1. Department of Ecology and Evolution, The University of Chicago, Chicago, IL
2. Present address: Division of Biological Systems and Engineering, Lawrence Berkeley National Laboratory; Department of Molecular Biology and Cell Biology, University of California, Berkeley
3. Department of Neurobiology, The University of Chicago, Chicago, IL
4. Present address: Committee on Genetics, Genomics & Systems Biology, The University of Chicago, Chicago, IL
5. Department of Biology, Wayne State University, Detroit, MI
6. Department of Statistical Genetics and Bioinformatics, North Carolina State University, Raleigh, NC