1. Department of Biology, University of York, York YO10 5YW
2. Plant-Soil Interactions, Scottish Crop Research Institute, Invergowrie, Dundee DD2 5DA
3. Department of Molecular and Cell Biology, University of Aberdeen Institute of Medical Sciences, Foresterhill, Aberdeen AB25 2ZD,Scotland
4. NERC Centre for Population Biology, Imperial College at Silwood Park, Ascot SL5 7PY, England