1. Howard Hughes Medical Institute Harvard University Cambridge Massachusetts 02138 USA
2. Museum of Comparative Zoology Harvard University Cambridge Massachusetts 02138 USA
3. Department of Organismic and Evolutionary Biology Harvard University Cambridge Massachusetts 02138 USA
4. Current address: Department of Zoology and Physiology University of Wyoming Laramie Wyoming 82071 USA
5. Current address: Department of Integrative Biology University of Wisconsin–Madison Madison Wisconsin 53706 USA
6. Department of Molecular and Cellular Biology Harvard University Cambridge Massachusetts 02138 USA
7. Center for Brain Science Harvard University Cambridge Massachusetts 02138 USA