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SPOROCARPS
According
to Daniels and Trappe (1979), sporocarps are irregular,
2-8 x 3-15 mm in size, sometimes fused into larger masses, dull
brownish yellow to brown in color; they arise from a basal pad of pale grayish
yellow, loose mycelium with a few interspersed spores; no peridium is formed.
WHOLE SPORES
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SHAPE:
Globose, subglobose, sometimes ovoid.
SIZE DISTRIBUTION:60-160 µm, mean = 129 µm (n = 120). Six-month-old
pot cultures of two accessions (IT104, KS210) failed to produce sporocarps on
sudangrass in a low organic matter (< 1%) sandy soil. Instead, numerous single
spores form abundantly around roots amongst masses of extramatrical hyphae (see
photos below). Repeated subcultures on different hosts (leek, corn) did
not change this sporulation behavior.
SUBCELLULAR STRUCTURE OF SPORES
SPORE WALL: Consisting of two layers (L1 and L2) that differentiate consecutively as spores develop, composite thickness 5-10 µm.
L1: An
outer layer adherent to the layer beneath it (L2), semi-permanent, subyhyaline,
0.5-1 µm thick; no reaction in Melzer's reagent.
L2: A layer consisting of thin adherent sublayers (or laminae),
pale yellow-brown (0-10-20-0) to orange-brown (0-40-100-0) in color; 4-9 µm
thick.
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SHAPE:
Cylindrical to slightly
flared (see photos above).
WIDTH: 4-7
µm (mean = 5.4 µm).
COMPOSITE WALL THICKNESS:
1-2.2 µm at spore base
WALL
STRUCTURE: Two layers (L1 and L2) continuous with the two layers
of the spore wall.
L1: The outer layer, subhyaline, usually present only near
point of attachment of mature spores.
L2: Inner
layer continuous with L2 of the spore wall, pale orange-brown (0-20-40-0).
OCCLUSION:
Recurved septum positioned at the innermost sublayer of the laminate
layer (L2), giving the appearance of being "inserted" (Daniels and
Trappe, 1979).
A germ tube emerges from the lumen of the subtending hypha.
All fungal structures (arbuscules, intraradical hyphae) stain only faintly in Direct Blue (or Acid Fuchsin), regardless of host species. No vesicles were observed in cultures grown at least 4 months. Coiled hyphae were common near entry points.
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Sporocarps obtained directly from Maria Harrison were typical of the species, and contained not only mature thick-walled spores but also subhyaline to pale yellow spores with very thin spore walls. The latter spores were embedded in the hyphal matrix.
When sporocarps were used as inoculum, only spores borne singly or in small aggregates were observed in 3-month-old sudangrass cultures. These spores bore little resemblance to those in mature sporocarps, being white to pale orange-brown, with a much thinner spore wall. This pattern suggests a somewhat dimorphic habit similar to that described for Glomus ambisporum and Glomus heterosporum (Smith and Schenck, 1985).
Mycorrhizal morphology (faint staining, hyphal coiling, absence of vesicles) more closely resembles that of fungi in Paraglomaceae than that of Glomaceae, suggesting this species may also be divergent from other taxa in the Glomus clade.
REFERENCES
Berch, S. M. and J. A. Fortin. 1983. Lectotypification of Glomus macrocarpum and proposal of new combinations: Glomus australe, Glomus versiforme, and Glomus tenebrosum (Endogonaceae). Can. J. Bot. 61:2608-2617.
Daniels, B. A. and J. M.Trappe. 1979. Glomus epigaeus sp. nov., a useful fungus for vesicular-arbuscular mycorrhizal research. Can. J. Bot. 57:539-542.
Daniels, B. A. and J. A. Menge. 1980. Secondary sporocarp formation by Glomus epigaeus, a vesicular-arbuscular mycorrhizal fungus, in long-term storage. Mycologia 72:1235-1238.
Smith, G. S. and N. C. Schenck. 1985. Two new dimorphic species in the Endogonaceae: Glomus ambisporum and Glomus heterosporum. Mycologia 77:566-574.