1. Department of Microbiology and Immunology, University of Miami School of Medicine, Miami, FL 33136, USA
2. Max-Plank–Institut für Biologie, Abteilung Immungenetik, Corrensstr. 42, D–72076 Tübingen, Germany
3. Coordination Center for Research and Education, The Graduate University for Advanced Studies, Hayama, Kanagawa 240–01, Japan
4. Department of Biology, Queen's University, Kingston, Ontario, Canada K7L 3N6, Canada
5. Department of Ecology and Evolutionary Biology, Princeton University, Princeton, NJ 08544–1003, USA