1. Department of Biology, West Virginia University , Morgantown, WV 26506 , USA
2. Department of Forest Ecology & Genetics, CIFOR-INIA, CSIC , Madrid 28040 , Spain
3. Warnell School of Forestry and Natural Resources, Department of Genetics, and Department of Plant Biology, University of Georgia , Athens, GA 30602 , USA
4. Department of Energy Joint Genome Institute , Berkeley, CA 94720 , USA
5. Poplar Innovations , Camas, WA 98607 , USA
6. Biosciences Division, Center for Bioenergy Innovation, Oak Ridge National Laboratory , Oak Ridge, TN 37830 , USA