Research at the Interface of Agriculture and Genetics

Susan Ratcliffe

Extension Specialist, Assistant Professor, Department of Crop Sciences

Ph.D., University of Illinois

Evolution in an Illinois Cornfield?

Natural selection plays the central role in shaping the biological world, yet its glacial pace makes the evolutionary change occurring around us appear all but invisible. Occasionally, exceptional circumstances permit us to witness the process of natural selection. Such circumstances exist today in east central Illinois; the behavior of the western corn rootworm, Diabrotica virgifera virgifera LeConte, an important pest of corn is changing under intensive selection by crop rotation circumventing the most cost effective and environmentally benign management tool. Western corn rootworm rotation resistance is fundamentally a problem of movement: egg laying females now disperse from cornfields to deposit their eggs in soybeans and other crops rotated with corn. Although the scientific literature is replete with examples of insects becoming resistant to insecticides, including corn rootworms, the adaptation to a cultural practice is exceedingly rare.

The debate within the entomological community rages regarding the fundamental explanation for the egg-laying shift of western corn rootworms away from the restrictive relationship with corn to include that of other crops, primarily soybeans. One theory suggests that the rigid cultural practice of crop rotation has placed selection pressure on the east central Illinois population of western corn rootworms for at least the last two decades eventually triggering the behavioral abandonment of corn as the primary egg-laying site. Another contemporary theory suggests that the agronomic trend towards earlier corn planting dates (along with the use of earlier maturing corn hybrids) has resulted in corn becoming a less attractive egg-laying target as compared with nearby soybean fields late in the summer. In the mid-1980s, WCR larval injury was confirmed in seed-production cornfields near Piper City, Illinois, located in Ford County. Seed production cornfields are generally harvested earlier than commercial cornfields. This area of Illinois may have served as the evolutionary cradle of this new WCR variant because of the intensity of crop rotation and the early-maturing characteristics of nearby seed-production cornfields.

A diverse group of researchers with expertise in entomology, atmospheric science, mathematical modeling, and genetics have been jointly funded by C-FAR to explore the collapse of crop rotation as a cultural control practice. Based on ecological and behavioral research conducted by Dr. Michael Gray, Professor, Department of Crop Sciences, University of Illinois and Dr. Joseph Spencer, Assistant Professional Scientist, Center for Economic Entomology, Illinois Natural History Survey, differences in flight, feeding, and ovipositional preferences occur in western corn rootworm populations. From the standpoint of explaining the movement of WCR between corn and soybean fields, corn developmental stages and soybean herbivory appear to influence this behavior. Dr. Spencer and his graduate student, Tim Mabry, have observed increased activity and flightiness following even brief periods of soybean herbivory. Graduate research assistants, Silvia Rondon and Chris Pierce, with Dr. Gray have found western corn rootworms in east central Illinois will lay eggs in a variety of cropping systems and early-planted corn appears to be a less attractive ovipositional site. Microarray profiling will allow us to evaluate gene expression variation between the two populations and possibly identify the mechanisms controlling these behaviors. Hopefully, we can return the promise of crop rotation as a viable pest management tool to producers throughout the eastern Corn Belt by linking specific genes to environmentally induced behavior.

Drs. Lei Liu, Director of Bioinformatics, Jose Pardinas, Director of High Throughput Sequencing and Genotyping at the W.M. Keck Center for Comparative and Functional Genomics, Susan Ratcliffe, Assistant Professor, Department of Crop Sciences, and Hugh Robertson, Professor, Department of Entomology, University of Illinois Urbana-Champaign plan to generate a western corn rootworm Expressed Sequence Tag (EST) database. Dr. Pardinas and Rachel Schwartz, Research Specialist, constructed the library from heads of gravid females exhibiting the shift in egg-laying behavior and from those not exhibiting the shift in behavior. Each population is tagged to preserve its population identity. Sequencing from the normalized library is currently underway and we currently have 5,500 unique sequences from the sequencing of 12,000 clones. Following a library subtraction, an additional 5,000 clones are being sequences and we estimate that will result in an additional 2,000 unique sequences. Dr. Liu will continue supervise gene annotation of the western corn rootworm EST and we predict many of the WCR ESTs will have high similarity with Drosophila and honey bee genes. Based on data from Dr. Gene Robinson's honey bee project, mircoarray profiling of the western corn rootworm EST should result in expression differences between the two population types. Dr. Mark Band, Director of Functional Genomics, W.M. Keck Center will collaborate with other members of the group on this portion of the project to identify candidate genes for future research based on gene expression. Additional collaborative relationships are in the developmental stage to maximize the EST resource.

This project was supported by a Sentinel Grant from the Illinois Council on Food & Agricultural Research (C-FAR).