Molecular organization of ribosomal RNA genes clustered at variable chromosomal sites in Triturus vulgaris meridionalis (Amphibia, Urodela)

The ribosomal RNA genes of Triturus vulgaris meridionalis (Amphibia, Urodela) show the peculiar feature of being clustered not only at the nucleolar organizer, present in the species at a definite chromosome location, but also at "additional ribosomal sites" which are highly variable in number and chromosomal distribution among individuals. The additional ribosomal sites are most often found at specific chromosome regions, such as telomeres, C-bands and centromeres, in virtually all the chromosomes. With increasing numbers of additional clusters, the genomic dosages of ribosomal RNA genes are found to increase over a tenfold range, though not linearly. At a molecular level, the ribosomal DNA repeats differ in size because of discrete variations in the length of the non-transcribed spacers. However, the resulting length heterogeneity of the gene family is rather limited within a single genome as well as within the species. Many of the ribosomal loci appear to be internally homogeneous with respect to the repeat length. Moreover, separate clusters from distant genomic regions can share the same size class of ribosomal repeats even in the same specimen. The nucleolar organizer is mostly endowed with "shorter" ribosomal repeating units, ranging in size from 13.7 X 10(3) to 15.2 X 10(3) base-pairs. The additional ribosomal sites are characterized by the occurrence of "longer" repeats, ranging in size from 16.2 X 10(3) to 19.7 X 10(3) base-pairs. The "shorter" class of ribosomal repeats is always detected in the amplified ribosomal DNA, suggesting that the nucleolar organizer locus is involved in the amplification process in most oocytes. "Longer" ribosomal repeats are also detectable in the amplified ribosomal DNA of a few females.