1. Department of Biological Sciences, Purdue UniversityWest Lafayette, Indiana 47906
2. Department of Organismic and Evolutionary Biology, Harvard UniversityCambridge, Massachusetts 02138
3. Department of Agronomy, Iowa State UniversityAmes, Iowa 50011
4. Waksman Institute, Rutgers The State University of New JerseyNew Jersey 08855-0758, Fax: 732-445-0072