1. Department of Plant and Microbial Biology, University of California, Berkeley, CA, 94720-3102
2. Département Génétique et Ecologie Evolutives Laboratoire Ecologie, Systématique et Evolution, CNRS-UPS-AgroParisTech, Université de Paris-Sud, 91405 Orsay, France, and Department of Microbiology and Immunology, Montana State University, Bozeman, MT 59717
3. Department of Plant and Microbial Biology, University of California, Berkeley, CA 94720-3102
4. INRA, UMR BGPI, Campus International de Baillarguet, 34398 Montpellier, France