Restriction map and polymorphisms of nuclear ribosomal genes of Populus balsamifera

Balsam poplar (Populus balsamifera) clones from five populations, which were collected along a transect from northern Wisconsin to the northern tree line, were evaluated for polymorphisms in nuclear ribosomal DNA. For this purpose, a restriction map was constructed using four six-cutter enzymes in single and double digests of genomic DNA. After electrophoretic separation on agarose gels and Southern transfer, blots were hybridized to non-radioactively labeled heterologous rDNA probes of soybean. Among populations, variation was detected in the length of the intergenic spacer between the tandem repeats of the coding regions and in the degree of methylation of one restriction enzyme recognition site. Based on a comparison of the derived restriction map of balsam poplar and other poplars, high homology was evident in the rDNA coding regions among species, whereas the intergenic spacer varied slightly in both length and number of restriction sites.