Mycorrhizae Biology Laboratory | LBM
Morphological aspects for the identification of AMF
Since the beginning of taxonomic studies, one of the criteria used to classify AMF was the mode of development or genesis of the spores (asexual formation), distinct from other soil fungi, such as Endogone, for example. Tulasne & Tulasne (1845), when describing the genus Glomus, took as a basis mainly the morphology of the sporocarps and spores. In his review, Thaxter (1922) grouped genera now included among AMF in Endogonaceae, based on the similarity between the sporocarps of Endogonone and Sclerocystis.
However, it was only in 1974 that Gerdemann & Trappe created the morphological basis used to this day by taxonomists for taxonomic studies of AMF. The criteria for identifying these fungi were adopted: shape, color and size of the spores; way of insertion of the hypha into the spore, number, thickness and color of the layers of the spore wall. In addition to these characters, common to all AMF, the authors also adopted, for species that formed sporocarps, the shape, size and color, in addition to the arrangement of the spores in this structure.
Nine years after this celebrated review, the basis for more detailed studies of the AMF spore wall was created. Walker (1983) proposed the systematization for descriptions of AMF species, creating names and describing four types of glomerospore “wall” (= layer): evanescent, laminated, unitary and membranous. In addition to this terminology, the author also proposed a representative scheme for each type of wall and its arrangement in the spore, calling it Murography, which has since been widely used by taxonomists to describe new species, when new characteristic “walls” were also suggested. in the group (figure 1).
Figure 1 – Currently, 9 types of walls are recognized (sensu Walker 1983). G – Germinative, P – Peridium,
Berck & Koske (1986), Walker (1986) and Morton (1986) described, respectively, the expansive, the leathery and the amorphous wall. With the exception of the leathery wall (less affected), the others are characterized by showing a strong reaction to the resin (polyvinyl alcohol lactoglycerol - PVLG) used to preserve the glomerospores (Koske & Tessier 1983).
In a review of the morphological characters of glomerospores, Morton (1988) admitted some confusion in the interpretation of the wall, caused by the use of resin and preservatives and showed the variation in the reaction promoted by these substances, leaving some aspects related to this structure doubtful, such as such as the distinction between the leathery, amorphous, unitary and membranous “walls”, in addition to the variable size of these layers (Figure 2).
The resin reaction was also observed in the laminated layer of Scutellospora biornata Spain, Sieverd. & Toro (Spain et al. 1989b), which expands in contact with the PVLG. Such a reaction had not been previously mentioned for any AMF species.
Spain et al. (1989a) described the germinal “wall”, found only in Gigaspora, and characterized by forming papillae on the inner part of the spore wall. It is in this layer that the germ tubes appear, as detailed in an ultrastructural study by G. albida Schenck & Smith (Maia et al. 1994). Spain et al. (1989a) further stated that in Gigaspora spores there is a distinct separation between the germinal and laminated layers. However, this layer is contested by other authors (Redecker et al. 2007).
Some wall types, as well as the description of AMF species, were contested by Spain (1990). According to the author, the reaction to the resin or preservatives changes the true morphology of the glomerospore “walls”, which should also be observed in water and not just in PVLG. Although this suggestion has not been widely accepted by taxonomists, it is appropriate because it allows visualization of the real morphological aspect of the walls and even layers not observed with the preparation of the resin material (Spain 2003).
A new type of wall, called notching wall, characterized by forming notches on the surface (similar to striations) and on the edge of broken spores, with “V”-shaped fissures, was described by Koske & Gemma (1995) . Since then, this wall has only been referred to in Scutellospora hawaiiensis Koske & Gemma.
In addition to traditional morphological studies that normally constitute descriptions of new species (Oehl et al. 2002; 2003; 2004), reevaluations of those already described (Blaszkowski 1997; Goto et al. 2008), new genera (Sieverding & Oehl 2004; Palenzuela et al 2008), ultrastructure (Maia 1995; Maia & Kimbrough 1993; Maia et al. 1994; Mosse 1970a; Mosse 1970b) and ontogenetic studies (Franke & Morton 1994, Bentivenga & Morton 1995, Morton 1995, Stürmer & Morton 1999; Stürmer & Morton 2000, de Souza et al. 2005) have contributed extensively to the understanding of the subcellular structures of the AMF spore wall. These studies range from evaluating the development of sporocarpic species (Almeida & Schenck 1990b; Wu 1993) to that of ectocarpic species (Giovannetti et al. 1991; Franke & Morton 1994; Bentivenga & Morton 1995; Morton 1995; Stürmer & Morton 1997 and 1999; Dalpé et al. 2002). This information was used by Morton ( http://invam.caf.wvu.edu ) to propose new terminology for describing AMF species.
AMF spores may have two walls (= wall groups sensu Walker 1983); spore wall, and inner or germinal wall, respectively (Stürmer & Morton (1999); http://invam.caf.wvu.edu ). Recently, Sieverding & Oehl (2006) suggested a change in this terminology: the spore wall (sensu Morton) was designated as the outer spore wall; the central layer of the wall was called the “middle wall” and the innermost layer was called the “inner wall”.
This diverse terminology for AMF descriptions has caused some confusion among researchers, especially those who are not familiar with this specific group of fungi. An example can be seen in the description of Archaeospora trappei made by Morton ( http://invam.caf.wvu.edu ), where the term “endospore” stands out to designate the inner layer of the wall of this species. On the other hand, Blaszkowski ( http://www.lrz-muenchen.de/~schuessler/amphylo/) states that Archaeospora trappei has a germ wall, instead of an endospore.
Endospore is a term used by mycologists to designate a structure that is generally thin and late formed during sporogenesis, while germinal wall is the term designated for walls that have a germination function. According to Jeffries et al. (2007), the strong association of internal flexible layers with the formation of the germinal plate consolidates the preference for using the term “germ wall”. However, in both cases, germination is implied, which makes the terms practically functional synonyms.
Figure 2. AF Wall types in glomerospores. A- Laminated in Scutellospora sp . B- Expansive laminate in D. biornata . C- Unitary in Scutellospora sp . D- Flexible or membranous, in A. morrowiae . E- Expansive evanescence, in F. halonatum . F- Amorphous, in A. foveata .
Ornamentation on the spore wall
Ornaments can be classified according to location, and can be found in the outer layer or in one of the inner layers (in the second or third layer of the glomerospore wall) or according to type, and can be projections (thorns, warts, papillae, tubercles and cones) or depressions (circular, irregular, labyrinthine, hexagonal and pentagonal) (Figure 3).
Variations in the ornamentation of the glomerospore wall are not frequently documented (Morton 1988), but A. tuberculata spores varied in height (1 micron in young spores to 2.5 microns in mature spores). In this way, AMF spore wall ornamentation may eventually be influenced by spore development.
Morton (1988) stated that only layers with a laminated, unitary and membranous appearance present ornamentation. However, in general, ornamentations occur in laminated and unitary layers, with an exception in flexible layers. According to Hu (2002), the first layer of G. spinosum glomerospores is covered by hair-like protuberances. This layer is similar to that found in the glomerospores of G. pansihalos and F. coronatum . A comparison of these species could help elucidate the nature of the outer layer of G. spinosum .
Glomerospores of Diversispora insculpita and F. verruculosum present ornamentation distinct from other AMF species (Blaszkowski et al. 2004; Blaszkowski & Tadych 1997), which occur on the inner surface of the second layer and the third layer of the spore wall, respectively. In D. insculpita, perforations are observed on the internal surface, giving the impression that the ornamentation is similar to that observed in glomerospores of A. scrobiculata . In F. verruculosum the glomerospores have warts on the inner surface of the third layer of the wall.
Recently, Cuenca & Herrera-Peraza (2008) described a new species of Scutellospora with a distinct ornamentation pattern, similar to a fingerprint, on the surface of the glomerospores of C. striata , which constitutes a new ornamentation pattern for this diverse group of fungi. .
Figura 3 A-O. A- Acaulospora alpina. B- Archaeospora undulata. C- A. excavata. D- A. foveata. E- A. bireticulata. F- Acaulospora sp. G- A. denticulata. H- A. rehmii. I- A. tuberculata. J- D. biornata. K- Scutellospora sp1. L- Scutellospora sp2. M- R. gregaria. N- R. verrucosa. O- F. multiforum.