide refinement of current resistance management practices to extend the sustainable use of transgenic maize producing Cry insecticidal proteins.