N.C. Schenck & S.M. Sm.
SPORES single in the soil; pale yellow (4A3) to greyish orange (5B5); globose to subglobose; (95-)135(-190) µm diam; sometimes ovoid; 95-110 x 110-160 µm; with one subtending hypha.
Layer 1 mucilagenous, hyaline, (1.5-)2.1(-2.8) µm thick, tightly adherent to layer 2, staining pink (12A4) to purplish red (14A8) in Melzer’s reagent, usually absent or highly deteriorated in mature spores.
Layer 2 semiflexible, smooth, hyaline, (2.2-)3.3(-3.9) µm thick, non-reactive in Melzer’s reagent.
Layer 3 laminate, smooth, pale yellow (4A3) to greyish orange (5B5), (2.9-)5.9(-8.1) µm thick, composed of tightly adherent sublayers, <0.5 µm thick.
Layer 4 flexible, smooth, hyaline, (0.4-)0.5(-0.6) µm thick, usually separating from layer 3 in crushed spores, but attached to the inner surface of the laminate layer of a subtending hypha.
SUBTENDING HYPHA pale yellow (4A3) to grayish orange (5B5); straight to curved; cylindrical or funnel-shaped; (8.3-)12.1(-15.4) µm wide at the spore base.
Wall of subtending hypha pale yellow (4A3) to grayish orange (5B5); (2.7-)3.7(-4.7) µm thick at the spore base; composed of three layers (shwl1-3) continuous with layers 1-3 of the spore wall; layer 1 rarely present in mature spores.
Pore closed by spore wall layer 4.
GERMINATION. Not observed.
MYCORRHIZAE. The mycorrhizae produced by this fungus in one-species pot cultures with Plantago lanceolata L. as the plant host consisted of arbuscules, vesicles, as well as intra- and extraradical hyphae. Arbuscules were numerous, evenly distributed along the roots and stained pale violet (17A3) to greyish violet (17C5) in 0.1% trypan blue. Vesicles were ellipsoid, 45-85 x 55-105 µm and stained pale violet (18A3) to bluish violet (18A7) in trypan blue. Intraradical hyphae grew parallel to each other and to the root axis, were 2.6-9.1 µm wide, and stained pale violet (17A3-A5). The hyphae frequently formed coils, especially at entry points. The coils were 10.0-13.7 x 18.6-25.1 µm and stained pale violet (17A3) to greyish violet (17C6).
In roots of P. lanceolata
Glomus claroideum has been the third most frequently occurring arbuscular
mycorrhizal fungal species among those recovered from 199 root-rhizosphere
soil mixtures taken from under 10 plant species cultivated in 106 sites of
the Western Pomerania province (Blaszkowski et al. 2003; Iwaniuk and Blaszkowski,
unpubl.). When spores recovered from field-collected root-rhizosphere soil
mixtures and trap cultures were considered, this fungus ranked first in respect
of the number of spores produced. Glomus
claroideum has also occurred in dune soils of the Vistula Bar (54o21’N,
19o14’E), the Bledowska Desert (50o22’N, 19o34’E), and soils
of the Tuchola Forests (53o46’N, 17o42’E-5340’N, 17o54’E;
Blaszkowski et al. 2002 a, b; Tadych and Blaszkowski 2000).
Apart from the sites of occurrence of Gl. claroideum mentioned above, in Poland, this fungus has also been recognized near Chrzanów (50º8’N, 19º24’E; Turnau et al. 2001). However, this record was based only on results of molecular investigations of mycorrhizal roots of Fragaria vesca L.; no spores were revealed.
Glomus claroideum has originally been described from spores recovered from under Glycine max (L.) Merr. cultivated in Florida (Schenck and Smith 1982). According to Walker and Vestberg (1998), this fungus is widely distributed in Northern Europe, whereas only two reports exist of its presence in the southern hemisphere. Blaszkowski et al. (2001) found spores of Gl. claroideum in maritime sand dunes adjacent to Tel Aviv (32º4’N, 34º46’E), Israel. Saito and Vargas (1991) revealed Gl. claroideum spores in cultivated soils of Japan.
NOTES. Most juvenile spores of Gl. claroideum have a two-layered wall with layers 1 and 2 present. Each next layer (layers 3 and 4) starts to form when the differentiation of the preceding layer was completed. This pattern of differentiation of the subcellular structure of a spore corresponds to that revealed in all Glomus spp. so far investigated ontogenetically (e. g., Blaszkowski and Tadych 1997; Stürmer and Morton 1997).
The most distinctive property of Gl. claroideum spores is their innermost, flexible wall layer. This layer easily separates from the penultimate laminate layer in most crushed spores coming from living pot cultures. However, in spores recovered from field-collected soils, it usually tightly adheres to the laminate layer and, thereby, is invisible in most crushed spores.
The semiflexible layer 2 may be also easily omitted. It is thin, usually tightly adheres to the laminate layer 3, and does not stain in Melzer’s reagent. This layer deteriorates with age as well, although its longevity is much higher than that of layer 1.
Layer 3 is permanent and coloured. It consists of very thin, coloured sublayers, sometimes separating in vigorously crushed young and overmatured spores. This makes an impression of the presence of an additional layer in spore wall structure of this fungal species. Examination of many restrainedly crushed spores of different maturity excludes the five-layered version of the wall structure of this species.
The lumen of subtending hypha in some spores of Gl. claroideum is closed by a plug, although the spore content is held by the continuous innermost spore wall layer forming an endospore.
Examination of the structure and biochemical properties of spore wall of specimens of different maturity readily divides these species into three groups. The first group represents Gl. luteum, which produces spores with both the outermost mucilagenous layer and the innermost flexible layer of Gl. claroideum (Kennedy et al. 1999). The second group includes Gl. clarum, Gl. caledonium, and Gl. etunicatum, which the outermost mucilagenous layer links (Morton 1996; Stürmer and Morton 1997). Glomus geosporum and Gl. versiforme form the third group; the outermost layer of their spores sloughs, but is non-reactive in Melzer’s reagent (Morton 2000; Walker 1982).
The properties distinguishing Gl. claroideum and Gl. luteum are the thicker and more persistent layer 2 and the thicker layer 3 of spores of the latter species (Kennedy et al. 1999). The thicker laminate layer 3 causes Gl. luteum spores to be darker coloured than those of Gl. claroideum.
Apart from the outermost mucilagenous layer, the laminate structural layer of Gl. caledonium is still overlaid by two permanent, rigid, hyaline layers (Morton 1996), whereas only one, impermanent, semiflexible layer exists between the laminate layer and the outermost mucilagenous one of Gl. claroideum spores.
Although Gl. geosporum differentiates an innermost layer adhering to the inner surface of the laminate structural layer as Gl. claroideum does, this layer in the former species is coloured, rigid, and rarely separates from the laminate layer (Morton 2000; Walker 1982; vs. it is colourless, flexible, and usually separates from the laminate layer in Gl. claroideum). Additionally, mature spores of Gl. geosporum usually are darker coloured than those of Gl. claroideum.
Glomus versiforme also differs from all the other fungal species compared here in properties of its mycorrhizae. They stain only faintly in the commonly used stains (Blaszkowski, pers. observ.; Morton 2000), whereas the staining reaction of those of the other fungal species is intensive (Blaszkowski, pers. observ.; Kennedy et al. 1999; Morton 2000).
Walker and Vestberg (1998) considered Gl. maculosum Miller & Walker, Gl. fistulosum Skou & Jakobsen, and Gl. multisubstensum Mukerji et al. to be synonyms of Gl. claroideum.
In the recently proposed classification of arbuscular mycorrhizal fungi (Schüßler et al. 2001), Gl. claroideum together with Gl. clarum and Gl. etunicatum are within group B, whereas Gl. caledonium and Gl. geosporum represent group A of the order Glomerales. Glomus versiforme is a member of the family Diversisporaceae C. Walker and Schuessler in the order Diversisporales C. Walker and Schuessler (Walker and Schüßler 2004).
Blaszkowski J., Adamska I., Czerniawska B. 2002a. Arbuscular mycorrhizal fungi (Glomeromycota) of the Vistula Bar. Acta Mycol. 37, 39-62.
Blaszkowski J., Adamska I., Czerniawska B. 2003. Glomus claroideum and G. spurcum, arbuscular mycorrhizal fungi (Glomeromycota) new for Poland and Europe, respectively. Acta Soc. Bot. Pol. 72, 149-156.
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Walker C. 1982. Species in the Endogonaceae: a new species (Glomus occultum) and a new combination (Glomus geosporum). Mycotaxon 15, 49-61.
Walker C., Schüßler A. 2004. Nomenclatural clarifications and new taxa in the Glomeromycota. Mycol. Res. 108, 979-982.
Walker C., Vestberg M. 1998. Synonymy amongst the arbuscular mycorrhizal fungi: Glomus claroideum, G. maculosum, G. multisubstensum and G. fistulosum. Ann. Bot. 82, 601-624.