Taxonomy and Ecology Of Inland Sand Dune ... - Mycorrhizae

17D. DIFFICULTIES IN VAMFUNGAL TAXONOMYCurrent VAM fungal taxonomy hasmany limitations. Among theselimitations are the following: almost allspecies are asexual; there is an absenceof an extensive fossil record, VAM fungican not be grown on synthetic media;there are many overlaps of speciescharacteristics; there are indeterminatecharacteristics for somatic hyphae andspores; there is a lack of comprehensivesynoptic keys for species and genera;and there is tendency to describe newspecies rather than carefully studying theevolutionary relationships amongspecies (Morton, 1988). Even thoughthe earliest land plants had VAM fungipresent in their underground organs(Stubblefield et al., 1987a; Stubblefieldet al., 1987b; Pirozynski and Dalpé,1989), these fossil fungi are notamenable to careful spore wall analysisneeded for establishing systematicrelationships. The inability to cultureVAM fungi on synthetic media (Mosseet al., 1981) creates difficulties inrepeating experiments using a specificVAM fungus. Living cultures may notbe available or the classification of thefungus may be inaccurate. In manystudies, VAM fungal identification isbased on a broad species concept. Forexample, the Glomus fasciculatumcomplex is cited as a species in manystudies. The abundant citation of thisspecies gives the impression that thisfungus is present everywhere and thatthis species has considerable genetic andphysiological plasticity (Morton, 1988).A more restricted interpretation of thisspecies has led to the segregation ofseveral new species that were previouslyidentified as Glomus fasciculatum(Walker and Koske, 1987). From anecological point of view, manyresearchers have shown that eachindividual species differs significantly inits ability to promote growth withchange in soil fertility, soil pH, anddrought (Abbott and Robson, 1978;Allen and Boosalis, 1983; Hayman andTavares, 1985). In fact there are evenVAM fungal ecotypes that are adaptedto soils containing high concentrationsof heavy metals, aluminum or salt (Allenand Cunningham, 1983; Gildon and