Inter- and intra-isolate rRNA large subunit variation in Glomus coronatum spores.

•  High levels of variation are reported in the large subunit (LSU) rRNA gene, D2 region of Glomus coronatum, a well characterized species of arbuscular mycorrhizal fungus (AMF). •  Clones (435) containing the D2 regions from 7 isolates of G. coronatum were investigated for intra- and inter-isolate sequence variation using PCR-single-strand conformational polymorphism (PCR-SSCP) as a prescreen before sequencing. Isolates of G. mosseae, G. constrictum and G. geosporum, three species of AMF with similar spore ontogeny and morphology, were also analysed. •  Analysis of 138 representative sequences indicated that most were unique; this variation could not be attributed to DNA polymerase or cloning artefacts. Only 13 sequences were found in more than one isolate. Neighbour-joining analysis showed that most sequences from G. coronatum formed a main group although several sequences from G. mosseae and G. constrictum clustered with G. coronatum. •  There was greater than expected variation in the LSU D2 region sequences from G. coronatum. The four Glomus species, closely related by spore morphology, might represent part of a genetic continuum. Implications for the concept of species in AMF, the use of rRNA sequences to estimate biodiversity and in situ detection in field ecology are discussed.