POLYMORPHISM OF THE NUCLEOLUS ORGANIZER REGION (NOR) ON THE PUTATIVE SEX-CHROMOSOMES OF ARCTIC CHAR (SALVELINUS-ALPINUS) IS NOT SEX-RELATED

Polymorphism of the nucleolus organizer region (NOR) on the putative s ex chromosomes of Arctic char (Salvelinus alpinus) was examined using conventional cytogenetic and molecular techniques. Variation was obser ved in the number, size and position of rDNA loci on the sex pair. Flu orescence in situ hybridization (FISH) analyses showed that the sex ch romosomes of Arctic char lack the repetitive DNA sequences (Mbol/Bglll family) that are a prominent feature of the sex chromosomes of lake t rout (S. namaycush). Southern analyses of genomic DNAs using an rDNA f ragment as probe revealed extensive restriction fragment length polymo rphism (RFLP) Variation among individuals. Despite the presence of var iation in all aspects of this rDNA locus, no sex-specific differences were detected. Repetitive DNAs (multicopy rDNA as in Arctic char or ta ndem repetitive DNA as in lake trout) appear to play important but dif ferent roles in the evolution of the sex chromosomes in these species.