Molecular Ecology (2004) 13, 3179–3186 doi: 10.1111/j.1365-294X.2004.02295.x © 2004 Blackwell Publishing Ltd Blackwell Publishing, Ltd. Community structure of arbuscular mycorrhizal fungi in undisturbed vegetation revealed by analyses of LSU rDNA sequences SØREN ROSENDAHL and EVA H. STUKENBROCK Department of Microbiology, Biological Institute, University of Copenhagen, Øster Farimagsgade 2D, DK-1353 Copenhagen K, Denmark Abstract Arbuscular mycorrhizal fungi (AMF) form a mutualistic symbiosis with plant roots and are found in most ecosystems. In this study the community structure of AMF in a clade of the genus Glomus was examined in undisturbed costal grassland using LSU rDNA sequences amplified from roots of Hieracium pilosella. Roots were sampled from May to November along eight 30-m transects, 30 –120 m apart. Phylogenetic analysis of the sequences revealed 11 phylogenetic clusters within the clade of Glomus. The phylogenetic clusters were patchily distributed within the area; time had no influence on the distribution pattern. The domin- ant cluster covered up to 10 m along the transect, whereas other clusters formed what can be interpreted as small individual mycelia. Four of the phylogenetic clusters included known species; the other clusters, including the dominant sequence types, were unknown. The dominant phylogenetic cluster enclosed nine haplotypes, and analyses of genetic diversity of this phylogenetic cluster showed that the total diversity could be found within single root fragments, suggesting that the multiple sequences were derived from a single individual. Keywords: arbuscular mycorrhiza, community, Glomus, genetic variation, LSU rDNA, spatial variance Received 9 March 2004; revision received 7 June 2004; accepted 23 June 2004 Introduction Arbuscular mycorrhizal fungi (AMF) form a mutualistic relationship with most plant species, and are important for plant diversity and productivity (Van der Heijden et al . 1998). The fungi are known to promote phosphorus uptake and protect plants against various types of stress. The fungi belong to Glomeromycota, a distinct monophyletic lineage separate from other terrestrial fungi (Schüssler et al . 2001; Schwarzott et al . 2001). Arbuscular mycorrhiza is believed to represent an ancient association between plants and fungi, and seems to have evolved with the first land plants (Berbee & Taylor 1993). Although AMF can be regarded as one of the most abundant groups of organisms, found in almost all ter- restrial ecosystems, only ~150 species have been described (Morton & Benny 1990). This low number of species may cover an immense functional and genetic diversity within the described species (Bever et al . 2001). Also, several reports indicate that in some vegetation systems, a large number of nonsporulating cryptic species may occur. This hypothesis was suggested by Sparling & Tinker (1975) to explain why only few spores could be found in permanent grasslands. More recent community studies of AMF using molecular markers from ribosomal genes (rDNA), have confirmed that it is possible to find several nucleotide sequences from fungi that have their origin within Glomeromycota, but are without known spores (Helgason et al . 1999; Husband et al . 2002a). In agricultural soils soil manipulation, e.g. tillage, will have a major impact on fungal communities ( Jasper et al . 1991). Although the relative abilities of different AMF to spread by spores and mycelia are unknown, it is possible that these disturbed systems favour sporulating species, whereas nonsporulating species that proliferate as mycelia may be more abundant in undisturbed systems. This should result in a nonuniform spatial distribution of the fungi (Bever et al . 2001). By contrast, temporal variation in Correspondence: Søren Rosendahl. Fax: +45 353 22321; E-mail: soerenr@bot.ku.dk