31 Airborne Migration of Obligate Nomads Demonstrates Gene Flow Across Eurasia Eckhard Limpert 1 *, Klaus Ammann 2 , Pavel Bartos 3 , Werner K. Graber 4 , Gerhard Kost 5 , Jacques G. Fuchs 1 ,4 1 Phytopathology, Institute of Plant Sciences, Swiss Federal Institute of Techno- logy Zurich (ETH), 8092 Ziirich, Switzerland 2 Botanical Garden, Bern, Switzerland 3 Research Institute of Crop Production, 161 06 Praha Ruzyne, Czech Republic 4 Paul Scherrer Institute, 5232 Villigen, Switzerland 5 Philipps-University Marburg, Special Botany and Mycology, Karl-von-Frisch Str., 35032 Marburg, Germany *Present address: Lecturer Aerobiology, University Zurich, Scheuchzerstr. 210, 8057 Zurich, Switzerland, E-mail eckhard.limpert@bluewin.ch 31.1 Abstract Understanding migration is important for the adequate use of biological resources. A new level of understanding is demonstrated with cereal pathogens recognized to be obligate nomads of the atmosphere. From basic reasoning, a hypothesis is put forward: virulence complexity, i.e. the number of virulences per pathogen geno- type, is expected to increase in the direction of predominant winds. The hypothesis was confirmed by all of a variety of data from own investigations and from the lit- erature and by modelling. For instance, virulence complexity of cereal mildews and rusts increased from western to eastern Europe and as far as Siberia by ap- proximately one to two virulences per 1,000 km. The impact of our findings for general population genetics and gene flow across Europe and Asia and for further geographical areas is supposed to be considerable and worth elucidating further. Obligate nomad is a novel term that appears to be advantageous in several re- spects of population biology and life. Therefore, the consideration of obligate no- madism is extended to a spectrum of cases including plants, fungi and animals, as well as to neonomads and invasive plant species as a consequence of our present traffic and civilization. Most often obligate nomads are forced to migrate as sub- strates are ephemeral and ecological niches exist for a short period of time only. Chances and risks for health and the use of biological resources are discussed. 31.2 Migration and Obligate Nomads Migration and biological resources have always been of major importance for hu- man society. Repeatedly, migrating pests and pathogens on plants gave rise to D. Werner (ed.), Biological Resources and Migration © Springer-Verlag Berlin Heidelberg 2004