Despite advances in mycorrhizal identification, the goal of elucidating the structure and development of mycorrhizal communities remains elusive. Fruit body production can be sporadic, morphological typing of mycorrhizae is subject to variation with environmental conditions or host, and cultural studies are labor intensive and miss fungi that cannot be isolated. Molecular techniques for identification of fungal symbionts can supplement these techniques and offer an approach that is rapid, is independent of environmental variation, and can be applied directly to large numbers of samples. Molecular approaches to mycorrhizal community analysis attempt to distinguish taxonomic groups so they can be monitored and their interactions studied. Initial characterization of community structure involves enzymatic amplification of DNA directly from mycorrhizal roots using fungus-specific primers, followed by restriction endonuclease digestion to produce taxon-specific restriction fragment patterns. Comparison of these patterns with those obtained from fungal fruit bodies or reference cultures facilitates identification of fungal symbionts. Phylogenetic relationships of fungi that cannot be matched to reference isolates can be inferred by sequencing enzymatically amplified DNA. Future directions that will result from molecular approaches include development of sampling strategies, resolution of species complexes, field observations of host specificity, elucidation of the dynamics of replacement processes (succession), and determination of the role of dispersal in community development. As additional techniques are developed for population analysis, resolution of questions related to genetic structure, variation, and gene flow will become feasible.RCsumC : En depit des progrks sur l'identification des mycorhizes, l'objectif de comprendre la structure et le dCveloppement des communautCs mycorhiziennes demeure difficile. La production des fructifications peut &tre sporadique, la caracttrisation des mycorhizes est sujette aux variations des conditions du milieu et de I'hBte, et les Ctudes culturales exigent beaucoup de travail et ntgligent les champignons qu'on ne sait pas cultiver. Les techniques molCculaires pour l'identification des symbiotes fongiques peuvent supplCer a ces dCficiences, et ouvrent une avenue rapide, indCpendante des variations du milieu, qui peut &tre appliquCe directement a un grand nombres dlCchantillons. Les mtthodes molCculaires appliquCes a l'analyse des communautCs visent distinguer les groupes taxonomiques, de facon a ce qu'on puisse les suivre et Ctudier leurs interactions. La caractCrisation initiale de la structure communautaire implique l'amplification enzymatique de l'ADN, directement 21 partir des racines mycorhizCes, en utilisant une amorGe spCcifique au champignon, suivi d'une digestion par des endonuclCases de restriction pour produire des patrons de restriction spCcifiques aux taxons. Une comparaison de ces patrons avec ceux obtenus a partir de fructifications de champignons ou de cultures de ...

The impacts of simulated climate change (warming and fertilization treatments) on diazotroph community structure and activity were investigated at Alexandra Fiord, Ellesmere Island, Canada. Open Top Chambers, which increased growing season temperatures by 1-3 degrees C, were randomly placed in a dwarf-shrub and cushion-plant dominated mesic tundra site in 1995. In 2000 and 2001 20N:20P2O5:20K2O fertilizer was applied at a rate of 5 gm(-2) year(-1). Estimates of nitrogen fixation rates were made in the field by acetylene reduction assays (ARA). Higher rates of N fixation were observed 19-35 days post-fertilization but were otherwise unaffected by treatments. However, moss cover was significantly positively associated with ARA rate. NifH gene variants were amplified from bulk soil DNA and analyzed by terminal restriction fragment length polymorphism analysis. Non-metric multidimensional scaling was used to ordinate treatment plots in nifH genotype space. NifH gene communities were more strongly structured by the warming treatment late in the growing season, suggesting that an annual succession in diazotroph community composition occurs.

Some views of mutualism, where the fitness of two symbiotic partners is higher in association than when apart, assume that they necessarily evolve towards greater benefit for the partners. Most mutualisms, however, seem prone to conflicts of interest that destabilize the partnership. These conflicts arise in part because mutualistic outcomes are conditional, depending upon complex interactions between environmental, developmental, and genotypic factors. Mutualisms are also subject to exploitation or cheating. Although various compensating mechanisms have been proposed to explain how mutualism can be maintained in the presence of exploiters, none of these mechanisms can eliminate exploitation. In this paper we explore various compensating mechanisms in mycorrhizas, examine the evidence for exploitation in mycorrhizas, and conclude that mycorrhizal mutualisms exhibit characteristics that are more consistent with a concept of reciprocal parasitism. We propose that researchers should not assume mycorrhizas are mutualistic based upon structural characteristics or limited functional studies showing bilateral exchange and should view mycorrhizas as occupying a wider range on the symbiotic continuum, including commensalism and antagonism. We recommend that comparative studies of mycorrhizas incorporate other types of root associations that have traditionally been considered antagonistic.Key words: mycorrhizas, mutualism, exploiters, compensating mechanisms, symbiotic continuum.

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Arctic air temperatures are expected to rise significantly over the next century. Experimental warming of arctic tundra has been shown to increase plant productivity and cause community shifts and may also alter microbial community structure. Hence, the objective of this study was to determine whether experimental warming caused shifts in soil microbial communities by measuring changes in the frequency, relative abundance and/or richness of nosZ and nifH genotypes. Five sites at a high arctic coastal lowland were subjected to a 13-year warming experiment using open-top chambers (OTCs). Sites differed by dominant plant community, soil parent material and/or moisture regimen. Six soil cores were collected from each of four replicate OTC and ambient plots at each site and subdivided into upper and lower samples. Differences in frequency and relative abundance of terminal restriction fragments were assessed graphically by two-way cluster analysis and tested statistically with permutational multivariate analysis of variance (ANOVA). Genotypic richness was compared using factorial ANOVA. The genotype frequency, relative abundance and genotype richness of both nosZ and nifH communities differed significantly by site, and by OTC treatment and/or depth at some sites. The site that showed the most pronounced treatment effect was a wet sedge meadow, where community structure and genotype richness of both nosZ and nifH were significantly affected by warming. Although warming was an important factor affecting these communities at some sites at this high arctic lowland, overall, site factors were the main determinants of community structure.