Until recently, arbuscular mycorrhizal (AM) fungi have generally been classified in the Zygomycota (Order Glomales), but they do not form the zygospores characteristic of zygomycota, and all ‘glomalean’ fungi form mutualistic symbioses. Recent molecular studies have suggested a separate phylum is appropriate for the AM fungi, the Glomeromycota, and this is the position taken by the AFTOL study (Redecker & Raab, 2006), and which we follow.
Traditionally, taxonomy of AM fungi has been based on characteristics of the relatively large (40 to 800 µm diameter) multinucleate spores. There is no evidence that the Glomeromycota reproduce sexually. Studies using molecular marker genes have detected little or no genetic recombination so it is assumed generally that the spores are formed asexually. Clearly, there is a severe limit to the morphology that can be used in taxonomy and fewer than 200 species (morphotypes) are recognisable. The spores have a layered wall and features of this can be used to describe morphospecies. Similarly, spores may be formed singly, in clusters or aggregated in so-called sporocarps and the mode of spore formation has been important in describing genera and families (CLICK HERE for illustration of most of these morphologies).
This is a very challenging group to study. AM fungi are obligate symbionts and none of these fungi has been cultivated without their plant hosts. Pure fungal biomass can be obtained only from cultures in transformed plant roots that can be cultivated in tissue culture, but only a small number of AM species are available in this form. Most samples can be contaminated by numerous other microorganisms, including fungi from other phyla, so progress with multigene phylogenies has been slow and the nuclear-encoded ribosomal RNA genes have remained the only widely accessible molecular markers.
Yet, in ecological terms, this is possibly the most important group of fungi because AM fungi form endomycorrhizal associations with about 80% of land plants. The association is essential for plant ecosystem function because the plants depend on it for their mineral nutrient uptake, which is efficiently performed by the mycelium of the fungal symbionts that extends outside the roots. Within root cells AM fungi form hyphal coils or the typical tree-like structures, the arbuscules.
Some also produce storage organs, termed vesicles (hence, another frequently used name for them – vesicular-arbuscular mycorrhizas or VAM-fungi). In phylogenetic terms they are important because they are the oldest unambiguous fungi known from the fossil record (see the Fossil Fungi section in Chapter 2). Put these two facts together and you get the suggestion that early colonisation of the land surface on Earth was promoted by the success of this plant-fungal symbiosis. The multigene phylogeny of fungi established by the AFTOL project places the Glomeromycota in a basal position as a sister group of Asco- and Basidiomycota (see the cladogram illustrated in Chapter 2).
Ten genera are recognised currently in the Glomeromycota. Names to look out for include Glomus, which is the largest genus in the phylum, with more than 70 morphospecies (spores, typically with layered wall structure, are formed by budding from a hyphal tip), placed in the Family Glomeraceae, Order Glomerales. It might be appropriate here to explain a specific point of nomenclature: why is Glomus classified in the Glomerales? The International Code of Botanical Nomenclature requires the name of a family or order to be formed from the genitive singular of the name of an included genus. The genitive of Glomus is Glomeris, and so, by International agreement, the name of the Family should be Glomeraceae and the Order Glomerales (rather than ‘Glomales’). For a reminder of the characteristic word-endings used for the principal taxonomic ranks of fungi today visit our ‘Some terminology explained’ Resources Box CLICK HERE.
Gigaspora and Scutellospora are closely related genera in the family Gigasporaceae; their spores form on a bulbous sporogenous cell and germinate through a newly formed opening in the spore wall. These two genera do not form vesicles within roots, and the mycelium external to the root bears ‘auxiliary cells’ of unknown function. For the family Acaulosporaceae the diagnostic feature is the formation of spores next to a ‘saccule’ that collapses during spore maturation and eventually disappears; the included genera are Acaulospora and Entrophospora.
Geosiphon pyriformis (family Geosiphonaceae) is the only member of the phylum that is symbiotic with a cyanobacterium. It forms an endosymbiosis with Nostoc punctiforme, the photobionts being harboured in fungal bladders up to 2 mm in size (Redecker & Raab, 2006).
Because of the difficulties (outlined above) resulting from their obligate symbiosis and inability to culture them, at the moment there is no clear notion of what constitutes a species in the Glomeromycota. Environmental studies using molecular markers have revealed a great deal of diversity suggesting that the number of 200 or so described morphospecies might considerably underestimate the true diversity of the Glomeromycota.
Updated December 16, 2016