GLOMUS AGGREGATUM EMENDED: A DISTINCT TAXON IN THE GLOMUS FASCICULATUM COMPLEX

The description of Glomus aggregatum is emended, broadening the range of spore characteristics encompassed to include mature spores possessing one or two walls and forming inner spores by internal proliferation from subtending hyphae. Glomus aggregatum is one of several species in the G. fasciculatum "complex. " The species is very similar to a fossil species, Rhizophagites butleri, that had been placed in synonymy with Glomus fasciculatum by Gerdemann and Trappe. Glomus aggregatum is frequently recovered from maritime and lacustrine dunes where it sporulates in the soil, in roots, in dead spores of other VA species, in decaying organic matter, and between the cleavage planes of mica particles. In 1943, Rosendahl described a new fossil genus and species of endogonaceous fungus, Rhizophagites butleri Rosendahl, based on collections from Pleistocene deposits in Minnesota and from peat and glacial clays from Alberta. Butler (1939) had illustrated and described but not named the species earlier, assigning it to the extant genus Rhizophagus. The species was characterized by spores that measured 73-103 x 79-124 jum, possessed one or two separable walls, and formed spores by internal proliferation. Spores of a fungus similar to those of R. butleri have been isolated frequently during investigations of the vesicular-arbuscular mycorrhizae (VAM) associated with dune-colonizing plants of Lake Huron, Lake Michigan, Hawaii, and the Atlantic Coast ofthe U.S. and Canada. Spores typically were bounded by a single wall 1-3 ixm thick, sometimes by two walls, and occasionally contained spores formed by the same type of internal proliferation as described for R. butleri. Attempts to obtain type material of R. butleri for comparison were unsuccessful. Until Walker's recent study ofthe type material ofGlomus fasciculatum (Thaxter) Gerd. & Trappe (Walker, pers. comm.), the spores of the R. butleri-like species from the dunes were referable to G. fasciculatum (sensu lato), which included among its synonyms R. butleri (Gerdemann and Trappe, 1974). Schenck and Smith (1982) recently described a small-spored VAM species, G. aggregatum Schenck & Smith, from a citrus nursery in Florida. Although the original description mentioned only spores with two thin walls, examination of the type material revealed that many spores in the collection possessed but a single wall and that some were formed by internal proliferation. Schenck (pers. comm.) believed that the formation of the second wall was a late stage in the development of all spores ofthe species and represented the wall structure ofthe mature spore. Thus, he and Smith described the two-walled condition as a char? acteristic of the species. Rosendahl had earlier reached a similar conclusion re? garding the spore wall of R. butleri. Examination of many collections of the R. butleri-like fungus from the dunes and of the type material of G. aggregatum indicated that both were of the same species and that the majority of spores in many collections possess just a single wall. Accordingly, the description of G. aggregatum is emended to indicate the