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CLASSIFICATION OF ARBUSCULAR MYCORRHIZAL FUNGI

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The fungi forming arbuscules in roots of terrestrial plants always created great taxonomic problems, mainly because of difficulties to extract their spores from the soil and to maintain the fungi in living cultures.

Peyronel (1923) first both discovered the regular occurrence of associations of spores and sporocarps of the Endogonaceae with vesicular-arbuscular mycorrhizae of plants and suggested the fungi to be the originators of the mycorrhizae.

The first monograph of the family Endogonaceae Paol. treated in the order Mucorales (Zygomycetes) has been prepared by Thaxter (1922). Fungi of this family were located in four genera producing spores in sporocarps: Endogone Link: Fries, Glaziella Berk., Sclerocystis Berk. & Br., and Sphaerocreas Sacc. & Ellis. In 1935, Zycha transferred the one species of Sphaerocreas to Endogone. The existing genera included both chlamydosporic and zygosporic species. Thaxter (1922) and Godfrey (1957) considered chlamydosporic species to be anamorphs of those producing zygospores, following the finding of both types of spores in sporocarps of Glomus fasciculatum (at that time known as Endogone fasciculata and E. microcarpa).

Many valuable data of the biology of fungi of the family Endogonaceae have been received from their studies in pot cultures. The mode of germination of spores of these fungi, their life cycles, subcellular spore structure, and the manner of colonization of roots have been recognized (Mosse 1959, 1970). It has been found that some taxa formed in plant roots vesicular-arbuscular mycorrhizae, whereas the mycorrhizae of other species lacked vesicles. Still other species have either been associated with ectomycorrhizae in the field (Gerdemann and Trappe 1974) or have produced ectomycorrhizae in one-species cultures (Fassi 1965; Walker 1985). The first key to recognition of the types of the endogonaceous spores isolated has been prepared by Mosse and Bowen (1968).

In 1974, Gerdemann and Trappe revised the Endogonaceae. In the family Endogonaceae further maintained in the order Mucorales, 44 species in seven genera were characterized. Among them, many taxa were redefined, and two genera (Acaulospora, Gigaspora) and 12 species were described as new. The genus Endogone contained 11 species with zygospores arranged in sporocarps. Two of them, E. lactiflua Berk. & Broom and E. flammicorona Trappe & Gerd., have produced ectomycorrhizae (Fassi 1965; Gerdemann and Trappe 1974; Walker 1985). According to Gerdemann and Trappe (1974), the other Endogone species were ectomycorrhizal or saprotrophic fungi.

The genus Glomus with 19 species (then with two varieties of Gl. macrocarpus, i. e., Gl. macrocarpus var. macrocarpus and Gl. macrocarpus var. geosporus), originally erected by Tulasne and Tulasne (1845), and the genus Sclerocystis with four taxa contained species forming chlamydospores blastically at hyphal tips. In contrast to sporocarpic Sclerocystis spp., the chlamydospores of members of the genus Glomus have been considered to occur mainly in loose aggregates or singly in the soil, although the genus also included species forming compact sporocarps with or without a peridium. Species of the genus Glomus have been found to form vesicular-arbuscular or only arbuscular mycorrhizae.

The distinctive property of members of the genus Sclerocystis was the production of chlamydospores arranged in a single layer around a central, spore-free plexus.

The newly erected genera Acaulospora and Gigaspora have been defined by Gerdemann and Trappe (1974) as forming azygospores singly in the soil, although no parthenogenetical process of spore development was observed. Acaulospora spp. produced spores laterally on the neck of a sporiferous saccule, and species of the genus Gigaspora formed spores terminally at the tip of an inflated, bulbous sporogenous hypha.

The genera Glaziella and Modicella Kanouse were represented by chlamydosporic and sporangial species, respectively.

In 1979, Ames and Schneider erected a new genus in the Endogonaceae, Entrophospora with E. infrequens, a species earlier existing in the genus Glomus, as Gl. infrequens (Hall 1977). Spores of E. infrequens were formed inside the neck of a sporiferous saccule.

In the same year, Benjamin (1979) redefined the order Mucorales, remaining in it only fungi being mainly saprotrophs or nonhaustorial parasites that reproduce asexually by sporangia, sporangiola, merosporangia, and sometimes by chlamydospores, arthrospores, and yeast-like cells. Consequently, he transferred the family Endogonaceae to the order Endogonales erected by Moreau (1953).

In 1979, Walker proposed to establish a next genus in the Endogonaceae, Complexipes C. Walker with one species, C. moniliformis C. Walker. The ornamented Glomus-like spores of C. moniliformis were isolated from the root zone of Parthenocisus quinquefolia (L.) Planch. growing in a pine plantation. No properties of mycorrhizae of this fungus were recognized. The spores morphologically resembled “crenulate spores” found by Mosse and Bowen (1968) in New Zealand and Australia and those of the E-strain fungi associated with ectomycorrhizae of trees of the genus Pinus (Mikola 1965; Wilcox et al. 1974).

Danielson (1982) suggested C. moniliformis to belong to the Ascomycota. Young and Wilcox (1984) classified the E-strain fungi and C. moniliformis to a new species of the genus Tricharina Eckblad, T. mikolae Chin S. Yang & H.E. Wilcox (Discomycetes, Ascomycota). Subsequently, Yang and Korf (1985) segregated from the genus Tricharina a new taxon, Wilcoxina Chin S. Yang & Korf gen. nov. (Pezizales) with anamorphic chlamydospores of the E-strain fungi and C. moniliformis.

Since 1984, the genera Glaziella and Modicella were not considered to be members of the family Endogonaceae. The presence of septa in fragments of sporocarps of Glaziella aurantiaca (Berk. & Curtis) Cooke has decided that the fungus has first been transferred to Deuteromycotina (Gibson 1984) and then to a new order, Glaziellales Gibson with one family, Glaziellaceae Gibson (Gibson et al. 1986). The genus Modicella has been transferred to the Mortierellaceae (Trappe and Schenck 1982).

In 1986, Walker and Sanders separated from the genus Gigaspora, containing species with spores lacking an inner germination wall having no physical contact with their main, structural wall, the genus Scutellospora with fungi forming spores having at least one inner germination wall.

In 1990, Morton and Benny located soil-borne fungi forming arbuscules in roots of terrestrial plants in a new order, Glomales (orthographically corrected to Glomerales by Schüßler et al. 2001), consisting of two suborders, Glomineae J.B. Morton & Benny and Gigasporineae J.B. Morton & Benny. The former suborder consisted of the type family Glomaceae, with the genera Glomus and Sclerocystis, and the Acaulosporaceae fam. nov., containing the genera Acaulospora and Entrophospora. The latter suborder was proposed to include the Gigasporaceae fam. nov. with the genera Gigaspora and Scutellospora.

Apart from differences in the mode of formation of spores and their subcellular structure, the taxa of the suborder Glomineae distinguished the ability to form vesicles that did not occur in mycorrhizae of members of the suborder Gigasporineae.

Fungi of the genus Endogone remained in the family Endogonaceae Paol. emend. J.B. Morton & Benny of the order Endogonales Moreau ex R. K. Benj. emend. J.B. Morton & Benny containing fungi forming only zygospores in compact sporocarps.

In 1990, Almeida and Schenck concluded that, except for Sclerocystis coremioides, a continuum of morphological properties exists between sporocarpic Glomus species and the other members of the genus Sclerocystis. As a result, the five-species genus Sclerocystis was reduced to a single-species one.

In 2000, Redecker et al., utilizing both morphological and molecular data, transferred S. coremioides to the genus Glomus and, thereby, eliminated the genus Sclerocystis from the Kingdom Fungi.

In 2001, based on data from molecular, morphological and biochemical investigations, Morton and Redecker erected two new families in the order Glomales (now Glomerales), i. e., Archaeosporaceae and Paraglomaceae (now Paraglomeraceae). Each of these families was phylogenetically distant from each other and from other glomalean families, despite similarities in mycorrhizal morphology. The family Archaeosporaceae contained one genus, Archaeospora, with three species forming atypical Acaulospora-like spores from the neck of a sporiferous saccule. Two of these species, Ar. gerdemannii and Ar. leptoticha, were considered to dimorphic, forming Glomus-like spores as well. The genus Paraglomus in the family Paraglomaceae (now Paraglomeraceae) consisted of two species producing spores indistinguishable from those of Glomus species.

The classification of arbuscular fungi used in the website presented here is that of Schüßler et al. (2001) with emendations of Oehl and Sieverding (2004), Walker and Schüßler (2004), Sieverding and Oehl (2006), Spain et al. (2006), Walker et al. (2007a, b), and Palenzuela et al. (2008).

GLOMEROMYCOTA C. Walker & Schuessler
	Glomeromycetes Cavalier-Smith
		Archaeosporales C. Walker & Schuessler
			Ambisporaceae C. Walker, Vestberg & Schuessler
				Ambispora Spain, Oehl & Sieverd.
			Archaeosporaceae J.B. Morton & D. Redecker emend. Oehl & Sieverd.
				Archaeospora J.B. Morton & D. Redecker
				Intraspora Oehl & Sieverd.
			Geosiphonaceae Engler. & E. Gilg emend. Schuessler
				Geosiphon (Kütz.) F. Wettst.
		Diversisporales C. Walker & Schuessler
			Acaulosporaceae J.B. Morton & Benny
				Acaulospora Gerd. & Trappe emend. S.M. Berch
				Kuklospora Oehl & Sieverd.
				Acaulospora Gerd. & Trappe emend. S.M. Berch
			Diversisporaceae C. Walker & Schuessler
				Diversispora C. Walker & Schuessler
				Otospora Oehl, J. Palenzuela & N. Ferrol
			Entrophosporaceae Oehl & Sieverd.
				Entrophospora R.N. Ames & R.W. Schneid. emend. Oehl & Sieverd.
			Gigasporaceae J.B. Morton & Benny
				Gigaspora Gerd. & Trappe emend. C. Walker & F.E. Sanders
				Scutellospora C. Walker & F.E. Sanders
			Pacisporaceae C. Walker, Blaszk., Schuessler & Schwarzott
				Pacispora Oehl & Sieverd.
		Glomerales J.B. Morton & Benny
			Glomeraceae Piroz. & Dalpé
				Glomus Tul. & C. Tul.
		Paraglomerales C. Walker & Schuessler
			Paraglomaceae J.B. Morton & D. Redecker
				Paraglomus J.B. Morton & D. Redecker

In this system of classification, fungi forming or considered to form arbuscular mycorrhizae are placed in four orders, i. e., Archaeosporales, Diversisporales, Glomerales, and Paraglomerales, comprising ten families and thirteen genera, belonging to the class Glomeromycetes of the phylum Glomeromycota (Oehl and Sieverding 2004; Palenzuela et al. 2008; Schüßler et al. 2001; Sieverding and Oehl 2006; Spain et al. 2006; Walker and Schüßler 2004; Walker et al. 2007a, b). Geosiphon pyriformis of the family Geosiphonaceae (Archaeosporales) does not form arbuscular mycorrhizae. It forms endocytosymbioses with cyanobacteria (Noctos sp.) and was included into the Glomeromycota based on only its close molecular relationship.

Molecular investigations showed that fungi of the phylum are more closely related to members of the phyla Ascomycota and Basidiomycota than to those of the phylum Zygomycota, as, e. g., Gerdemann and Trappe (1974), Lawrynowicz (1979), and Morton and Benny (1990) earlier believed.