Genome specificity of rDNA spacer fragments from Oryza sativa L

The intergenic spacer derived from a cloned rDNA unit from a cultivated rice was dissected into several subclones, which were used as probes to analyze sequence homologies between rDNA spacers from wild rice belonging to genome types AA, BB, CC, EE, FF, BBCC, and CCDD. This analysis allowed us to detect several regions with different degrees of homology. A series of 250-260 bp repeats is located in the central part of the AA spacer. This sequence cross-hybridizes with the BB, CC, BBCC, and FF genomes, but is absent in the EE and CCDD genomes. Regions proximal to 25S and 18S sequences are well conserved in all genomes. Finally, two adjacent sequences of 61 bp and 94 bp, located downstream from the repeats, have been found to have a narrow genomic specificity, restricted respectively to AA and FF genomes for the first one and to AA for the second. These data provide new information on the evolution of the ribosomal RNA gene spacer within a complex of related species, and add to our knowledge on the relationship between the various rice genomes.