Distribution of mating type alleles in the wheat pathogen Mycosphaerella graminicola over spatial scales from lesions to continents J. Zhan, a, * G.H.J. Kema, b C. Waalwijk, b and B.A. McDonald a a ETH Zentrum/LFW, Phytopathology Group, Institute of Plant Sciences, Universit€ atstrasse 2, Z€ urich, CH-8092, Switzerland b Plant Research International, B.V.P.O. Box 16, Wageningen, 6700 AA, The Netherlands Received 10 September 2001; accepted 28 January 2002 Abstract A total of 2035 Mycosphaerella graminicola strains collected from 16 geographic locations on four continents were assayed for the mating type locus. RFLP fingerprints were used to identify clones in each population. At the smallest spatial scale analyzed, both mating types were found among fungal strains sampled from different lesions of the same leaf as well as from different pycnidia in the same lesion. At larger spatial scales, the two mating types were found at equal frequencies across spatial scales ranging from several square meters to several thousand square kilometers. Though the absolute frequencies of the two mating types sometimes varied for different sampling units within the same spatial scale in the hierarchy (plots within a field, fields within a country, or different continents of the world), none of the differences were statistically significant from the null hypothesis of equal frequencies for the two mating types. The evolutionary forces likely to maintain the even distribution of the two mating types in this pathogen were discussed. Ó 2002 Elsevier Science (USA). All rights reserved. Keywords: Mating type; Mycosphaerella graminicola; Balancing selection; Sexual selection; PCR amplification; RFLPs; Population genetics; Geographic distribution 1. Introduction Sexual reproduction in heterothallic fungi involves the temporary joining of two fungal strains carrying compatible mating types, followed by meiosis and ex- change of genetic information between the individuals. For these fungi, sexual reproduction is possible only when two compatible mating types are available at the same geographic location at the same point in time. When two fungal strains of opposite mating type come together, they detect each other’s presence in response to the mating pheromone produced by unlike mating types (Coppin et al., 1997; Egel, 1992; Ferris et al., 1997; Kronstad and Staben, 1997). The double pheromone– receptor interaction initiates a signal transduction pathway that changes the pattern of gene expression and leads to mating. Over the last decade, the genes that govern the mating type status of fungal individuals have been studied in- tensively (recent reviews include Coppin et al., 1997; Hiscock and Kues, 1999; Kronstad and Staben, 1997; Turgeon, 1998). The majority of these studies focused on the molecular characterization of mating type genes (e.g., Sugimoto et al., 1991; Travis et al., 1991; Turgeon et al., 1995) or used mating type genes as tools to study the evolutionary development of mating systems (e.g., Beatty et al., 1994; Glass et al., 1990; Sharon et al., 1996) and the phylogenetic relationships among organisms (e.g., Turgeon, 1998). Studies on the geographic distri- bution of mating types and the evolutionary forces governing this distribution are more limited. Given the central role of the geographic distribution of mating types to the life cycle of heterothallic fungi and of mating systems to the evolution and population biology of fungi, this knowledge is important in understanding and predicting the population genetics, dynamics, and evolutionary potential of fungi. Fungal Genetics and Biology 36 (2002) 128–136 www.academicpress.com * Corresponding author. Fax: +41 1632-1572. E-mail address: jiasui.zhan@ipw.agrl.ethz.ch (J. Zhan). 1087-1845/02/$ - see front matter Ó 2002 Elsevier Science (USA). All rights reserved. PII:S1087-1845(02)00013-0