|DENNISON, TESIA - Iowa State University|
|YANDEAU-NELSON, MARNA - Iowa State University|
Submitted to: PLOS ONE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/1/2019
Publication Date: 4/16/2019
Citation: Lopez, M.D., Dennison, T., Ward, T.M., Yandeau-Nelson, M.D., Abel, C.A., Lauter, N.C. 2019. Development and application of a quantitative bioassay to evaluate maize silk resistance to corn earworm herbivory among progenies derived from Peruvian landrace Piura. PLoS One. 14(4):e0215414. https://doi.org/10.1371/journal.pone.0215414.
Interpretive Summary: Corn earworm, Helicoverpa zea, a serious caterpillar pest of corn and other agricultural crops. Corn earworm eggs are laid by moths on emerged corn silks; the larvae begin feeding and subsequently tunnel down the silk channel where they feed on the developing kernels and cob tissue, causing serious yield and quality losses. Because the larval stage is accessible to insecticides for only a short time, pesticide applications must be timed carefully, and applied multiple times for control to be achieved. Corn with genetic sources of native resistance would be beneficial because eliminating pesticide applications reduces both financial and environmental costs. A bioassay is a laboratory test for insect feeding, and can be used to test silks for resistance to corn earworm feeding in a controlled environment. In this paper, we report the successful development of a scaled-up bioassay that is suitable for use in genetic studies of corn earworm resistance in maize. We demonstrate the utility of the bioassay by using it to characterize the genetic inheritance of a novel corn earworm resistance factor first discovered in Piura 208, a Peruvian Landrace of corn. Results from this study demonstrate that this source of resistance is amenable to deployment in corn improvement breeding, offering the opportunity to improve crop health and reduce negative environmental impact.
Technical Abstract: Corn earworm, Helicoverpa zea (Boddie), Noctuidae, (CEW) is a major insect pest of corn (Zea mays spp. mays L.), whereon its larvae cause substantial yield and quality losses by feeding on silks, kernels, and cob tissue. The long-term goal of this work is to elucidate the genetic and biochemical basis of a previously reported non-maysin source of CEW resistance first observed in silk tissue of Piura 208, a Peruvian Landrace of maize (PI 503849). To enable execution of future genomic and metabolomic experiments, we developed a quantitative CEW bioassay and tested it on four small populations that contrast alleles from Piura 208 and GT119, a susceptible maize inbred that also produces no maysin. In replicated analyses of two populations of F1:2 families, corn genotype accounts for 84% (p<0.0001) and 68% (p<0.0001) of the variance in CEW larval weights, demonstrating both the success of the quantitative bioassay and the strength of the Piura 208 resistance mechanism. Corresponding analysis of two populations of BC1:2 families revealed no statistically significant effects of corn genotype, suggesting multigenic inheritance of resistance and/or a failure to capture key resistant alleles in backcrossing, two possibilities whose contributions can’t be quantified in the study. Technical factors in bioassay performance were assessed, primarily by analyzing 1600 CEW larvae raised on control diet (meridic with no corn silks added). Minor, but statistically significant contributions to CEW weight gain variance were revealed for factors in the preparation, incubation and evaluation phases of the bioassay, demonstrating the importance of randomization, stratification, replication, and variable-tracking across the many steps of the quantitative CEW bioassay. Together, these findings indicate that the novel CEW resistance originating in Piura 208 is experimentally tractable using the bioassay as described and alleles already captured in the F1:2 families. Plans for breeding future analysis populations are discussed.