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Archaeosporaceae
Morton and Redecker (2000)

Studies of other molecular characters, such as monoclonal antibody specificities (Wright et al., 1986) and fatty acid profiles (Graham et al., 1995) have led to the long-held suspicion that members of this family were phylogenetically distant from species in Acaulospora and Glomus, but all of these data did not contain enough phylogenetic information to determine it's position relative to other glomalean taxa. Even morphological characters were atypical for the Acaulospora-like spores (Morton and Benny, 1990). Small subunit (18S) ribosomal DNA sequences finally provided the character set to position this family is ancestral to both Acaulosporaceae and Glomaceae (Redecker et al., 2000; Sawaki et al., 1997). The molecular phylogenetic tree suggests that members of this family and Paraglomaceae are more closely related to a nonmycorrhizal fungus, Geosiphon pyriforme, a putative Zygomycete fungus which is the mycobiont in a symbiosis with a Nostoc cyanobacterium (the photobiont).

Vegetative structures consist of those inclusive in the suborder Glomineae. There are several characteristics unique to this family, however, in mycorrhizal structures.

All species in this family can be distinguished from other glomalean families (including Paraglomaceae) by the signature nucleotide sequence: TCTCKKYTTCGGYSGAGTCC at position 227 of the 18S rDNA gene (Morton and Redecker, 2000). The degeneracy in this sequence reflects variation among species in the group and has the positive benefit of providing a more lenient diagnostic tool for discovering other species

Arbuscules generally stain very faintly (more so than in Acaulosporaceae) in conventional acidic stains. The photo at right has been enhanced digitally to better visualize arbuscule structure. Patchily distributed within roots of corn, red clover and sudangrass (culture hosts used thus far). Narrow trunk hypha (< 4 µm), with fine branching near tips.
Vesicles have not been observed in pot cultures of any isolates of species to date. Loss of these structures by their absence is difficult to prove, but the hypotheses remain that these structures may not have evolved in this group (they are ancestral to other glomalean families) or were lost.
Intraradical hyphae tend to stain slightly less faintly than arbuscules and usually are the structures that identify foci of colonization. Spread both intra- and inter-cellularly, the former 3-10 µm wide and often coiled and the latter 2-8 µm wide, with some coiling and irregular branching.
Extraradical hyphae generally are thin (2-3 µm wide), but profuse around roots. They often remain attached to roots in entry point regions.
Species produce asexual spores spores which:
Originate from the neck of a sporiferous saccule that is formed terminally on a fertile hypha. Saccules often cease to expand at the onset of spore formation, occurring singly and more rarely in loose aggregates.
May be sessile (no hyphal attachment), but of the three species described thus far, two form spores on a "pedicel" or branch hypha.
Subcellular structure consists solely of layers of a spore wall (3-4) of which the outermost layer is continous with the monolayer wall of the neck of the "sporiferous saccule". Spores do not form thin flexible "germinal walls" common to all species of Acaulosporaceae.
One or more layers, and even a septum, may occlude the opening in the spore to the lumen of a subtending hypha or the neck of the "sporiferous saccule".
Germination is poorly documented in any members of this group, although germ tubes have been observed independently (Morton, Spain) emerging from the lumen of the pedicel of acaulosporoid morphs (much like Glomus)

One genus is recognized:

Archaeospora

REFERENCES

Graham, J.H., N. C. Hodge, and J. B. Morton. 1995. Fatty acid methyl ester profiles for characterization of glomalean fungi and their endomycorrhizae.  Applied and Environmental Microbiology 61:58-64.

Morton, J. B. and G. L. Benny. 1990. Revised classification of arbuscular mycorrhizal fungi (Zygomycetes): A new order, Glomales, two new suborders, Glomineae and Gigasporineae, and two new families, Acaulosporaceae and Gigasporaceae, with an emendation of Glomaceae. Mycotaxon 37:471-491.

Morton, J. B. and D. Redecker. 2001. Two new families of Glomales, Archaeosporaceae and Paraglomaceae, with two new genera Archaeospora and Paraglomus, based on concordant molecular and morphological characters. Mycologia (in press).

Redecker, D., J. B. Morton, and T. D. Bruns. 2000. Ancestral lineages of arbuscular mycorrhizal fungi (Glomales). Molecular Phylogenetics and Evolution 14:276-284.

Sawaki, H, K. Sugawara, and M. Saito. 1998. Phylogenetic position of an arbuscular mycorrhizal fungus, Acaulospora gerdemannii , and its synanamorph Glomus leptotichum, based upon 18S rRNA gene sequence. Mycoscience 39:477-480.

Wright, S. F., J. B. Morton, and J. E. Sworobuk. 1987. Identification of a vesicular-arbuscular mycorrhizal fungus by using monoclonal antibodies in an enzyme-linked immunosorbent assay. Applied and Environmental Microbiology 53:2222-2225.