Beech roots are simultaneously colonized by multiple genets of the ectomycorrhizal fungus Laccaria amethystina clustered in two genetic groups S. HORTAL* 1 , L. K. TROCHA,† C. MURAT,* I. J. CHYBICKI,‡ M. BUE ´ E*, M. TROJANKIEWICZ,‡ J. BURCZYK‡ and F. MARTIN* *UMR INRA-UHP Interactions Arbres ⁄ Micro-organismes, INRA-Nancy, Route d’Amance, 54280 Champenoux, France, †Instytut Dendrologii, Polska Akademia Nauk, ul. Parkowa 5, 62-035 Ko ´rnik, Poland, ‡Uniwersytet Kazimierza Wielkiego (UKW), Department of Genetics, ul. Chodkiewicza 30, 85-064 Bydgoszcz, Poland Abstract In this study, we characterize and compare the genetic structure of aboveground and belowground populations of the ectomycorrhizal fungus Laccaria amethystina in an unmanaged mixed beech forest. Fruiting bodies and mycorrhizas of L. amethystina were mapped and collected in four plots in the S ´ wie ˛ tokrzyskie Mountains (Poland). A total of 563 fruiting bodies and 394 mycorrhizas were successfully genotyped using the rDNA IGS1 (intergenic spacer) and seven simple sequence repeat markers. We identified two different genetic clusters of L. amethystina in all of the plots, suggesting that a process of sympatric isolation may be occurring at a local scale. The proportion of individuals belonging to each cluster was similar among plots aboveground while it significantly differed belowground. Predominance of a given cluster could be explained by distinct host preferences or by priority effects and competition among genets. Both aboveground and belowground populations consisted of many intermingling small genets. Conse- quently, host trees were simultaneously colonized by many L. amethystina genets that may show different ecophysiological abilities. Our data showed that several genets may last for at least 1 year belowground and sustain into the next season. Ectomycorrhizal species reproducing by means of spores can form highly diverse and persistent belowground genets that may provide the host tree with higher resilience in a changing environment and enhance ecosystem performance. Keywords: genet, genetic structure, Laccaria, mycorrhizas, population genetics, simple sequence repeat markers Received 3 March 2011; revision received 24 January 2012; accepted 1 February 2012 Introduction Soil microbes drive major biogeochemical processes and contribute to the maintenance of plant productivity and species richness in most terrestrial ecosystems (van der Heijden et al. 2008). Among these, ectomycorrhizal fungi are particularly important because they live in symbiotic association with the roots of trees and may play a key role in the adaptation of forests to changing environmental conditions. They promote water and nutrient uptake in plants and protect them against soil borne pathogens (Read & Perez-Moreno 2003; Smith & Read 2008). Stress tolerance in host plants is, at least in some cases, mediated by the habitat-specific adaptations of fungal symbionts (Rodriguez & Redman 2008). Thus, understanding the dynamics of ectomycorrhizal soil populations can help us understand how forests func- tion, evolve and respond to environmental changes. However, despite the key ecological role of ectomycor- rhizal fungi, little is known about the population biol- ogy of most species (Douhan et al. 2011). Correspondence: Sara Hortal, Fax: +34950277100; E-mail: sara.hortal@eeza.csic.es 1 Current address: Estacio ´n Experimental de Zonas A ´ ridas, CSIC, Carretera de Sacramento s ⁄ n, E-04120 La Can ˜ ada de San Urbano, Almerı ´a, Spain. Ó 2012 Blackwell Publishing Ltd Molecular Ecology (2012) doi: 10.1111/j.1365-294X.2012.05515.x