1. University of Iceland, Institute of Biology, Grensásvegi 12, Reykjavík 108, Iceland, ,
2. Institute for Experimental Pathology, Keldur, Reykjavík 112, Iceland,,
3. Agricultural Research Institute, Keldnaholt, Reykjavík 112, Iceland,,
4. John Innes Centre, Norwich Research Park, Colney, Norwich NR4 7UH, UK