Patterns of genetic variation in Phialocephala fortinii across a broad latitudinal transect in Canada Melissa M. PIERCEY, Sean W. GRAHAM and Randolph S. CURRAH* Department of Biological Sciences, University of Alberta, Edmonton, AB, T6G 2E9, Canada. E-mail : rcurrah@ualberta.ca Received 27 February 2003; accepted 30 April 2004. Dark septate root endophytes (DSE) are an artificial assemblage of fungi that have darkly pigmented, septate hyphae and that are frequent or distinctive intracellular associates of roots of apparently healthy plants. Based on isolates obtained from the roots of Salix spp., the distribution of a common DSE fungus, Phialocephala fortinii, was examined along a latitudinal transect in Canada running from the high arctic to the 49x N parallel. Non-sporulating isolates were provisionally identified as P. fortinii through analysis of DNA sequence data of the ITS2 region of rDNA. P. fortinii was isolated frequently from boreal and arctic habitats, but rarely from grassland habitats. Patterns of genetic variation were examined through analysis of amplified fragment length polymorphisms (AFLP). All AFLP profiles were unique with the majority of genetic variation occurring among individuals within the collecting sites at each latitude. Neighbour-joining analysis of genetic distances yielded eight well-supported clusters, three of which included individuals from more than one latitude. Some linkage disequilibrium, possibly due to partial clonality, was detected. INTRODUCTION Phialocephala fortinii is a member of the dark septate endophytes or DSE (Haselwandter & Read 1982), a group of ascomycetous fungi found in apparently healthy roots of many plants worldwide. These taxa generally produce olivaceous to mouse grey mycelium in culture (Jumpponen & Trappe 1998a) and are diffi- cult to identify because they sporulate rarely or spor- adically, and often only after extended periods of incubation (4–6 months) at cool temperatures. P. fortinii is a phialidic hyphomycete allied with the Helotiales (LoBuglio, Berbee & Taylor 1996, Hambleton 1998, Jumpponen & Trappe 1998a, Gru¨ nig et al. 2002a) that has been isolated from the roots of 51 different plant species from 18 families (Jumpponen & Trappe 1998a, Piercey 2003) and is best known from regions where soils are cool and rich in organic debris (Addy, Hambleton & Currah 2000). The role that P. fortinii plays as a root endophyte is unclear but its high frequency of isolation in these habitats indicates the association is unlikely to be coincidental. In vitro studies to determine the nature of the symbiosis have been inconclusive, even though several authors have used different plant species, fungal isolates and growing conditions during axenic resyntheses (Jumpponen & Trappe 1998b); interactions among these three vari- ables have yielded results that indicate mutualism, parasitism or no effect on host fitness (e.g. Wilcox & Wang 1987, Stoyke & Currah 1991, Fernando & Currah 1996). While there is at least rudimentary information regarding the habitat of this fungus, patterns of global distribution are unknown. Studies of this aspect of the biology of the fungus have been problematic because colonial morphology is quite variable and exhibits con- siderable overlap with other species of fungi that have dematiaceous hyphae. Colonies may have abundant aerial mycelium, or little aerial mycelium but abundant submerged mycelium, solid or feathery margins, and numerous sclerotia or none at all (Stoyke 1991, Addy et al. 2000). The genetic variability of the taxon has also been indicated by various types of genetic analy- sis, including PCR/RFLP (Stoyke, Egger & Currah 1992), random amplified polymorphic DNA (RAPD: Jumpponen 1999), ribosomal DNA sequence analysis (Addy et al. 2000), and ISSR-PCR (Gru¨nig, Sieber & Holdenrieder 2001). All of these studies focused pri- marily on isolates from relatively small areas ; none attempted to determine if patterns of variation are dis- cernable across continental distances. * Corresponding author. Mycol. Res. 108 (8): 955–964 (August 2004). f The British Mycological Society 955 DOI: 10.1017/S0953756204000528 Printed in the United Kingdom.