1. Sainsbury Laboratory, University of Cambridge, Bateman Street, Cambridge CB2 1LR, UK.
2. INRA-CNRS-ENS Lyon-UCB Lyon 1, Laboratoire de Reproduction et Développement des Plantes, 46 Allée d’Italie, 69364 Lyon Cedex 07, France.
3. Division of Biology, California Institute of Technology, Pasadena, CA 91125, USA.
4. Howard Hughes Medical Institute, California Institute of Technology, Pasadena, CA 91125, USA.
5. Computational Biology and Biological Physics Group, Department of Astronomy and Theoretical Physics, Lund University, S-221 00 Lund, Sweden.