Arbuscular mycorrhizal fungi (AMF, phylum Glomeromycota) could be the most widespread symbiotic organisms in nature. They can form symbioses with the roots from a majority of higher plant species. The research on AMF species diversity has attracted much attention because of the widespread distribution of these organisms in various types of ecosystems around the world and because of their great application potential in the areas of agriculture, forestry and environmental sciences. Recent study has shown that in these ecosystems the diversity and community structure of AMF can have significant effects on the diversity and productivity of plant communities, giving this research on AMF species diversity even greater significance.
Despite their widespread world distribution, fewer than 250 morphological species of AMF have been reported to date. However, evidence is now accumulating that the overall AMF global diversity has been severely underestimated. In natural or non-natural ecosystems, the species diversity of AMF could be affected by such factors as the species composition of host plants, human disturbances and by a variety of environmental factors. On the other hand, our understanding of AMF species diversity depends to a large extent on the development of methodology and on the application of new techniques. For a long time, the obligate symbiosis character of AMF and the methodology limitations have greatly hampered the research progress on AMF species diversity.
For many years, investigations of AMF species diversity depended on the morphological identification of AMF spores isolated from the rhizospheric soil of host species. However, the species diversity of AMF spores cannot truly reveal or reflect the diversity of the AMF species colonizing host plants, and the morphological identification of AMF species has relied too much on the research experiences of the investigators. The more recent application of the PCR-based molecular method in AMF species diversity studies has much improved our knowledge of AMF species diversity. On the one hand, the PCR molecular method can directly detect the AMF species diversity within the roots of host plants. On the other hand, as its procedures are more standardized, it can provide more reliable and comparable results. However, there are also several drawbacks of the molecular method. Firstly, the design of the AMF specific primer is dependent on the published DNA sequences; therefore the discovery of new AMF species is hampered. Secondly, large scale investigation of AMF diversity by molecular methods is still expensive, though the price for Sanger sequencing procedures has dropped significantly in recent years. Thirdly, the molecular method can only provide the results of AMF "taxa" diversity, not of AMF species diversity.
For many years, these drawbacks hampered the further development of the field of AMF species diversity. In recent years, the improvement in methodology (e.g. the proposed DNA barcode region for AMF) and the development of second generation sequencing technology (e.g. the 454 pyrosequencing technology) have provided excellent opportunities to strengthen our knowledge of AMF species diversity. We believe that within the next few years there will be huge progress in elucidating AMF species diversity. In this review article, research advances in AMF classification systems, in understanding of global species diversity levels and affecting factors to AMF species diversity, and in development of methodology for studying AMF species diversity are described and further research fields that need focus are also analyzed.