1. Department of Biology, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599
2. Department of Biology, Indiana University, Bloomington, Indiana 47405
3. Sainsbury Laboratory, John Innes Centre, Colney, Norwich NR4 7UH, United Kingdom
4. Department of Microbiology and Immunology and Curriculum in Genetics, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599