GERMINATION.
Unknown.
MYCORRHIZAE. According to Sieverding (1988), Am. callosa formed vesicular-arbuscular mycorrhizae in one-species pot cultures with Desmodium spp. and Sorghum bicolor (L.) Moench. as the host plants.
PHYLOGENETIC POSITION. Results of phylogenetic analyses of sequences of spores of the glomoid morph of the fungus indicated it to belong in a monophyletic clade along with Am. fennica, Am. gerdemannii, Am. leptoticha, members of the newly erected family Ambisporaceae (Walker 2008, Walker et al. 2007b), and G. pyriformis (Geosiphonaceae), to which the clade containing Ar. trappei, the type species of the genus Archaeospora, represents a sister lineage (Walker et al. 2007a).
DISTRIBUTION. The holotype of Am. callosa (treated as 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) and Japan (Walker et al. 2007a).
NOTES.
The description of Am. callosa 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 Am. callosa and, thereby, made their observation difficult or impossible.
Ambispora callosa has originally been described as Glomus callosum (Sieverding 1988) and the acaulosporioid morph of this fungus is not known to date.
Of the species of the Glomeromycota of colourless or light coloured glomoid spores, Am. callosa produces largest spores. Other species of arbuscular fungi forming colourless or light coloured glomoid spores of a size slightly lower, but more or less overlapping with the lower size range of spores of Am. callosa are Gl. albidum, Gl. eburneum, and Gl. lacteum. However, the spore surface of none of the three latter species is ornamented with warts as that of 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 Am. callosa is permanent, similarly as in Gl. eburneum and Gl. lacteum. Moreover, the structural laminate layer of spores of Am. callosa is much thicker (2.5-10.5 µm thick) 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 Am. callosa 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 and Diversispora spurca. Compared with spores of Am. callosa, 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 Am. callosa (Morton 2002).
Spores of D. spurca also are much smaller [(75-)86(-110) µm diam; Blaszkowski 2003] than those of Am. callosa, 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 (Blaszkowski 2003; vs. two tightly adherent layers in Am. callosa).
Ambispora callosa may also be confused with Pacispora scintillans, a fungus producing hyaline to white glomoid spores of an ornamented upper 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 (Blaszkowski 2003 vs. (180-)220-280(-300) µm diam in Am. callosa (Sieverding 1988)], their warts are much larger (1.7-5.7 x 0.7-0.9 µm; Blaszkowski 2003 vs. <0.5 µm high; Blaszkowski, pers. observ.; Sieverding 1988), and their subcellular structure is more complex. While the subcellular structure of spores of Am. callosa consists of only 2-layered spore wall, that of Pac. scintillans includes a 3-layered spore wall and a 3-layered germinal inner wall with its middle layer staining deep red (11B8) in Melzer's reagent (Blaszkowski 2003; vs. nonreactivity of any of the spore wall layers of Am. callosa in this reagent; Sieverding 1988).
REFERENCES
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Morton J. B. 2002. International Culture Collection of (Vesicular) Arbuscular Mycorrhizal Fungi. West Virginia University: http://www.invam.caf.wvu.edu/.
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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.
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Walker C., Rhodes L. H. 1981. Glomus albidus: a new species in the Endogonaceae. Mycotaxon 12, 509-514.
Walker C., Vestberg M., Demircik F., Stockinger H., Saito M., Sawaki H., Nishmura I., Schüßler A. 2007. Molecular phylogeny and new taxa in the Archaeosporales (Glomeromycota): Ambispora fennica gen. sp. nov., Ambisporaceae fam. nov., and emendation of Archaeospora and Archaeosporaceae. Mycol. Res. 111, 137-13.
Walker C., Vestberg M., Schüßler A. 2007. Nomenclatural clarifications in Glomeromycota. Mycol. Res. 111, 253-255.
