1. Department of Biology, University of York, Heslington, York, YO10 5DD, UK.
2. Canadian Institute for Advanced Research—Program in Evolutionary Biology, Department of Biochemistry and Molecular Biology, Dalhousie University, Halifax, Nova Scotia, B3H 4H7 Canada.
3. Institut fur Pflanzenbiologie, Universitat Zurich, CH-8008 Zurich, Switzerland.