SPORES formed singly in the soil or in loose clusters at the tip of fascicle-like branched hyphae. Spores subhyaline to white; usually globose; (180-)220-280(-300) µm diam; rarely ovoid; 230-270 x 205-260 µm; with one subtending hypha. Spores usually covered with debris and soil particles and, thereby, dull when observed under a dissecting microscope. Spores soft, plastic, not breaking readily when mounted in PVLG and crushed by applying pressure to the cover slip.
SUBCELLULAR
STRUCTURE OF SPORES consists of a spore wall composed of two permanent, tightly adherent layers (swl1 and 2).
Layer
1, forming the spore surface,
unit, subhyaline to white, 0.5-1.5 µm thick including its ornamentation; ornamentation of the upper surface of crowded minute warts, <0.5 µm high and ca. 0.5 µm diam.
GERMINATION.
Unknown.
MYCORRHIZAE. According to Sieverding (1988), Gl. callosum formed vesicular-arbuscular mycorrhizae in one-species pot cultures with Desmodium spp. and Sorghum bicolor (L.) Moench. as the host plants.
DISTRIBUTION. The holotype of Gl. callosum has been selected from spores extracted from the greenhouse pot culture no. GTZ-21 with the host plant S. bicolor grown at the Institute für Pflanzenbau und Tierhygiene in den Tropen und Subtropen, University of Göttingen, Germany. The spores used to establish this culture were originally isolated from an experimental field (plot 3a) of the agricultural school Mushweshwe, near Bukavu, South-Kivu province of Zaire. Additionally, spores of this fungus have been found in soils of Rwanda (Sieverding 1988). There is no other literature report of a finding of this fungus in other regions of the world.
NOTES.
The description of Gl. callosum presented above was made based on the original description (Sieverding 1988) and observations of spores crushed in PVLG [slides no. 3983 (5) and 3796] provided by Dr. E. Sieverding, Institute for Plant Production and Agroecology in the Tropic and Subtropics, University of Hohenheim, Germany. The spores were poorly preserved and most diagnostic characters of this fungus were difficult to compare with those originally described by Sieverding (1988). Additionally, debris and soil particles adherent to the spore surface in all the slides provided obscured the most important structures of Gl. callosum and, thereby, made impossible their observation.
Of the species of the genus Glomus of colourless or light coloured spores, Gl. callosum produces largest spores. Other Glomus spp. forming colourless or light coloured spores of a size slightly lower and more or less overlapping with the lower size range of spores of Gl. callosum are Gl. albidum C. Walker & L.H. Rhodes, Gl. eburneum L.J. Kenn. et al., and Gl. lacteum S.L. Rose & Trappe. However, the spore surface of none of the three latter species is ornamented with warts that ornament spores of the former fungus (Kennedy et al. 1999; Rose and Trappe 1980; Sieverding 1988; Walker and Rhodes 1981). Additionally, the wall layer 1 of spores of Gl. albidum, forming their surface, sloughs with age, whereas the outer layer of the spore wall of Gl. callosum is permanent, similarly as in Gl. eburneum and Gl. lacteum. Moreover, the structural laminate layer of spores of Gl. callosum is much thicker (2.5-10.5 µm) than that of Gl. albidum (0.5-2.0 µm thick; Walker and Rhodes 1981), Gl. eburneum (1.2-3.8 µm thick; Kennedy et al. 1999), and Gl. lacteum (3-5 µm thick; Rose and Trappe 1980). Finally, spores of Gl. callosum usually associate debris and soil particles, a phenomenon not found in the other species compared here. However, this property is variable and thus of little diagnostic value. The tendency to bind soil debris also occurs in two other species of arbuscular fungi forming glomoid and hyaline to light coloured spores, i. e., Gl. viscosum Nicol. and Diversispora spurca (C.M. Pfeiff., C. Walker & Bloss) C. Walker & Schuessler. Compared with spores of Gl. callosum, those of Gl. viscosum are much smaller [(50-)82(-120) µm diam when globose vs. (180-)220-280(-300) µm diam], do not have an ornamented surface (vs. ornamented with fine warts), and their wall consists of three layers, of which the semi-flexible middle one does not occur in the spore wall of Gl. callosum (Morton 2002).
Spores of D. spurca also are much smaller [(75-)86(-110) µm diam; Błaszkowski 2003] than those of Gl. callosum, are not ornamented, and have a wall consisting of three layers with the middle one and the adherent outer layer easily separating from the laminate inner layer in crushed spores (Błaszkowski 2003; vs. two tightly adherent layers in Gl. callosum).
Glomus callosum may also be confused with Pacispora scintillans (S.L. Rose & Trappe) Sieverd. & Oehl, a fungus producing hyaline to white glomoid spores of an ornamented surface. However, compared with spores of the former species, those of the latter fungus are at least 2-fold smaller [(70-)107(-165) µm diam when globose (Błaszkowski 2003 vs. (180-)220-280(-300) µm diam in Gl. callosum (Sieverding 1988)], their warts are much larger (1.7-5.7 x 0.7-0.9 µm; Błaszkowski 2003 vs. <0.5 µm high; Błaszkowski, pers. observ.; Sieverding 1988), and their subcellular structure is more complex. While the subcellular structure of spores of Gl. callosum consists of only 2-layered spore wall, that of Pac. scintillans includes a 3-layered spore wall and a 3-layered germination inner wall with its middle layer staining deep red (11B8) in Melzer's reagent (Błaszkowski 2003; vs. nonreactivity of any of the spore wall layers of Gl. callosum in this reagent; Sieverding 1988).
REFERENCES
Błaszkowski J. 2003. Arbuscular mycorrhizal fungi (Glomeromycota), Endogone, and Complexipes species deposited in the Department of Plant Pathology, University of Agriculture in Szczecin, Poland. http://www.agro.ar.szczecin.pl/~jblaszkowski/.
Kennedy L. J., Stutz J. C., Morton J. B. 1999. Glomus eburneum and G. luteum, two new species of arbuscular mycorrhizal fungi, with emendation of G. spurcum. Mycologia 91, 1083-1093.
Morton J. B. 2002. International Culture Collection of (Vesicular) Arbuscular Mycorrhizal Fungi. West Virginia University: http://www.invam.caf.wvu.edu/.
Rose S. L., Trappe J. M. 1980. Three new endomycorrhizal Glomus spp. associated with actinorrhizal shrubs. Mycotaxon 10, 413-420.
Sieverding E. 1988. Two new species of vesicular arbuscular mycorrhizal fungi in the Endogonaceae from tropical high lands of Africa. Angew. Bot. 62, 373-380.
Walker C., Rhodes L. H. 1981. Glomus albidus: a new species in the Endogonaceae. Mycotaxon 12, 509-514.
