rDNA units are highly polymorphic in Scutellospora castanea (glomales, zygomycetes)

The ribosomal DNA (rDNA) units in the glomalean zygomycete fungus Scutellospora castanea were analyzed. Dot-blot assays allowed an estimation of 75 copies per genome. After constructing a genomic library in a phage lambdaEMBL3 vector, 13 rDNA clones were screened and explored. PCR experiments confirmed their nature and allowed homologous probes to be obtained. Restriction-fragment length polymorphism (RFLP) analysis and hybridizations with 18 s and 25 s probes allowed their grouping into nine families. The 18 s gene from these 13 clones was partially sequenced. The resulting 550 bases sequences were analyzed, and a phylogenetic tree was inferred. This revealed that two clones contain one highly divergent rDNA family (rUSc1) by comparison with other known 18 s sequences from the database. A phylogenetic tree was constructed with the entire 18 s sequences of rUSc1, rUSc3 and those of seven species representative of the glomalean fungi, Glomus, Entrophospora, Acaulospora, Scutellospora and Gigaspora. This tree confirmed that the rUSc1 sequence is the neighbor of 18 s sequences from Glomus (Glomineae), while rUSc3 remained in the group of the Gigaspora and Scutellospora (Gigasporineae). A specific primer, rUSc1-1, was generated from the ITS region of rUSc1, and used for PCR amplification from single spores, depicting the presence of rUSc1 in the genome of S. castanea at a lower frequency than other units.