rDNA units are highly polymorphic in Scutellospora castanea (Glomales, Zygomycetes)

The ribosomal DNA (rDNA) units in the glomalean zygomycete fungus Scutellos pora castanea were analyzed. Dot-blot assays allowed an estimation of 75 co pies per genome. After constructing a genomic library in a phage lambda EMB L3 vector, 13 rDNA clones were screened and explored. PCR experiments confi rmed 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 Is s gene fr om these 13 clones was partially sequenced. The resulting 550 bases sequenc es were analyzed, and a phylogenetic tree was inferred. This revealed that two clones contain one highly divergent rDNA family (rUSc1) by comparison w ith other known 18 s sequences from the database. A phylogenetic tree was c onstructed with the entire 18 s sequences of rUSc1, rUSc3 and those of seve n species representative of the glomalean fungi, Glomus, Entrophospora, Aca ulospora, Scutellospora and Gigaspora. This tree confirmed that the rUSc1 s equence is the neighbor of 18 s sequences from Glomus (Glomineae), while rU Sc3 remained in the group of the Gigaspora and Scutellospora (Gigasporineae ). A specific primer, rUSc1-1, was generated from the TTS region of rUSc1, and used for PCR amplification from single spores, depicting the presence o f rUSc1 in the genome of S. castanea at a lower frequency than other units. (C) 1999 Elsevier Science B.V. All rights reserved.