ABSTRACT
The emphasis within this chapter is on molecular phylogeny, which is comparatively a recent trend among mycorrhiza researchers. Chronologically, the classical approaches precede the use of molecular phylogenetic methods. In accordance with this fact, a summary of historical development, and a highly abridged version of classical taxonomy have been provided. The classical taxonomy of arbuscular mycorrhizas began with Thaxter's (1922) revision of fungi belonging to Endogoneceae. At that time, involvement of many species belonging to this group of fungi in symbiosis was yet unclear. Around the same period, Peyronel (1923, 1924) grouped these fungi under the order Endogonales. Detailed investigation and characterization of individual AM fungal species became a trifle easier after Gerdemann and Nicholson (1963) standardized the wet sieving and decanting technique. This technique became frequent with mycorrhizasts, and lead to the realization that soil-borne AM fungi are worldwide in distribution and common with most plant species. Gerdemann and Trappe (1974) then delineated Endogonales into 7 genera and described in all 31 zygomycetous mycorrhizal species in their monograph. Trappe (1982) states that a spurt in the interest to classify and study AM fungi led to the identification of additional 70 plus AM species in 1970s and 1980s. Later, Berch and Trappe (1987), in their revision on the classification of AM fungi, stated that 107 species existed. In fact, considering the zeal and rapidity of reports on new AM fungal species (e.g. Schenk and Smith, 1982; Walker and Trappe, 1980; Walker et al. 1984; Smith and Schenk, 1985; Berch and Trappe, 1985; Koske, 1985; Warcup, 1985), it was forecasted that the list may reach over 200 species by 1990 (Trappe, 1982; 1987). However, at present, slightly over 180 AM fungal species have been identified and described (Walker and Trappe, 1993; Morton et al. 1993,1995; Morton and Bentivenga, 1994; Trappe and Schenck, 1982, see Table 1.2).
