1. X. Cui, Department of Zoology and Genetics, Iowa State University, Ames, IA 50011, USA.
2. R. P. Wise, Field Crops Research, USDA Agricultural Research Service, and Department of Plant Pathology, Iowa State University, Ames, IA 50011, USA.
3. P. S. Schnable, Department of Agronomy and Department of Zoology and Genetics, Iowa State University, Ames, IA 50011, USA.