MOLECULAR ANALYSIS OF A NOR SITE POLYMORPHISM IN BROWN TROUT (SALMO-TRUTTA) - ORGANIZATION OF RDNA INTERGENIC SPACERS

Using restriction endonuclease mapping, we have analyzed the organizat ion of rDNA (DNA coding for ribosomal RNA (rRNA)) units in the salmoni d fish Salmo trutta, as an initial step toward understand the molecula r basis of a nucleolar organizer region (NOR) site polymorphism detect ed in this species. The size of the rDNA units ranged between 15 and 2 3 kb, with remarkable variation both within individuals and between po pulations. Three regions of internal tandem repetitiveness responsible for this length polymorphism were located to the intergenic spacers. NOR site polymorphic individuals showed a higher number of length clas ses, in some cases forming a complete 1 kb fragment ladder. The amount of rRNA genes was as much as 8-fold higher in polymorphic individuals compared with standard individuals. All individuals from the most pol ymorphic population showed a 14-kb insertion of unknown nature in a sm all proportion (below 25%) of the 28S rRNA genes.