Currently 270 species of Glomeromycota have been described (Oehl et al. 2014), representing a significant increase since the review by Morton & Benny (1990) who used around 149 species (Schenck & Pérez 1990). There are divergent diversity estimates for fungi ranging from 1.5 million species (Hawksworth 1991) to 5.1 million (Blackwell 2012), which take into account different evaluation parameters such as ecological or molecular aspects. Despite these striking differences in methodology, the wide richness of species in the kingdom of fungi is evident, where the vast majority remains unknown as only 2 to 5% of species are described (Kirk et al. 2012). However, there is still a certain contradiction between the diversity data. For example, around 65% of the described fungal species can be synonymous (Hawksworth 2004), that is, around 105,000 fungal species can already be known, considering only data contained in the Fungal Dictionary (Kirk et al 2001).

       Estimating the diversity of AMF is still a major problem in mycorrhizology as there is currently great divergence among researchers regarding the mode of reproduction of these fungi, which until the last decade were recognized as exclusively asexual (Morton 1990). However, new genetic evidence shows that these fungi have preserved meiotic machinery in their genome, showing compelling evidence of sexuality (Tisserant et al. 2013). This drastically changes the initial paradigm that AMF would be a group of fungi that are not very diverse since they would be asexual organisms. Considering them as sexual, and ancestral, are strong factors to believe that they are a megadiverse group as they would have had a long evolutionary history (450 to 550 million years) for numerous speciation processes. The last compilation of descriptions was made by Schenck & Pérez (1990), with data on 149 species, distributed in six genera: Acaulospora (28), Entrophospora (3), Gigaspora (7), Glomus (75), Sclerocystis (13) and Scutellospora (23). Since then, many new species have been described and others have become synonymous (Almeida & Schenck 1990; Walker & Vestberg 1998; Oehl et al. 2002 and 2003).

      According to de Souza (2007), the apparent small richness of species known in the phylum Glomeromycota is paradoxical, mainly due to the ancestral origin of these fungi, the high diversity of plants with which they are associated, the long period of co-evolution of mycorrhizal symbiosis and the broad distribution of AMF in terrestrial ecosystems. All these arguments indicate diversification much greater than that known for this phylum. However, to better understand the mechanisms that govern the diversification of a group of organisms, it is necessary to understand the biology of the group and the processes related to speciation in Glomeromycota, which will be discussed in the topic regarding species conceptualization.

            Based on data obtained by Bever et al. (2001) in studies carried out in an area of ​​1 hectare, with 50 plant species, where 37 species of AMF were found, 1/3 of which are not yet known to science, de Souza et al. (2008) estimated AMF diversity to be between 37,000 and 78,000 species. Therefore, currently only 0.05% of the species in this group of fungi are known, based on the supposed existence of 37,000 species.

            According to Peixoto et al. (2006), one third of Brazilian biological diversity is known and only 20% of the planet's total species are cited in Brazil. Therefore, it is plausible to estimate high diversity of AMF also in Brazil.

The difficulty in estimating the diversity of Glomeromycota is related to the difficult access to the fungus, which is basically due to obligatory mycotrophism and hypogeal nature, in addition to the lack of broad knowledge about the ecology and biology of these fungi (Bever et al. 2001; de Souza et al. 2008). Furthermore, species descriptions are based on morphological evidence from glomerospores extracted from native soil or crops, but there are species that are not observed in spore form (Clapp et al. 2002). All these problems related to the diversification of this group also promote serious conceptual difficulties. Defining species for AMF is still a difficult task given the morphological limitations (with broad diversification in the same species) and also the molecular ones (which show the segregation of species considered similar).