Soil tillage affects the community structure of mycorrhizal fungi in maize roots

In this study we tested whether communities of arbuscular mycorrhizal fungi (AMF) colonizing the roots of maize (Zea mays L.) were affected by soil tillage practices (plowing, chiseling, and no-till) in a long-term field experiment carried out in Tanikon (Switzerland). AMF were identified in the roots using specific polymerase chain reaction (PCR) markers that had been developed for the AMF previously isolated from the soils of the studied site. A nested PCR procedure with primers of increased specificity (eukaryotic, then, fungal, then AMF species or. species-grouop specific) was used. Sequencing of amplified DNA confirmed that the DNA obtained from the maize roots was of AMF origin. Presence of particular AMF species or species-group was scored as a presence of a DNA product after PCR with specific primers. We also used single-strand conformation polymorphism analysis (SSCP), of amplified DNA samples to-check if the amplification of the DNA from maize roots matched the expected profile for a particular AMF isolate with a given specific primer pair. Presence of the genus Scutellospora, in maize roots was strongly reduced in plowed and chiseled soils. Fungi from the suborder Glomineae were more prevalent colonizers of maize roots growing in plowed soils, but were also present in the roots from other tillage treatments. These changes in community of AMF colonizing maize roots might be due to (1), the differences in tolerance to the tillage-induced disruption of the hyphae among the different AMF species, (2) changes in nutrient content of the soil, (3) changes in microbial activity, or (4) changes in weed populations in response to soil tillage. This is the first report on community composition of AMF in the roots of a field-grown crop plant (maize) as affected by soil tillage.

Evolution of uni- and bifactorial sexual compatibility systems in fungi.

Mating systems, that is, whether organisms give rise to progeny by selfing, inbreeding or outcrossing, strongly affect important ecological and evolutionary processes. Large variations in mating systems exist in fungi, allowing the study of their origin and consequences. In fungi, sexual incompatibility is determined by molecular recognition mechanisms, controlled by a single mating-type locus in most unifactorial fungi. In Basidiomycete fungi, however, which include rusts, smuts and mushrooms, a system has evolved in which incompatibility is controlled by two unlinked loci. This bifactorial system probably evolved from a unifactorial system. Multiple independent transitions back to a unifactorial system occurred. It is still unclear what force drove evolution and maintenance of these contrasting inheritance patterns that determine mating compatibility. Here, we give an overview of the evolutionary factors that might have driven the evolution of bifactoriality from a unifactorial system and the transitions back to unifactoriality. Bifactoriality most likely evolved for selfing avoidance. Subsequently, multiallelism at mating-type loci evolved through negative frequency-dependent selection by increasing the chance to find a compatible mate. Unifactoriality then evolved back in some species, possibly because either selfing was favoured or for increasing the chance to find a compatible mate in species with few alleles. Owing to the existence of closely related unifactorial and bifactorial species and the increasing knowledge of the genetic systems of the different mechanisms, Basidiomycetes provide an excellent model for studying the different forces that shape breeding systems.

Evolution of the P-type II ATPase gene family in the fungi and presence of structural genomic changes among isolates of Glomus intraradices.

BACKGROUND: The P-type II ATPase gene family encodes proteins with an important role in adaptation of the cell to variation in external K+, Ca2+ and Na2+ concentrations. The presence of P-type II gene subfamilies that are specific for certain kingdoms has been reported but was sometimes contradicted by discovery of previously unknown homologous sequences in newly sequenced genomes. Members of this gene family have been sampled in all of the fungal phyla except the arbuscular mycorrhizal fungi (AMF; phylum Glomeromycota), which are known to play a key-role in terrestrial ecosystems and to be genetically highly variable within populations. Here we used highly degenerate primers on AMF genomic DNA to increase the sampling of fungal P-Type II ATPases and to test previous predictions about their evolution. In parallel, homologous sequences of the P-type II ATPases have been used to determine the nature and amount of polymorphism that is present at these loci among isolates of Glomus intraradices harvested from the same field. RESULTS: In this study, four P-type II ATPase sub-families have been isolated from three AMF species. We show that, contrary to previous predictions, P-type IIC ATPases are present in all basal fungal taxa. Additionally, P-Type IIE ATPases should no longer be considered as exclusive to the Ascomycota and the Basidiomycota, since we also demonstrate their presence in the Zygomycota. Finally, a comparison of homologous sequences encoding P-type IID ATPases showed unexpectedly that indel mutations among coding regions, as well as specific gene duplications occur among AMF individuals within the same field. CONCLUSION: On the basis of these results we suggest that the diversification of P-Type IIC and E ATPases followed the diversification of the extant fungal phyla with independent events of gene gains and losses. Consistent with recent findings on the human genome, but at a much smaller geographic scale, we provided evidence that structural genomic changes, such as exonic indel mutations and gene duplications are less rare than previously thought and that these also occur within fungal populations.

Rapid genotypic change and plasticity in arbuscular mycorrhizal fungi is caused by a host shift and enhanced by segregation.

Arbuscular mycorrhizal fungi (AMF) are among the most abundant symbionts of plants, improving plant productivity and diversity. They are thought to mostly grow vegetatively, a trait assumed to limit adaptability. However, AMF can also harbor genetically different nuclei (nucleotypes). It has been shown that one AMF can produce genotypically novel offspring with proportions of different nucleotypes. We hypothesized that (1) AMF respond rapidly to a change of environment (plant host) through changes in the frequency of nucleotypes; (2) genotypically novel offspring exhibit different genetic responses to environmental change than the parent; and (3) genotypically novel offspring exhibit a wide range of phenotypic plasticity to a change of environment. We subjected AMF parents and offspring to a host shift. We observed rapid and large genotypic changes in all AMF lines that were not random. Genotypic and phenotypic responses were different among offspring and their parents. Even though growing vegetatively, AMF offspring display a broad range of genotypic and phenotypic changes in response to host shift. We conclude that AMF have the ability to rapidly produce variable progeny, increasing their probability to produce offspring with different fitness than their parents and, consequently, their potential adaptability to new environmental conditions. Such genotypic and phenotypic flexibility could be a fast alternative to sexual reproduction and is likely to be a key to the ecological success of AMF.

Effects of the relatedness of arbuscular mycorrhizal fungi on associated species ecology

More than 80 % of vascular plants in the world form symbioses with arbuscular mycorrhizal fungi (AMF). AMF supply plants with nutrients such as phosphate and nitrogen, and can also help the plants to take up water. Hence, the symbiosis can greatly influence the growth and the defence of plants. By modifying plant productivity and diversity, AMF are considered as keystone species in ecosystems, playing a role that ultimately affects many food webs. This is why mycorrhizal symbioses have been investigated for several decades by many research groups.¦However, a large part of the scientific research done on AMF symbiosis has focused on the interaction between one plant and one fungus. This situation is far from realistic, as in natural ecosystems, many different fungal strains and species are co-existing and interacting in a belowground network. The main goal of this PhD was to investigate first, the interaction occurring among different co-existing AMF depending on their genetic relatedness and second, the outcome of the interaction and their effects on associated species.¦We found that AMF genetic relatedness partly explains the interaction among AMF, and this was in agreement with theories made for completely different species. Briefly, we demonstrated that AMF isolates of the same species coexisted more easily when they were closely-related, whereas AMF from different species were more in competition in this case of high relatedness. We also demonstrated that coexistence and competition among AMF can mediate plant growth as well as herbivore behaviour, opening new insights in our understanding of AMF effects on ecosystem functioning.¦Overall, the results of the different experiments of this PhD highlight the necessity of using multiple AMF to understand their interactions. Even so, we demonstrated here that simple species richness is not enough to understand these interactions and genetic relatedness among the co-existing AMF is a parameter that must be taken into account.¦-¦Sur Terre, plus de 80 % des plantes vasculaires forment des symbioses avec des champignons endomycorhiziens à arbuscules (CEA). Ces CEA permettent aux plantes d'acquérir plus facilement des nutriments tels que des phosphates, des nitrates, ou simplement de l'eau. Ainsi, cette symbiose peut avoir un effet important à la fois sur la croissance mais aussi sur la défense des plantes. En modulant la productivité et la diversité des plantes, les CEA sont donc des espèces clefs dans l'écosystème. Leur présence peut avoir des répercussions sur l'ensemble des réseaux trophiques. C'est pourquoi de nombreuses équipes de recherches étudient ces symbioses mycorhizienes depuis plusieurs décennies.¦La plupart des études concernant ces symbioses se sont focalisées sur l'action d'une espèce de CEA sur une espèce de plante. Malheureusement, cette situation ne correspond pas à ce que l'on peut retrouver dans la nature, où de nombreuses souches et de nombreuses espèces de CEA coexistent et interagissent dans un réseau mycélien souterrain. Le principal but de cette thèse était d'étudier, premièrement les interactions entre les différent CEA en fonction de leur apparentement génétique, et deuxièmement, d'étudier l'effet de ces interactions fongiques sur l'écologie des espèces associées.¦Au cours des différentes expériences de cette thèse, nous avons démontré que l'apparentement génétique entre les CEA expliquait une part non négligeable de leurs interactions. En résumé, plus l'apparentement génétique entre des souches de CEA d'une même espèce sera grand, plus ces souches seront capables de coexister. En revanche, s'il s'agit d'espèces différentes de CEA, plus elles seront apparentées, plus la compétition sera grande entre elles. Nous avons également démontré que la coexistence et la compétition entre différents CEA peut modifier à la fois la croissance des plantes mais aussi le comportement de leur prédateurs, ce qui ouvre de nouvelles perspectives sur notre compréhension des effets des CEA dans le fonctionnement des écosystèmes.¦Globalement, les résultats de nos différentes expériences mettent en évidence la nécessité d'utiliser plusieurs souches ou espèces de CEA pour mieux comprendre leurs interactions. Quand bien même, nos expériences démontrent que le simple recensement du nombre d'espèces de CEA n'est pas suffisant pour comprendre les interactions et que l'apparentement génétique des CEA coexistants est un paramètre qui doit être pris en compte.

Characterization of the surface hydrophobicity of filamentous fungi.

A method for the quantitative analysis of the hydrophobicity of the mycelial mat of filamentous fungi based on contact angle measurements is presented. It was tested for a range of fungi belonging to the classes of basidiomycetes, ascomycetes and deuteromycetes. The measured contact angles of the mycelial mats ranged between hydrophilic (<30 degrees) for the deuteromycetes Fusarium oxysporum Fo47 GUS1 and Trichoderma harzianum P1[pZEGA1] and hydrophobic (>60 degrees) for the ascomycete Cladosporium sp. DSE48.1b and the basidiomycetes Paxillus involutus WSL 37.7, Hebeloma crustiliniforme WSL 6.2, Suillus bovinus WSL 48.1 and Laccaria bicolor WSL 73.1. For some fungi, variations in the hydrophobicity of the mycelium depending on the growth medium, the physiological state and the exposure to water were distinguished.

Arbuscular mycorrhizal fungi mediate below-ground plant-herbivore interactions: a phylogenetic study

Ecological interactions are complex networks, but have typically been studied in a pairwise fashion. Examining how third-party species can modify the outcome of pairwise interactions may allow us to better predict their outcomes in realistic systems. For instance, arbuscular mycorrhizal fungi (AMF) can affect plant interactions with other organisms, including below-ground herbivores, but the mechanisms underlying these effects remain unclear. Here, we use a comparative, phylogenetically controlled approach to test the relative importance of mycorrhizal colonization and plant chemical defences (cardenolides) in predicting plant survival and the abundance of a generalist below-ground herbivore across 14 species of milkweeds (Asclepias spp.). Plants were inoculated with a mixture of four generalist AMF species or left uninoculated. After 1month, larvae of Bradysia sp. (Diptera: Sciaridae), a generalist below-ground herbivore, colonized plant roots. We performed phylogenetically controlled analyses to assess the influence of AMF colonization and toxic cardenolides on plant growth, mortality and infestation by fungus gnats. Overall, plants inoculated with AMF exhibited greater survival than did uninoculated plants. Additionally, surviving inoculated plants had lower numbers of larvae in their roots and fewer non-AM fungi than surviving uninoculated plants. In phylogenetic controlled regressions, gnat density in roots was better predicted by the extent of root colonized by AMF than by root cardenolide concentration. Taken as a whole, AMF modify the effect of below-ground herbivores on plants in a species-specific manner, independent of changes in chemical defence. This study adds to the growing body of literature demonstrating that mycorrhizal fungi may improve plant fitness by conferring protection against antagonists, rather than growth benefits. In addition, we advocate using comparative analyses to disentangle the roles of shared history and ecology in shaping trait expression and to better predict the outcomes of complex multitrophic interactions.

Two novel MvaT-like global regulators control exoproduct formation and biocontrol activity in root-associated Pseudomonas fluorescens CHA0.

Pseudomonas fluorescens CHA0 protects various crop plants against root diseases caused by pathogenic fungi. Among a range of exoproducts excreted by strain CHA0, the antifungal compounds 2,4-diacetylphloroglucinol (DAPG) and pyoluteorin (PLT) are particularly relevant to the strain's biocontrol potential. Here, we report on the characterization of MvaT and MvaV as novel regulators of biocontrol activity in strain CHA0. We establish the two proteins as further members of an emerging family of MvaT-like regulators in pseudomonads that are structurally and functionally related to the DNA-binding protein H-NS. In mvaT and mvaV in frame-deletion mutants of strain CHA0, PLT production was enhanced about four- and 1.5-fold, respectively, whereas DAPG production remained at wild-type levels. Remarkably, PLT production was increased up to 20-fold in an mvaT mvaV double mutant. DAPG biosynthesis was almost completely repressed in this mutant. The effects on antibiotic production could be confirmed by following expression of gfp-based reporter fusions to the corresponding biosynthetic genes. MvaT and MvaV also influenced levels of other exoproducts, motility, and physicochemical cell-surface properties to various extents. Compared with the wild type, mvaT and mvaV mutants had an about 20% reduced capacity (in terms of plant fresh weight) to protect cucumber from a root rot caused by Pythium ultimum. Biocontrol activity was nearly completely abolished in the double mutant Our findings indicate that MvaT and MvaV act together as further global regulatory elements in the complex network controlling expression of biocontrol traits in plant-beneficial pseudomonads.

Taking the fungal highway: mobilization of pollutant-degrading bacteria by fungi.

The capacity of fungi to serve as vectors for the dispersion of pollutant-degrading bacteria was analyzed in laboratory model systems mimicking water-saturated (agar surfaces) and unsaturated soil environments (glass-bead-filled columns). Two common soil fungi (Fusarium oxysporum and Rhexocercosporidium sp.) forming hydrophilic and hydrophobic mycelia, respectively, and three polycyclic aromatic hydrocarbon degrading bacteria (Achromobacter sp. SK1, Mycobacterium frederiksbergense LB501TG, and Sphingomonas sp. L138) were selected based on the absence of mutual antagonistic effects. It was shown that fungal hyphae act as vectors for bacterial transport with mobilization strongly depending on the specific microorganisms chosen: The motile strain Achromobacter sp. SK1 was most efficiently spread along hyphae of hydrophilic F. oxysporum in both model systems with transport velocities of up to 1 cm d(-1), whereas no dispersion of the two nonmotile strains was observed in the presence of F. oxysporum. By contrast, none of the bacteria was mobilized along the hydrophobic mycelia of Rhexocercosporidium sp. growing on agar surfaces. In column experiments however, strain SK1 was mobilized by Rhexocercosporidium sp. It is hypothesized that bacteria may move by their intrinsic motilitythrough continuous (physiological) liquid films forming around fungal hyphae. The results of this study suggest that the specific stimulation of indigenous fungi may be a strategy to mobilize pollutant-degrading bacteria leading to their homogenization in polluted soil thereby improving bioremediation.

Contributions of the peroxisome and β-oxidation cycle to biotin synthesis in fungi.

The first step in the synthesis of the bicyclic rings of D-biotin is mediated by 8-amino-7-oxononanoate (AON) synthase, which catalyzes the decarboxylative condensation of l-alanine and pimelate thioester. We found that the Aspergillus nidulans AON synthase, encoded by the bioF gene, is a peroxisomal enzyme with a type 1 peroxisomal targeting sequence (PTS1). Localization of AON to the peroxisome was essential for biotin synthesis because expression of a cytosolic AON variant or deletion of pexE, encoding the PTS1 receptor, rendered A. nidulans a biotin auxotroph. AON synthases with PTS1 are found throughout the fungal kingdom, in ascomycetes, basidiomycetes, and members of basal fungal lineages but not in representatives of the Saccharomyces species complex, including Saccharomyces cerevisiae. A. nidulans mutants defective in the peroxisomal acyl-CoA oxidase AoxA or the multifunctional protein FoxA showed a strong decrease in colonial growth rate in biotin-deficient medium, whereas partial growth recovery occurred with pimelic acid supplementation. These results indicate that pimeloyl-CoA is the in vivo substrate of AON synthase and that it is generated in the peroxisome via the β-oxidation cycle in A. nidulans and probably in a broad range of fungi. However, the β-oxidation cycle is not essential for biotin synthesis in S. cerevisiae or Escherichia coli. These results suggest that alternative pathways for synthesis of the pimelate intermediate exist in bacteria and eukaryotes and that Saccharomyces species use a pathway different from that used by the majority of fungi.

Organisation of genetic variation in multinucleate arbuscular mycorrhizal fungi

Les Champignons Endomycorhiziens Arbusculaires (CEA) forment une symbiose racinaire avec environ 80% des espèces connues de plantes vasculaires. Ils occupent une position écologique très importante liée aux bénéfices qu'ils confèrent aux plantes. Des études moléculaires effectuées sur des gènes ribosomaux ont révélé un très grand polymorphisme, tant à l'intérieur des espèces qu'entre celles-ci. Ces champignons étant coenocytiques et multinucléés, l'organisation de cette variabilité génétique intraspécifique pourrait avoir différentes origines. Ce travail se propose d'examiner l'organisation et l'évolution de cette variabilité. Sur la base de fossiles, l'existence des CEA remonte à au moins 450 millions d'années. Cette symbiose peut donc être considérée comme ancienne. Les premières données moléculaires n'indiquant pas de reproduction sexuée, une hypothèse fut élaborée stipulant que les CEA seraient des asexués ancestraux. La première partie de cette thèse (chapitre 2) met en évidence l'existence de recombinaison dans différents CEA mais montre également que celle-ci est insuffisante pour purger les mutations accumulées. La reproduction étant essentiellement asexuée, on peut prédire que les nombreux noyaux ont probablement divergé génétiquement. En collaboration avec M. Hijri nous avons pu vérifier cette hypothèse (chapitre 2). Dans le chapitre 3 j'ai cherché à comprendre si le polymorphisme était également présent dans une population naturelle du CEA Glomus intraradices au niveau intraspécifique, ce qui n'avait encore jamais été examiné. En comparant les empreintes génétiques d'individus obtenus chacun à partir d'une spore mise en culture, j'ai clairement démontré que d'importantes différences génétiques existent entre ceux-ci. Un résultat similaire, portant sur des traits quantitatifs d'individus de la même population, a été trouvé par A. Koch. Les deux études en ensemble montre que le polymorphisme génétique dans cette population est suffisamment grand pour être important au niveau écologique. Dans le chapitre 4, j'ai cherché a examiner le polymorphisme des séquences du gène BiP au sein d'un individu. C'est la première étude qui examine la diversité génétique du génome de CEA avec un autre marqueur que l'ADN ribosomique. J'ai trouvé 31 types de séquences différentes du gène BiP issu d'un isolat de G. intraradices mis en culture à partir d'une seule spore. Cette variation n'était pas restreinte à des zones sélectivement neutres du BiP. Mes résultats montrent qu'il y a un grand nombre de variants non-fonctionnels, proportionnellement au faible nombre de copies attendues par noyau. Ceci va dans le sens d'une partition de l'information génétique entre les noyaux.<br/><br/>Arbuscular mycorrhizal fungi (AMF) are root symbionts with about 80% of all known species of vascular land plants. AMF are ecologically important because of the benefits that they confer to plants. Molecular studies on AMF showed that rDNA sequences were highly variable between species and within species. Because AMF are coenocytic and multinucleate there are several possibilities how this intraspecific genetic variation could be organized. Therefore, the organization and evolution of this variation in AMF were investigated in the present work. Based on fossil records the AMF symbiosis has existed for 450 Million years and is therefore considered ancient. First molecular data indicated no evident sexual reproduction and gave rise to the hypothesis that AMF might be ancient asexuals. The first part of this thesis (Chapter 2) shows evidence for recombination in different AMF but also indicates that it has not been frequent enough to purge accumulated mutations. Given asexual reproduction, it has been predicted that the many nuclei in AMF should diverge leading to genetically different nuclei. This hypothesis has been confirmed by an experiment of M. Hijri and is also included in chapter 2 as the results were published together. In chapter 3 I then investigated whether intraspecific genetic variation also exists in a field population of the AMF Glomus intraradices. Comparing genetic fingerprints of individuals derived from single spores I could clearly show that large genetic differences exist. A similar result, based on quantitative genetic traits, was found for the same population by A. Koch. The two studies taken together show that the genetic variation observed in the population is high enough to be of ecological relevance. Lastly, in chapter 4, I investigated within individual genetic variation among BiP gene sequences. It is the first study that has analyzed genetic diversity in the AMF genome in a region of DNA other than rDNA. I found 31 sequence variants of the BiP gene in one G. intraradices isolate that originated from one spore. Genetic variation was not only restricted to selectively neutral parts of BiP. A high number of predicted non-functional variants compared to a likely low number of copies per nucleus indicated that functional genetic information might even be partitioned among nuclei. The results of this work contribute to our understanding of potential evolutionary strategies of ancient asexuals, they also suggest that genetic differences in a population might be ecologically relevant and they show that this variation even occurs in functional regions of the AMF genome.

Linking the population genetics and ecology of arbuscular mycorrhizal fungi

Résumé Les champignons endomycorhiziens arbusculaires (CEA) ont co-évolué avec les plantes terrestres depuis plus de 400 millions d'années. De nos jours, les CEA forment une symbiose avec les racines de la majorité des plantes terrestres. Les CEA sont écologiquement importants parce qu'ils influencent non seulement la croissance des plantes, mais aussi leur diversité. Les CEA sont des biotrophes obligatoires qui reçoivent leur énergie sous forme de glucides issus de la photosynthèse des plantes. En contrepartie, les CEA apportent à leurs hôtes du phospore. Les CEA croissent et se reproduisent clonalement en formant des hyphes et des spores. De plus, les CEA sont coenocytiques et multigénomiques; le cytoplasme d'un CEA contient des noyeaux génétiquement différents. De nombreuses études ont démontré que différentes espèces de CEA agissent différentiellement sur la croissance des plantes. Malgré une conscience de plus en plus forte de l'existence d'une variabilité intraspécifique, la question de savoir si les populations de CEA sont génétiquement variables a été largement négligée. Dans le Chapitre 2, j'ai cherché à savoir si une population de CEA provenant d'un seul champ possède une diversité génétique. Cette étude a mis en évidence une importante variation génétique et phénotypique au sein d'individus de la même population. Des différences au niveau de traits de croissance, héritables et liés à la valeur sélective, indiquent que la variation génétique observée entre isolats n'est pas entièrement neutre. Dans le Chapitre 3, je montre que les différences génétiques entre isolats de CEA d'une population provoquent de la variation dans la croissance des plantes. L'effet des isolats dépend des conditions environnementales et varie de bénéfique à parasitique. Dans le Chapitre 4, je montre que des traits de croissance de CEA varient significativement dans des environnements contrastés. J'ai détecté de fortes interactions entre différents génotypes de CEA et différentes espèces de plantes. Ceci suggère que dans un environnement hétérogène, la sélection pourrait localement favoriser différents génotypes de CEA, maintenant ainsi la diversité génétique dans la population. Les résultats de ce travail aident à mieux comprendre l'importance écologique de la variation intraspécifique des CEA. La possibilité de pouvoir cultiver des individus d'une population de CEA au laboratoire nous a permis une meilleure compréhension de la génétique de ces champignons. De plus, ce travail est une base pour de futures expériences visant à comprendre l'importance évolutive de la diversité intraspécifique des CEA. Abstract Arbuscular mycorrhizal fungi (A1VIF) have co-evolved with land plants -for over 400 million years. Today, AMF form symbioses with roots of most land plants and are ecologically important because they alter plant growth and affect plant diversity. AMF are obligate biotrophs, obtaining their energy in form of plant-derived photosynthates. In return,- they supply their host plants with phosphorous. These fungi grow and reproduce clonally by hyphae and spores. They are coenocytic and multigenomic, harbouring genetically different nuclei in a common cytoplasm. Many studies have shown different AMF species differentially alter plant growth. Despite the increasing awareness of intraspecific variability the question whether there is any genetic variation among different individuals of the same population has been largely neglected. In Chapter 2, we investigated whether there is genetic diversity in a field population of the AMF G. intraradices. This work revealed that large genetic and heritable phenotypic variation exists in this AMF population. Differences in fitness-related growth traits among isolates suggest that some of the observed genetic variation is not selectively neutral. In Chapter 3, we show that genetic differences among isolates from the same population also cause variation in plant growth. The isolate effects on plant growth depended on the environmental conditions and varied from beneficial to detrimental. In Chapter 4, fitnessrelated growth traits of genetically different isolates were significantly altered in contrasting environments. we detected strong AMF isolate by host species interacfions which suggests that in a heterogeneous environment selection could locally favour different AMF genotypes, thereby maintaining high genetic diversity in the population. The results of this work contribute to the understanding of the ecological importance of intraspecific diversity in AMF. The possibility of culturing individuals of an AMF field population under laboratory condition gave new insights into AMF genetics and lays a foundation for future studies to analyse the evolutionary significance of intraspecific genetic diversity in AMF.