Glomus, the Largest Genus of the Arbuscular Mycorrhizal Fungi (Glomales), Is Nonmonophyletic

The community composition of arbuscular mycorrhizal fungi (AMF) was analyzed in roots of Gentiana verna, Gentiana acaulis, and accompanying plant species from two species-rich Swiss alpine meadows located in the same area. The aim of the study was to elucidate the impact of host preference or host specificity on the AMF community in the roots. The roots were analyzed by nested PCR, restriction fragment length polymorphism screening, and sequencing of ribosomal DNA small-subunit and internal transcribed spacer regions. The AMF sequences were analyzed phylogenetically and used to define monophyletic sequence types. The AMF community composition was strongly influenced by the host plant species, but compositions did not significantly differ between the two sites. Detailed analyses of the two cooccurring gentian species G. verna and G. acaulis, as well as of neighboring Trifolium spp., revealed that their AMF communities differed significantly. All three host plant taxa harbored AMF communities comprising multiple phylotypes from different fungal lineages. A frequent fungal phylotype from Glomus group B was almost exclusively found in Trifolium spp., suggesting some degree of host preference for this fungus in this habitat. In conclusion, the results indicate that within a relatively small area with similar soil and climatic conditions, the host plant species can have a major influence on the AMF communities within the roots. No evidence was found for a narrowing of the mycosymbiont spectrum in the two green gentians, in contrast to previous findings with their achlorophyllous relatives.Arbuscular mycorrhiza is an ancient symbiosis (26) between the majority of land plants and fungi from the phylum Glomeromycota (30). Arbuscular mycorrhizal fungi (AMF) colonize plant roots and contribute to the mineral nutrient uptake of the hosts in exchange for carbohydrates (32).The diversity of AMF can be evaluated using microscopic analysis of spore morphology or molecular methods. The production of spores is highly dependent on environmental conditions and the physiological status and life strategy of the particular mycorrhizal fungus. Molecular methods allow the identification of the symbiotic community currently colonizing the roots of an individual plant at any given time. The majority of recent molecular studies have used AMFspecific primers for nuclearly encoded rRNA genes (rDNA). However, identification of AMF on the species or even isolate level is complicated by the heterogeneity of rDNA within glomeromycotan spores and isolates. Several authors have shown that different variants of rDNA genes coexist within single AMF spores (15, 27). Hence, a conservative approach in the evaluation of phylogenetic analyses is advisable by, e.g., defining a well-supported monophyletic sequence cluster as an AMF phylotype.On the basis of the morphological features of their spores, only about 200 species of AMF have been described so far (http://www.lrz-muenchen.de/ϳschuessler/amphylo/). This small number of species was originally though...

Section: Methodsmentioning

confidence: 99%

Cooccurring Gentiana verna and Gentiana acaulis and Their Neighboring Plants in Two Swiss Upper Montane Meadows Harbor Distinct Arbuscular Mycorrhizal Fungal Communities

Sýkorová

Wiemken

Redecker

2007

“…A mixture of equal amounts of the AM1 (26), AM2, and AM3 primers was used as the reverse primer combination ( Table 1). The AM1 primer, originally designed for the specific amplification of Glomeromycota DNA, presents some mismatches at the priming site with taxa belonging to Glomus group B and Glomus group C (Diversisporaceae) (59). The primers AM2 and AM3 are modifications of the primer AM1 and aim to amplify DNA from taxa belonging to Glomus group B and Glomus group C (Diversisporaceae), respectively.…”

Section: Methodsmentioning

confidence: 99%

Seasonal Dynamics of Arbuscular Mycorrhizal Fungal Communities in Roots in a Seminatural Grassland

Santos-González

Finlay²,

Tehler

2007

126

Symbiotic arbuscular mycorrhizal fungi (AMF) have been shown to influence both the diversity and productivity of grassland plant communities. These effects have been postulated to depend on the differential effects of individual mycorrhizal taxa on different plant species; however, so far there are few detailed studies of the dynamics of AMF colonization of different plant species. In this study, we characterized the communities of AMF colonizing the roots of two plant species, Prunella vulgaris and Antennaria dioica, in a Swedish seminatural grassland at different times of the year. The AMF small subunit rRNA genes were subjected to PCR, cloning, sequencing, and phylogenetic analysis. Nineteen discrete sequence types belonging to Glomus groups A and B and to the genus Acaulospora were distinguished. No significant seasonal changes in the species compositions of the AMF communities as a whole were observed. However, the two plant species hosted significantly different AMF communities. P. vulgaris hosted a rich AMF community throughout the entire growing season. The presence of AMF in A. dioica decreased dramatically in autumn, while an increased presence of Ascomycetes species was detected.In temperate and boreal regions, seminatural grasslands are some of the most species-rich plant communities (37). Grasslands in Europe have been managed during the last millennia by grazing and haymaking. The total area and connectivity of grasslands have decreased dramatically during the last hundred years (77). This decline is one of the major threats to diversity for many groups of organisms (44).Arbuscular mycorrhizal fungi (AMF), belonging to the phylum Glomeromycota, are obligate symbiotic fungi forming mutualistic associations with the roots of most land plants. Increased access to low-mobility soil mineral nutrients has been considered to be the main beneficial effect of AMF on their host plants (61). Experimental greenhouse studies have shown that AMF diversity can affect the diversity and productivity of the host plant communities and, therefore, the functioning of the whole ecosystem (21,33,74,76,78). Other benefits to host plants arising from the presence of AMF include a reduction in the occurrence of pathogenic infections (5, 41), the improvement of water relations (9), and the limitation of heavy-metal uptake (39). The effects of AMF diversity and species composition are thought to arise through differential effects of different AMF taxa on the growth of individual plant species (47), but descriptive ecological data on the structure of AMF communities are still few in comparison with the available information on the structure and dynamics of plant communities (52).Investigations of the phenology, diversity, and functioning of natural AMF communities have traditionally been based on root colonization estimates and spore counts. The extension of the intraradical mycelia gives information on the mycorrhizal status of the plant but can provide hardly any information about the identities of the fungi. Soil spore anal...

“…AMF are putative ancient asexuals, belonging to the fungal phylum Glomeromycota (12,32), that form obligate symbiotic interactions with about 80% of land plants. AMF are known as important determinants of plant biodiversity, ecosystem variability, and productivity (36), yet very little is known about the evolution of their genes and genomes.…”

mentioning

confidence: 99%

High-Level Molecular Diversity of Copper-Zinc Superoxide Dismutase Genes among and within Species of Arbuscular Mycorrhizal Fungi

Corradi

Ruffner

Croll

et al. 2009

In the ecologically important arbuscular mycorrhizal fungi (AMF), Sod1 encodes a functional polypeptide that confers increased tolerance to oxidative stress and that is upregulated inside the roots during early steps of the symbiosis with host plants. It is still unclear whether its expression is directed at scavenging reactive oxygen species (ROS) produced by the host, if it plays a role in the fungus-host dialogue, or if it is a consequence of oxidative stress from the surrounding environment. All these possibilities are equally likely, and molecular variation at the Sod1 locus can possibly have adaptive implications for one or all of the three mentioned functions. In this paper, we analyzed the diversity of the Sod1 gene in six AMF species, as well as 14 Glomus intraradices isolates from a single natural population. By sequencing this locus, we identified a large amount of nucleotide and amino acid molecular diversity both among AMF species and individuals, suggesting a rapid divergence of its codons. The Sod1 gene was monomorphic within each isolate we analyzed, and quantitative PCR strongly suggest this locus is present as a single copy in G. intraradices. Maximum-likelihood analyses performed using a variety of models for codon evolution indicated that a number of amino acid sites most likely evolved under the regime of positive selection among AMF species. In addition, we found that some isolates of G. intraradices from a natural population harbor very divergent orthologous Sod1 sequences, and our analysis suggested that diversifying selection, rather than recombination, was responsible for the persistence of this molecular diversity within the AMF population.Positive Darwinian selection is a potential signature of genetic conflicts. This has been demonstrated in host-pathogen interactions using molecular evidence (3, 37) and is also supported by theoretical predictions (7,29,34,35). In sharp contrast, how genes evolve in a mutualistic environment remains largely unexplored. Strong selective constraints on genes that are specifically expressed during symbiotic stages are expected, and, indeed, slow rates of evolution have been predicted in conceptual models of mutualism because only weak selection pressures are expected in the host environment (8). Conversely, mutualism has been considered as a less harmonious interaction because the organisms involved have been predicted to evolve mechanisms to avoid overexploitation by their partners (20,24). Evolution of such mechanisms in the host could create selective pressures for change in the symbiont. In