1. Department of Molecular, Cellular, and Developmental Biology, Yale University, New Haven, CT 06520, USA.
2. Department of Genetics, Yale University, New Haven, CT 06520, USA.
3. Department of Ecology and Evolutionary Biology, Yale University, New Haven, CT 06520, USA.
4. Department of Mathematics, Yale University, New Haven, CT 06520, USA.
5. Genome Research Facility, NASA Ames Research Center, Mail Stop 239-11, Moffett Field, CA 94035, USA.