1. Crop Performance and Improvement Division; Rothamsted; Harpenden, Herts AL5 2JQ UK
2. Institut de Biotechnologie des Plantes, UMR CNRS 8618; Université de Paris XI; Orsay cedex 91405 France
3. Biomathematics and Bioinformatics Division; Rothamsted Research; Harpenden, Hertfordshire AL5 2JQ UK