1. Department of Genetics, Trinity College, Dublin 2, Ireland
2. School of Archaeology and Palaeoecology, Queen's University of Belfast, Belfast BT7 1NN, Northern Ireland
3. Department of Morphology, Genetics and Aquatic Biology, Norwegian College of Veterinary Medicine, PO Box 8146, Dep. N-0033 Oslo, Norway
4. Agricultural Research Institute, Keldnaholt, 112 Reykjavik, Iceland