1. Department of Biology, University of North Carolina, Chapel Hill, NC 27599, USA.
2. Howard Hughes Medical Institute, University of North Carolina, Chapel Hill, NC 27599, USA.
3. Curriculum in Genetics and Molecular Biology, University of North Carolina, Chapel Hill, NC 27599, USA.
4. Department of Microbiology and Immunology, University of North Carolina, Chapel Hill, NC 27599, USA.
5. Carolina Center for Genome Sciences, University of North Carolina, Chapel Hill, NC 27599, USA.
6. Two Blades Foundation, 1630 Chicago Avenue, Evanston, IL 60201, USA.
7. Department of Plant and Microbial Biology, 111 Koshland Hall, University of California, Berkeley, CA 94720–3120, USA.