Atlantic salmon 5S ribosomal DNA (5S rDNA) was amplified by the polymerase chain reaction, using as primers conserved sequences from the coding region of rainbow trout 5S rRNA. Two amplified products of different molecular weights were obtained, cloned, and sequenced, revealing them to be tandemly arranged. The nucleotide sequences differed between the two clones in the length of the nontranscribed spacer (NTS) and in three nucleotides of the coding sequence. By means of fluorescence in situ hybridization the 5S rDNA was chromosomally located in the heterochromatic arm of the pair bearing the satellite, adjacent to the major ribosomal DNA locus (rDNA).

The karyotype of the rainbow trout is characterized by a primitive XX/XY sex-determining chromosomal system. (Thorgaard et al., 1977). In the present study using FISH we have physically linked the 5S rRNA genes to the partially undifferentiated X chromosome pair. PCR amplified 5S rDNA was used for FISH and hybridization signals indicated that the genes were duplicated, present in one acrocentric and one metacentric pair of chromosomes. After analyzing several individuals, the female metaphases showed four fluorescent signals whereas males presented only three signals. Two of the three signals obtained in males corresponded to the metacentric pair whereas the single signal was mapped to the heterochromatin that cytologically differentiates the X chromosome from the Y chromosome. Double FISH experiments demonstrated that the 5S rDNA which is not sex linked is located at the NOR bearing arm close to the major ribosomal RNA genes (5.8S, 18S and 28S), similar to the situation observed in Atlantic salmon (Pendas et al., 1994a).