Location: Southern Insect Management Research Unit
2012 Annual Report
Objective 2: Determine gene flow and migration patterns by analyzing tobacco budworm and bollworm populations in temporal and spatial scales using genetic and/or empirical/mathematical approaches.
Objective 3: Identify possible mechanisms of resistance to Bt toxins by profiling gene expression patterns and develop a marker based genetic linkage map.
Corn fields were surveyed after harvest to assess the density and maturity of volunteer corn in the fall as well as the abundance of corn earworm in non-Bt corn at various growth stages. Greenhouse and field trials examining the efficacy of various crosses of Bt and non-Bt corn varieties on bollworm were conducted during last year and are being repeated during 2012. Helicoverpa zea colonies with increased tolerance to Cry2Ab and Cry 1Ac were established from insects collected from field locations in the Mississippi Delta. Back-cross genetic mapping populations were developed by mating these colonies with insects from a susceptible laboratory colony maintained at SIMRU and DNA extractions were completed from 96 insects for use in a marker based mapping study.
Nucleotide sequences from 14 DNA pools containing 36,500 clones of a bacterial artificial chromosome (BAC) library were obtained using high-throughput sequencing technology. The sequence reads were assembled to identify candidate Bt resistance genes and genetic markers associated with them. Nucleotide sequences from bollworm BAC clones and transcriptome are also being used to complete assembly and annotation of the bollworm genome by CSIRO, Australia.
Laboratory colonies of Bt resistant and susceptible Old World bollworm, Helicoverpa armigera, exposed to Cry1Ac were used to study the time course of midgut gene expression. RNA extracted from midguts of treated insects was used to obtain short nucleotide reads for RNA-Seq profiling. Validation of differential expression patterns observed in RNA-Seq experiments is in progress.
Computer model simulations examining the risk of corn earworm resistance development on corn expressing two Bt genes versus those expressing a single gene were carried out. At least 350 replicate simulations with randomly drawn parameters were completed for each of four risk assessments. When dual-gene Bt-cotton, planted with a natural refuge and single-gene corn planted with a 50% refuge was simulated, resistance to both toxins simultaneously never occurred within 30 years, but in 38.5% of simulations, resistance evolved to toxin present in single-gene Bt-corn (Cry1A). When both corn and cotton were simulated as dual-gene products, cotton with a natural refuge and corn with a 20% refuge, 3% of simulations evolved resistance to both toxins simultaneously within 30 years, while 10.4% of simulations evolved resistance to the Cry1A toxin.
Luttrell, R.G., Jackson, R.E. 2012. Helicoverpa zea and Bt Cotton in the United States. GM Crops and Food Biotechnology in Agriculture and the Food Chain. 3(3):213-227.
Perera, O.P., Blanco, C. 2011. Microsatellite variation in Helicoverpa zea (Boddie) populations in the southern United States. Southwest Entomology. 36(3):271-286.
Perera, O.P., Blanco, C.A., Ballard, L., Silva-Brandao, K.L., Domingues, F.A., Abel, C.A. 2011. Evaluation of anonymous and expressed sequence tag derived polymorphic microsatellite markers in the tobacco budworm Heliothis virescens (Lepidoptera: noctuidae). Southwestern Entomologist. 36(3):287-294.