1. Biology Dept., Open University, Walton Hall, Milton Keynes, MK7 6AA, UK,
2. Departments of Virology and Microbial Genetics, Institute of Medical Biology, University of Tromso, Norway
3. Institute for Applied Ecology, Postfach 6226, DE-79038 Freiburg, Germany
4. Foetal and Infant Toxipathology, University of Liverpool, Liverpool L69 3BX, UK
5. Postfach 130165, 53061 Bonn, Germany