Screening for resistance alleles to Cry1 proteins through targeted sequencing in the native and invasive range of Spodoptera frugiperda (Lepidoptera: Noctuidae)

Abstract

The fall armyworm, Spodoptera frugiperda (J. E. Smith), is a highly polyphagous pest native to the tropical Americas that has recently spread to become a global super-pest threatening food and fiber production. Transgenic crops producing insecticidal Cry and Vip3Aa proteins from Bacillus thuringiensis (Bt) are used for control of this pest in its native range. The evolution of practical resistance represents the greatest threat to sustainability of this technology and its potential efficacy in the S. frugiperda invasive range. Monitoring for resistance is vital to management approaches delaying S. frugiperda resistance to Bt crops. DNA-based resistance screening provides higher sensitivity and cost-effectiveness than currently used bioassay-based monitoring. So far, practical S. frugiperda resistance to Bt corn-producing Cry1F has been genetically linked to mutations in the SfABCC2 gene, providing a model to develop and test monitoring tools. In this study, we performed targeted SfABCC2 sequencing followed by Sanger sequencing to confirm the detection of known and candidate resistance alleles to Cry1F corn in field-collected S. frugiperda from continental USA, Puerto Rico, Africa (Ghana, Togo, and South Africa), and Southeast Asia (Myanmar). Results confirm that the distribution of a previously characterized resistance allele (SfABCC2mut) is limited to Puerto Rico and identify 2 new candidate SfABCC2 alleles for resistance to Cry1F, one of them potentially spreading along the S. frugiperda migratory route in North America. No candidate resistance alleles were found in samples from the invasive S. frugiperda range. These results provide support for the potential use of targeted sequencing in Bt resistance monitoring programs.

Exposure to Cry1 Toxins Increases Long Flight Tendency in Susceptible but Not in Cry1F-Resistant Female Spodoptera frugiperda (Lepidoptera: Noctuidae)

“…Resistance to Cry1F corn originally described in Puerto Rico [19] was described in the parental strain of 456LS4D and linked to an allele found at a very high frequency (0.4) in the island [23,24]. However, substantial genotyping efforts by multiple groups have failed to detect the resistance allele outside of Puerto Rico, even at Caribbean and migratory destinations in Florida [23,24,26,30]. While speculative, it is plausible that the lack of increased average flight distance in 456LS4D moths after sublethal exposure of larvae to Cry1F helps explain the lack of spread of this resistance allele through migratory pathways.…”

“…This lack of fitness costs may enable the maintenance of standing genetic variation in S. frugiperda populations, contributing to the fast development and continued high frequency of resistance observed in Puerto Rico, even after voluntary withdrawal of Cry1F corn from the local market [23,29]. Contrary to the expected migration of S. frugiperda island-hoping through the Caribbean into Florida [12], the allele linked to Cry1F resistance in Puerto Rico is not found elsewhere [24,26,30]. However, the potential effects of resistance to Cry1F on S. frugiperda flight tendency have not been addressed to date.…”