||Environmental Protection Agency, Cincinnati, OH. National Exposure Research Lab. ;Dynamac International, Inc., Rockville, MD.;Environmental Protection Agency, Washington, DC. Office of Research and Development.
Genetically modified (GM) crops have been used on a widespread commercial basis for over 10 years. The US EPA is charged with regulating GM crops which contain plantincorporated protectants (PIPs). Among these products, a relatively new form of Bt corn has a great potential to reduce the amount of traditional chemical pesticides used to control the major pest, Diabrotica virgifera virgifera, the western corn rootworm (WCR). The EPA requires commercial Bt corn growers to follow insect resistance management (IRM) plans to slow resistance development. Should insects develop resistance, growers will be forced to return to environmentally harmful chemical pesticides such as organophosphates and carbamates. To be effective, insect resistance management plans must be based on the best science available. Biological and genetic data for WCR are limited and thus pose a risk to successful deployment of IRM plans. This research attempts to more fully elucidate the underlying genetics of WCR that relate to development of resistance to Bt corn. The overarching goal is to assess the benefit to EPA of collecting detailed biological and genetic data on targeted pest organisms to better inform resistance development models that form the basis of IRM plans. Herein, we present two of several approaches we have undertaken to assess the genetic architecture of the western corn rootworm. One approach is based on analysis of genetic loci called microsatellites to assess the population genetic structure at neutral genes that are not expected to be under selection. These genetic markers tell us about basic population parameters of WCR, including gene flow patterns and effective population size. We evaluated microsatellite patterns for WCR specimens sampled across their US range: from Colorado to New York and from South Dakota to Texas, over three consecutive years. Effective population sizes estimated at most sites were small, with substantial amounts of immigration (generally greater than
10% immigrants). Two sites near the eastern and western edges of the WCR distribution expressed much smaller immigration (less than 1% immigrants). Allele frequencies at most sites were generally very similar, which is consistent with high levels of gene flow. However, sites in the Southwest (Texas, New Mexico) were differentiated from other sites, suggesting stronger barriers to dispersal. The second approach is to evaluate a candidate gene that is expected to be under selection for resistance to Bt corn. We isolated a cadherin-like gene from WCR midgets that is closely related to genes that have been shown to be associated with Bt resistance in Lepidoptera. The genomic sequence of the gene was analyzed, including the promoter region and all intron-exon boundaries. Two regions of the gene were identified that have strong homology to Bt binding regions of lepidopteran cadherins. Microsatellite sequences within the introns were used to develop genetic markers for monitoring allele frequencies in the field. Genetic structure for the cadherinlike locus was similar to neutral microsatellite loci, although differentiation between Midwestern and Southwestern areas appeared to be stronger. The allelic screen we have developed is a potentially powerful method for monitoring resistance development in the field.