Mycologist, Volume 19 , Part 3 August 2005. ©The British Mycological Society Printed in the United Kingdom. DOI: 10.1017/S0269915XO5003022 96 Keywords: arbuscular mycorrhizas, Glomeromycota, evolution, genetic structure, ecology. Introduction In 2001, the Kingdom Fungi gained a new Phylum, the Glomeromycota. Formerly, the (AM) fungi were placed in a separate order (Glomales) within the Zygomycota, but molecular evidence has shown that they are a distinct lineage (Schüßler et al., 2001). Recognition of a new phylum is noteworthy, but tells us little about the ecological and evolutionary significance of what is turning out to be a remarkable group of organisms. Mycorrhizas have been studied for over 100 years, but in the last 20 there has been an explosion of interest. Virtually all plants form mycorrhizal symbioses: probably only 5-10% are non- mycorrhizal. About two thirds form a distinctive type of mycorrhiza called an arbuscular mycorrhiza (AM), named from the distinctive hyphal structure formed within the cortical cells of roots (Figure 1). Other plant species form a variety of other types of mycorrhizal symbiosis, always involving quite different fungal partners (Finlay, 2004). What is most striking about the arbuscular mycorrhizal symbiosis is its ubiquity. Every major clade in the plant kingdom, from liverworts to monocots has AM members, which suggests that the symbiosis originated very early in land plant evolution, and has been lost in a few branches of the plant phylogenetic tree and replaced by other mycorrhizal types in a few other branches. Fossil evidence confirms this: rhizomes of Aglaophyton major from the Devonian Rhynie chert (400m years b.p.) contain easily recognisable arbuscules (Remy et al., 1994; Taylor et al., 1995; Figure 1). The picture is completed by molecular clock data that suggest the origin of the phylum is at least this old (Simon et al. 1993). Why was the AM symbiosis such an early evolutionary innovation? In modern plants, the major functional activity of the symbiosis is the exchange of carbon for phosphorus among the partners. The fungus is an obligate symbiont, acquiring all its carbon from the plant, while the plant receives phosphate, acquired by the fungus from the soil. The benefit to the plant is not as obvious as that to the fungus. Phosphate is an exceptionally immobile ion in soil, since it forms insoluble complexes with all the predominant soil cations, such as Fe 3+ , Al 3+ and Ca 2+ . Consequently, phosphate ions diffuse The ecology and evolution of the arbuscular mycorrhizal fungi THORUNN HELGASON & ALASTAIR FITTER Department of Biology, University of York, PO Box 373, York, YO10 5YW. Tel: +44 (0)1904 328614, Fax: +44 (0)1904 328505. Email: th7@york.ac.uk; ahf1@york.ac.uk The Glomeromycota is the newest fungal phylum. These are the arbuscular mycorrhizal (AM) fungi that form symbioses with the majority of land plant species. Fossil and molecular evidence suggest this is an ancient symbiosis, that may have been instrumental in enabling plants to colonise terrestrial habitats. The AM fungi gain carbon from their plant host, and the primary benefit to plants is thought to be the acquisition of phosphate, a highly immobile ion in soil. However, they are thought to have many other ecological benefits. Host specificity has been considered to be low in this group, as most fungi in culture will colonise most plants, but recent evidence suggests that some AM fungi show a degree of specificity. We review the evidence revealing the genetic structure of this ancient group of fungi. Although a consensus has not yet been reached, we suggest that understanding the evolution and genetic structure of this enigmatic group may be the key to understanding how they function in ecosystems.