Location: Plant Genetics Research2013 Annual Report
1a. Objectives (from AD-416):
1) Identify molecular markers associated with resistance to Cry3Bb1 Bt corn in replicated colonies of WCR populations that are reared on isoline or Bt corn. 2) Test the dominance of resistance and fitness costs to Bt corn. 3) Develop colonies with resistance to Cry34Ab1 + Cry35Ab1. 4) Evaluate the potential for cross resistance between current Bt products and stacked events.
1b. Approach (from AD-416):
A genome scan using a large number of SNP markers will be used to identify markers associated with the resistant phenotype. Crosses of resistant and susceptible lines will be developed and evaluated. We will use techniqes of Meihls et al. (2008) and Lefko et al. (2008) to develop a set of resistant colonies in both Ames and Columbia. We hope to be able to evaluate for cross resistance between Cry3Bb1 resistance and Cry34Ab1 + Cry35Ab1 resistance. This will depend on successful MTAs, but Syngenta has already agreed that we can evaluate for cross resistance between MIR 604 resistance and resistance to Event 5307.
3. Progress Report:
This work is related to sub-objective 1.A of the parent project: “Develop colonies with resistance to Cry34/35Ab1 and test the effectiveness of different refuge types to delay resistance” and contributes to Component 2A of the National Program 304 Action Plan by developing increased knowledge of the biology, ecology, behavior, genetics of pests, and plant traits conferring pest resistance. Transgenic corn has been highly effective in rootworm management until recently. In fourteen attempts, using all four proteins that are targeted for control of corn rootworm, laboratory selection efforts found resistance to the targeted proteins in just a few generations. Given these laboratory results, it is not surprising that insect resistance has developed over time in the field to the transgenic product planted over the largest land area. It is critical to stay ahead of the curve. ARS scientists in Columbia, MO conducted research aimed at understanding the nature of resistance, cross resistance, and whether or not refuges work in delaying resistance for low-dose rootworm products. This work will serve as an alert to the U.S. corn industry of potential problems. Our cross resistance work will help growers and industry to understand the transgenic options for management. In both Missouri and Iowa, a non-diapausing strain of western corn rootworm was divided into four strains: 1) Isoline, 2) Block, 3) Blend, and 4) Bt, and was maintained in the lab for 15 generations. Each strain experienced a different selection regime using two types of corn seed: Cry34Ab1 / Cry35Ab1 corn and non-Bt corn. For the Isoline strain, larva were reared on 100% non-Bt corn; for the Blend strain, larva were reared on a mix of 10% non-Bt corn and 90% Bt corn; for the Bt strain, larva were reared on 100% Bt corn. For the Block strain, larva were reared on Bt corn; however, adults emerging from these seedling mats were mixed with adults emerging from the Isoline strain in a 1:9 ratio (to replicate field conditions when a block refuge is present). During unselected generations, all strains were reared on non-Bt corn. In Iowa, F1, F3, F4, F5, F8, F9, F10, and F11, F13 and F15 were selected generations and F2, F6, F7, F12, F12 were unselected generations. In Missouri, alternate generations were selected. Measurements were taken for survival and developmental rate during each generation of selection. Plant-based bioassays were conducted following selection in the F5, F10, and F14. Adaption to Bt corn was significantly delayed with a block refuge, compared with the Bt strain, and adaptation to the blend was intermediate. Simulation studies to optimize the strategy for genome scanning for rotation-resistance in corn rootworm showed that additional genetic markers are needed to supplement the currently available markers to achieve the desired genetic power to detect selection. Specifically, the number of genetic markers should be increased 4- to 10-fold. During the past year, several projects including genome and transcriptome sequencing have generated data that will be mined for additional markers. High-throughput genotyping with a commercially available assay is pending. Consultation with a genotyping service provider indicated that a second type of assay will allow a greater number of marker loci to be analyzed per unit cost. Together, these developments will result in a much richer data set to be generated than was previously anticipated.