(S.L. Rose & Trappe) Sieverd. & Oehl
SPORES single in the soil; hyaline to orange white (6A2); globose to subglobose; (70-)107(-165) µm diam; rarely ovoid; 100-130 x 110-150 µm.; with one subtending hypha.
SUBCELLULAR STRUCTURE OF SPORES composed of a spore wall and one, inner germination wall.
Layer 1 permanent, rigid, hyaline to orange white (6A2); (0.7-)1.2(-1.5) µm thick; ornamented mainly with warts, 1.7-5.7 x 0.7-1.9 µm, rarely with ridges, 0.5-0.8 µm high, tightly adherent to layer 2; occasionally separating from layer 2 in vigorously crushed spores.
Layer 2 laminate, smooth, hyaline, (2.0-)3.5(-4.5) µm thick.
Layer 3 flexible to semiflexible, hyaline, 0.5-0.8 µm thick, usually tightly adherent to layer 2 and, thereby, difficult to see.
Germination wall composed of three layers (gwl1-3).
Layer 1 flexible, hyaline, (0.2-)0.7(-1.2) µm thick, easily separating from layer 2.
Layer 2 flexible, coriaceous, hyaline, (1.2-)1.8(-2.8) µm thick, staining dull red (9B4) to deep red (11B8) in Melzer’s reagent.
Layer 3 flexible, hyaline, up to 0.5 µm thick, always tightly adherent to layer 2 and, hence, extremely difficult to resolve.
SUBTENDING HYPHA hyaline; straight or slightly curved; cylindrical, sometimes slightly constricted at the spore base; (6.4-)8.7(-12.5) µm wide at the spore base.
GERMINATION. A germ tube develops from a germination shield and penetrates the spore wall.
Arbuscules in roots of P. lanceolata
MYCORRHIZAE. In one-species cultures with Plantago lanceolata L. as the plant host, Pac. scintillans formed mycorrhizae composed of arbuscules, vesicles, as well as intra- and extraradical hyphae staining intensively in 0.1% trypan blue. Additionally, one of the ca. 150 cultures established from single spores showed that Pac. scintillans forms auxiliary cells morphologically resembling those of species of the genus Scutellospora, i. e., they are knobby and not spiny as in Gigaspora spp. The auxiliary cells of Pac. scintillans were produced both at the ends of extraradical hyphae of and inside mycorrhizal roots of P. lanceolata. Intraradical auxiliary cells also occurred in mycorrhizae of S. reticulata (Koske, D.D. Mill. & C. Walker) C. Walker & F.E. Sanders (Declerk et al. 2004).
Arbuscules, vesicles, and hyphae in roots of P. lanceolata
Extrararadical auxiliary cells
Intraradical auxiliary cells
NOTES. The wall of the most juvenile Pac. scintillans spores consisted of only one layer (layer 1). Then, layer 2 originated due to synthesis first a very thin, <0.5 µm thick, flexible structure (sublayer) and subsequently gradual addition of next sublayers. Layer 3 probably formed after the differentiation of layer 2, as in other members of the Glomeromycota examined ontogenetically. However, this phenomenon was not observed by the author of this website. When the spore wall was completely differentiated, the inner germination wall layers 1-3 started to form successively. The beginning of the differentiation of the germination wall layer 2 signaled its staining reaction in Melzer's reagent. The formation of the germination shield on the surface of the inner germination wall ended the ontogenetic development of Pac. scintillans spores.
As Blaszkowski (2003), Morton (1993), and Walker (1992) emphasized, spores of Gl. chimonobambusae C.G. Wu et Y.S. Liu, Gl. dominikii Blaszk., and Gl. scintillans Trappe are untypical for members of the family Glomeraceae in the manner of their origination (determined only in Gl. dominikii; Blaszkowski 2003), germination (revealed in Gl. dominikii and Gl. scintillans; Blaszkowski 2003; Walker et al. 2004), and subcellular structure. First, all the three species form spores with two walls, a spore wall and an inner wall resembling in morphology and the function performed a germination wall of some fungi of the family Acaulosporaceae (e. g., Entrophospora infrequens (I.R. Hall) R.N. Ames & R.W. Schneid.) and the subgroup of genus Scutellospora represented by S. persica (Koske & C. Walker) C. Walker & F.E. Sanders (Morton 1995). All Glomus spp. have only a spore wall. Second, the spore wall of these fungi consists of three layers, of which the outermost one, forming the spore surface, is a permanent structure and ornamented with small processes, and the innermost layer is flexible to semiflexible and colourless. The outermost spore wall layer of all Scutellospora spp. also is permanent and the wall of spores of most Acaulospora and Scutellospora spp. comprises a third layer similar to that of Gl. dominikii and Gl. scintillans. In contrast, the outermost spore wall layer of most species of the genus Glomus sloughs with age and generally is smooth when young. Additionally, only few Glomus species, e. g., Gl. claroideum N.C. Schenck & S.M. Sm., produce spores with a thin, flexible innermost layer. Except for A. thomii Blaszk., in the other Acaulospora and Entrophospora spp. described to date, the outermost spore wall layer sloughs and rarely is present in mature spores. The outermost layer of the spore wall of A. thomii is a semipermanent structure, i. e., it is more or less deteriorated at maturity, but usually is present in even old spores. The ornamentation of spores of Gl. chimonobambusae, Gl. dominikii, and Gl. scintillans highly resembles that of four of the five species of the subgroup of Scutellospora characterized by Morton (1995). Third, the middle layer of the germination wall of spores of Gl. chimonobambusae, Gl. dominikii, and Gl. scintillans (1) is surrounded with a thin, smooth, flexible layer, (2) has properties of the coriaceous layer sensu Walker (1986), (3) encloses a thin flexible innermost layer; such a layer probably also occurs in other Scutellospora spp. (Blaszkowski, pers. observ.), and (4) stains purple in Melzer's reagent. Thus, the position, as well as the morphological, physical and biochemical properties of this layer are reminiscent of those of the middle layer of the innermost germination wall of, e. g., S. nodosa Blaszk. (Blaszkowski 1991). Additionally, the inner wall of spores of Gl. dominikii, Gl. scintillans, and probably Gl. chimonobambusae, as the microphotographs of the original paper describing the fungus (Wu et al. 1995) suggest, has no physical contact with the spore wall and origins independently and de novo after the full differentiation of the spore wall, similarly as in all members of the family Acaulosporaceae and the genus Scutellospora. Forth, the most unique structure of spores of Gl. dominikii and Gl. scintillans is a germination shield, from which a germ tube grows (Blaszkowski 2003). A pre-germination structure also occurs in spores of fungi of the genera Acaulospora and Entrophospora (called a germination orb) and Scutellospora (called a germination shield). The germination shields of Gl. dominikii and Gl. scintillans are uniform, plate-like structures and, thereby, most resemble those of fungi of the genus Scutellospora. In members of the genus Acaulospora and Entrophospora, the germination shields are composed of a centrifugally rolled hypha (Blaszkowski 2003; Morton 2002). However, the germination shields of Scutellospora spp. are permanent structures, whereas those of Gl. dominikii and Gl. scintillans deteriorate with age and usually are difficult or impossible to recognize in older spores, as the germination orbs of spores of Acaulospora and Entrophospora spp. Finally, Blaszkowski (unpubl. data) recently produced a one-spore culture of G. dominikii, in which mycorrhizal roots of P. lanceolata were associated with auxiliary cells, morphologically similar to those formed by species of the genus Scutellospora. Neither Acaulospora and Entrophospora spp. nor members of the other genera of AMF produce auxiliary cells.
In summary, spores of Gl. dominikii, Gl. scintillans, and Gl. chimonobambusae possess characters of both the fungi of the family Acaulosporaceae and the genus Scutellospora. However, the number of the common characters of the group represented by G. dominikii with those of the Acaulosporaceae and Scutellospora spp. clearly showed a close relationship of the former group with fungi of the genus Scutellospora, especially with those of the subgroup represented by S. persica.
Following Blaszkowski's (2003), Morton (1993), and Walker (1992) suggestions, Oehl and Sieverding (2004) erected a new genus in the Glomeraceae, Pacispora Sieverd. & Oehl, with four species. The type species of this genus is Pac. scintillans S.L. Rose & Trappe emend. Sieverd. et Oehl comb. nov. This genus was established based on only morphological characters of spores. Although the action undertook was pertinent, the location of Pacispora in the family Glomeraceae was erroneous, as molecular analyses of two of the four species of this genus revealed (Walker et al. 2004). Walker's et al. (2004) studies indicated that Gl. dominikii and Gl. scintillans are congeneric and are more closely related to species of the genus Scutellospora than Acaulospora, but Gl. scintillans and Scutellospora spp. separate a statistically proved distance at the level of a family (Fig. 1). Thus, the methods of molecular biology confirmed the morphological similarities presented above. Consequently, Walker et al. (2004) erected a new family, Gerdemanniaceae C. Walker et al., with a new genus, Gerdemannia C. Walker et al. represented by the type species, Ge. scintillans (S.L. Rose & Trappe) C. Walker, Blaszk., Schuessler & Schwarzott comb. nov. Glomus chimonobambusae was transferred to Gerdemannia as Ge. chimonobambusae (C.G. Wu et Y.S. Liu) C. Walker et al. comb. new.
Unfortunately, Oehl's and Sieverding (2004) paper was published one month before that of Walker et al. (2004) and the genus Pacispora consequently had priority and must be used under Art. 11.3 of the Botanical Code. Recently, Walker and Schüßler (2004) introduced a new family name, Pacisporaceae C. Walker, Blaszk., Schuessler & Schwarzott, including the genus Pacispora.
The distinctly ornamented spores of Pac. scintillans are easy to separate from those of the other species of arbuscular fungi forming glomoid spores, i. e., spores similar to those of Glomus spp. However, when observed under a dissecting microscope, spores of Pac. scintillans with small warts or low ridges resemble smooth spores of Gl. eburneum L.J. Kenn. et al., Gl. diaphanum J.B. Morton & C. Walker, Gl. laccatum Blaszk., Gl. viscosum Nicol., and Paraglomus occultum (C. Walker) J.B. Morton & D. Redecker (Blaszkowski 1988b; Kennedy et al. 1999; Morton and Redecker 2001; Morton and Walker 1984; Walker 1982; Walker et al. 1995). Examination of the subcellular structure of spores under a compound microscope readily separates Pac. scintillans from the other species listed. None of them possesses a 3-layered germination wall with the middle layer showing a dextrinoid reaction.
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