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 ribosom al (18S + 28S and 5S) loci in amphibian species belonging to the advan ced family Salamandridae (genera Triturus, Salamandra, and Salamandrin a) and in the primitive hynobiid Salamandrella keyserlingii (family Hy nobiidae). In each analyzed karyotype the 5S rDNA sites appear to be s table, and definite in number, while an intraspecific variability both in number and chromosomal location of the 18S + 28S rDNA loci has bee n found in some Triturus species. In particular, an evolutionary trend toward a large intraspecific variability of the 18S + 28S rDNA loci h as been found in the T vulgaris species group. A structural analysis o f the ribosomal repetition units demonstrates the occurrence of a leng th polymorphism within the 18S + 28S rDNA repeats in the examined spec ies of the family Salamandridae; however, this polymorphism is rather limited, even in those Triturus species characterized by high intragen omic variability of the ribosomal sites. We show that in T vulgaris me ridionalis the variant repetition units actually segregate with indivi dual chromosomes. This implies that they are not intermingled in the r ibosomal clusters.