Global population structure and migration patterns suggest significant population differentiation among isolates of Pyrenophora tritici-repentis S. Gurung a , D.P.G. Short a , T.B. Adhikari b,⇑ a Department of Plant Pathology, University of California, Davis, c/o U.S. Agricultural Research Station, 1636 E. Alisal Street, Salinas, CA 93905, United States b Center for Integrated Pest Management, and Department of Plant Pathology, North Carolina State University, 840 Main Campus Drive, Partners II Building, Suite # 1400, Centennial Campus, Box 7253, Raleigh NC 27606, United States article info Article history: Received 29 August 2012 Accepted 17 January 2013 Available online 31 January 2013 Keywords: Gene flow Genetic diversity Linkage disequilibrium Tan spot Triticum aestivum L. abstract The global population structure and migration patterns of foliar wheat pathogen Pyrenophora tritici- repentis (PTR) were determined using 12 microsatellite loci. Analysis of 439 single-spore isolates of PTR from five continents (18 wheat-producing countries) showed high level of genetic diversity, and moderate to high population differentiation between continents. A high level of gene diversity (H S = 0.31 to 0.56) was observed within each population. Allelic richness indicated the European and the North American population have a high effective population size. Bayesian analyses showed five clus- ters where the inferred clusters did not represent geographical populations. Corrected standardized fix- ation index (G 00 ST ) estimates ranged from 0.042 to 0.265 between populations, indicating low to high genetic differentiation exists between populations. We found migration (gene flow) between old world (Europe) and new world (Americas) population; however, little migration was observed among other continents. The European population was the major source of immigrants for the North American, South American, Australian and the Asian populations. Significant (P < 0.001) linkage disequilibrium (LD) was detected in the Australian and the South American populations. In contrast, non-significant (P < 0.001) LD values were observed in the Asian, European and the North American populations. Overall, our find- ings demonstrate the population differentiation exits among the global populations and strict quarantine measures should be applied to prevent the accelerated global spread of this pathogen. Ó 2013 Elsevier Inc. All rights reserved. 1. Introduction The knowledge on pathogen origin, genetic structure and its dispersal mechanism is crucial to comprehend management strat- egies for any plant pathogen. To identify the centers of diversity and course of migration patterns or gene flow, the distribution of genetic diversity within and among populations on a global scale is necessary. Unlike other pathogens, few studies have endeavored to decrypt historical sources of populations and migration patterns of wheat pathogens on a large geographic scale. Global migration patterns of the two wheat pathogens, Mycosphaerella graminicola (Banke and McDonald, 2005) and Phaeosphaeria nodorum,(Stuken- brock et al., 2006) have been investigated and high migration pat- terns were reported, suggesting a lack of population differentiation on a global scale. Another ascomycete fungus, Pyrenophora tritici- repentis (PTR) (Died.) Drechs. (anamorph: Drechslera tritici-repentis (Died.) Shoem.) causes tan spot in wheat (Triticum aestivum L.) globally (Hosford et al., 1987). Severe disease epidemics have been reported in the US, Brazil, and Australia (Murray and Brown, 1987; Schilder and Bergstrom, 1995) and in some European countries (Cook and Yarham, 1989; Leisová et al., 2008). The primary inocu- lum can come from infected seed, infested crop residues, and in- fected over-wintering grasses (de Wolf et al., 1998). PTR produces both ascospores and conidia, and these spores function as primary and secondary sources of inoculum, respectively (Kru- pinsky, 1992). The teleomorph is formed on wheat straw during the winter, and the resulting ascospores play an important role in tan spot epidemics (Krupinsky, 1992). Conidia are dispersed over short distances by rain-splash, but the sexually produced ascospores can be wind disseminated up to 200 km (Maraite et al., 1992). It has been speculated that the dissemination of PTR on a continental scale occurred during shipments of infected grain (Friesen et al., 2006). However, little is known concerning the migration patterns and population genetic structure of PTR from broad geographic areas. Host-selective toxins (HSTs) such as Ptr ToxA, Ptr ToxB, Ptr ToxC and other two toxins Ptr ToxD, are produced by different patho- genic races of PTR (Effertz et al., 2002; Manning et al., 2002; Mein- hardt et al., 2003; Orolaza et al., 1995; Tomás and Bockus, 1987). Those toxins are accountable for necrosis and chlorosis symptoms on sensitive wheat genotypes. On the basis of presence and 1087-1845/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.fgb.2013.01.003 ⇑ Corresponding author. E-mail address: tika_adhikari@ncsu.edu (T.B. Adhikari). Fungal Genetics and Biology 52 (2013) 32–41 Contents lists available at SciVerse ScienceDirect Fungal Genetics and Biology journal homepage: www.elsevier.com/locate/yfgbi