Ribosomal RNA gene sequence diversity in arbuscular mycorrhizal fungi (Glomeromycota)

1. The existence of multiple rRNA sequences within AMF single spores and the high levels of variability found in these symbionts for highly conserved genes with low copy numbers have provoked intense controversy over the genetic structure of the Glomeromycota. Lately, two hypotheses have been proposed to explain the source of such a variation, mainly either at internuclear (multigenomic) or intranuclear (polyploidy) levels. 2. Evidence reported by numerous authors suggests that the magnitude of sequence variation, and the presence in single spores of sequences from apparently unrelated taxa within the Glomeromycota, makes the use of one, or very few sequences, from only one or two isolates of AMF insufficient for taxonomic purposes in some species groups. 3. A large-scale, collaborative study on genetic variation in AMF across different genes could help to address fundamental questions on the biology and genetic structure of these important symbionts.