Department of Biosystems Science and Engineering/Swiss Institute of Bioinformatics, ETH Zurich, 4058 Basel, Switzerland, Competence Center for Systems Physiology and Metabolic Diseases, ETH Zurich, 8093 Zurich, Switzerland, Institute of Evolutionary Biology and Environmental Studies/Swiss Institute of Bioinformatics, University of Zurich, 8057 Zurich, Switzerland, Institute for Molecular Systems Biology, 8093 Zurich, Switzerland and The Santa Fe Institute, Santa Fe, 87501 New Mexico, USA.