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- Materials and Methods
• The asexual ectomycorrhizal fungus Cenococcum geophilum, known for its wide host and habitat range, has been suggested to provide isolate-dependant drought protection to fine roots. However, little is known about its genetic structure at the fine scale.
• Genetic diversity and population structure of C. geophilum at the regional and stand scales was surveyed in five beech (Fagus silvatica) forests in northeastern France. The stands were selected for their contrasting climatic and edaphic features to assess the effect of environmental factors on population structure.
• The genetic diversity of C. geophilum was estimated using RAPD, PCR/RFLP of the rDNA internal transcribed spacer (ITS) and PCR/RFLP and sequencing of an anonymous sequence characterized amplified region (SCAR) on ectomycorrhizas and sclerotia-derived mycelial cultures.
• A high degree of genetic diversity was observed between and within beech stands in C. geophilum populations. These results suggest the occurrence of a high rate of mitotic or meiotic recombination and an effect of stand features on population structure.
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- Materials and Methods
At the soil–root interface, ectomycorrhizal (ECM) fungi are known to play a fundamental role in temperate and boreal forest ecosystems, by enhancing the hydro-mineral nutrition and affecting the host plant growth (Smith & Read, 1997). In these ecosystems, a single host tree may interact with hundreds of different species of ECM fungi (Taylor et al., 2000; Dahlberg, 2001), each species being represented by different genotypes (Bonello et al., 1998; Gherbi et al., 1999; Fiore-Donno & Martin, 2001) differing in their physiological abilities. This is thought to be important to ecosystem functionning (Debaud et al., 1995; Cairney, 1999).
Within the ectomycorrhizal communities, the ubiquitous and cosmopolitan ascomycete Cenococcum geophilum Fr. is one of the dominant and most frequent ectomycorrhizal types. Horton & Bruns (2001) emphasized that there are virtually no studies where this fungus was not detected. C. geophilum was found to be one of the prominent fungi in fennoscandian forests with a relative abundance of 15–18% (Dahlberg et al., 1997; Kåren et al., 1997). Furthermore, Kranabetter & Wylie (1998) have found C. geophilum to be the most frequent ECM fungus on naturally regenerated western hemlock seedlings in northwest British Columbia. Moreover, C. geophilum sclerotia, hypogeous resistant propagules, have been reported to be highly abundant in soils (Dahlberg et al., 1997). In northeastern France, the relative abundance of C. geophilum ectomycorrhizas has been estimated to be 47% in a beech forest (Blaise & Garbaye, 1983).
This abundant fungus could provide a functionnal asset to the colonized fine roots since several studies reported that C. geophilum is more resilient to drought stress than other ECM fungi (Mexal & Reid, 1973; Coleman et al., 1989; Neves Machado, 1995) and may protect roots from drought when it is involved in ectomycorrhizal symbiosis (Pigott, 1982a,b; Neves Machado, 1995). The resistance to drought stress however, varies between different isolates of C. geophilum (Coleman et al., 1989; Neves Machado, 1996) suggesting functionnal heterogeneity among C. geophilum populations. The genetic structure of C. geophilum populations may thus play a fundamental role in the adaptation of the host populations to drought.
Despite this remarkable set of ecological features, little is known about the genetic structure of C. geophilum populations. Surveys of isolates on a worldwide scale revealed a significant and unexpected genetic diversity for an asexual fungus (LoBuglio et al., 1991, 1996; Shinohara et al., 1999). Only a few studies have dealt with genetic diversity of C. geophilum on a detailed scale. Panaccione et al. (2001) showed that C. geophilum populations found in serpentine and nonserpentine soils were polymorphic. These authors distinguished 12 different genotypes among 13 local isolates.
In the present study, we have analysed regional and local genetic variability of C. geophilum and addressed the structuration of this species between different stands of Fagus silvatica L and different soil cores within each stand. Samples were collected in five different beech stands selected for their contrasting climatic and edaphic features in northeastern France. The genetic diversity has been studied by using RAPD, PCR/RFLP of the nuclear rDNA ITS and sequence analysis of a sequence characterized amplified region (SCAR1) on isolates originating from sclerotia. In addition, ectomycorrhizal tips were typed using PCR/RFLP of the ITS and of SCAR1.