ORGANIZATION OF NUCLEAR RIBOSOMAL DNA AND SPECIES-SPECIFIC POLYMORPHISM IN CLOSELY-RELATED FRAXINUS-EXCELSIOR AND F-OXYPHYLLA

The ribosomal DNA repeat units of two closely related species of the g enus Fraxinus, F. excelsior and F. oxyphylla, were characterized. The physical maps were constructed from DNA digested with BamHI, EcoRI, Ec oRV and SacI, and hybridized with three heterologous probes. The prese nce or the absence of an EcoRV restriction site in the 18s RNA gene ch aracterizes two ribosomal DNA unit types found in both species and whi ch coexist in all individuals. A third unit type appeared unique to al l individuals of F. oxyphylla. It carries an EcoRI site in the interge nic spacer. Each type of unit displayed length variations. The rDNA un it length of F. excelsior and F. oxyphylla was determined with EcoRV r estriction. It varied between 11kb and 14.5 kb in F. excelsior and bet ween 11.8 kb to 13.8 kb in F. oxyphylla. Using SacI restriction, at le ast ten spacer length variants were observed in F. excelsior, for whic h a detailed analysis was conducted. Each individual carries 2-4 lengt h variants which vary by a 0.3-kb step multiple. This length variation was assigned to the intergenic spacer. By using the entire rDNA unit of flax as probe in combination with EcoRI restriction, each species c an be unambiguously discriminated. The species-specific banding patter n was used to compare trees from a zone of sympatry between the two sp ecies. In some cases, a conflicting classification was obtained from m orphological analysis and the use of the species-specific rDNA polymor phism. Implications for the genetic management of both species are dis cussed.