Molecular cytogenetics of the ribosomal (18S + 28S and 5S) DNA loci in primitive and advanced urodele amphibians.

In the present work we performed a cytogenetic analysis of the ribosomal (18S + 28S and 5S) loci in amphibian species belonging to the advanced family Salamandridae (genera Triturus, Salamandra, and Salamandrina) and in the primitive hynobiid Salamandrella keyserlingii (family Hynobiidae). In each analyzed karyotype the 5S rDNA sites appear to be stable, and definite in number, while an intraspecific variability both in number and chromosomal location of the 18S + 28S rDNA loci has been found in some Triturus species. In particular, an evolutionary trend toward a large intraspecific variability of the 18S + 28S rDNA loci has been found in the T. vulgaris species group. A structural analysis of the ribosomal repetition units demonstrates the occurrence of a length polymorphism within the 18S + 28S rDNA repeats in the examined species of the family Salamandridae; however, this polymorphism is rather limited, even in those Triturus species characterized by high intragenomic variability of the ribosomal sites. We show that in T. vulgaris meridionalis the variant repetition units actually segregate with individual chromosomes. This implies that they are not intermingled in the ribosomal clusters.