Molecular Ecology (2006) 15, 2895–2904 doi: 10.1111/j.1365-294X.2006.02986.x © 2006 The Authors Journal compilation © 2006 Blackwell Publishing Ltd Blackwell Publishing Ltd Global migration patterns in the fungal wheat pathogen Phaeosphaeria nodorum EVA H. STUKENBROCK, SØREN BANKE and BRUCE A. M c DONALD Institute of Integrative Biology, Plant Pathology, ETH Zurich, LFW, Universitätstrasse 2, CH-8092 Zurich, Switzerland Abstract The global migration patterns of the fungal wheat pathogen Phaeosphaeria nodorum were analysed using 12 microsatellite loci. Analysis of 693 isolates from nine populations indi- cated that the population structure of P. nodorum is characterized by high levels of genetic diversity and a low degree of subdivision between continents. To determine whether genetic similarity of populations was a result of recent divergence or extensive gene flow, the microsatellite data were analysed using an isolation-with-migration model. We found that the continental P. nodorum populations diverged recently, but that enough migration occurred to reduce population differentiation. The migration patterns of the pathogen indi- cate that immigrants originated mainly from populations in Europe, China and North America. Keywords: directional gene flow, microsatellites, population divergence, population genetic structure, Septoria nodorum, sexual recombination, Stagonospora nodorum Received 3 December 2005; revision received 16 March 2006; accepted 30 March 2006 Introduction Fungal plant pathogens exhibit a diverse range of life histories and dispersal mechanisms that have important consequences for disease dynamics and pathogen per- sistence. Genetic markers have been widely applied to study population dynamics and migration patterns of plant pathogens at spatial scales ranging from single fields to continents (Burdon et al. 1982; Burt et al. 1997; Engel- brecht et al. 2004; Banke & McDonald 2005). Long-distance dispersal of plant pathogens is a common phenomenon that may occur either naturally by air-dispersed spores or via human-mediated movement of infected plant material and seeds. By analysing the distribution of genetic diversity within and among populations, it is possible to identify centres of diversity and patterns of migration (Beerli & Felsenstein 2001). The centre of origin of a pathogen is likely to consist of populations with more genetic variability than recently founded populations (Templeton et al. 1995). Patterns of global migration from the centre of origin into new territories have been described for a few plant pathogens such as Phytophthora infestans from Mexico to North America and Europe, and Mycosphaerella graminicola from the Middle East and Europe to ‘New World’ continents (Fry et al. 1992; Banke et al. 2004). A newly founded population often shares alleles with the source population of immigrants. For the population geneticist, it may be difficult to differentiate the relative importance of gene flow and persistence of variation in both populations for maintaining shared polymorphisms (Hey et al. 2004). In the case of plant pathogens, this know- ledge is needed to understand the processes underlying population divergence and evolution. An isolation-with- migration model was applied to genetic data to analyse gene flow between recently separated populations, e.g. of humans and fish (Nielsen & Wakeley 2001; Hey et al. 2004; Hey 2005), but to our knowledge the model has not yet been applied to studies of pathogens. Another factor that affects the ability of a pathogen to adapt to new environments is the mode of reproduction and mating system (McDonald & Linde 2002). The repro- ductive modes of fungal pathogens range from purely asexual to highly outcrossing (Milgroom 1996). Under asexual reproduction, combinations of alleles giving high fitness are kept together and selected clones carrying these allele combinations can increase to high frequencies. In the rice blast fungus Magnaporthe grisea, which is predom- inantly asexual (Babujee & Gnanamanickam 2000), this has Correspondence: Eva H. Stukenbrock, Fax: +41-44-632-1572; E-mail: eva.stukenbrock@agrl.ethz.ch