1. Department of Biology, Emory University, 1510 Clifton Road, Atlanta, GA 30322, USA.
2. Friedrich Miescher Institute, Maulbeerstrasse 66, CH-4058 Basel, Switzerland, and Institute for Experimental Ecology, Eidgenössische Technische Hochschule (ETH) Zürich, ETH Zentrum NW, CH-8092 Zürich, Switzerland.