1. Department of Ecology and Evolutionary Biology; Yale University; New Haven Connecticut 06520
2. Current Address: The Skirball Institute for Biomolecular Medicine; New York University School of Medicine; New York New York 10016
3. Department of Microbiology; University of New Hampshire; Durham New Hampshire 03824
4. Current Address: Department of Ecology; Evolution and Natural Resources; Rutgers; The State University of New Jersey; New Brunswick New Jersey 08901
5. Coli Genetic Stock Center; Department of Molecular; Cellular and Developmental Biology; Yale University; New Haven Connecticut 06520