The 5S rDNA of the bivalve Cerastoderma edule: nucleotide sequence of the repeat unit and chromosomal location relative to 18S-28S rDNA

The whole 5S rDNA repeated unit of the bivalve Cerastoderma edule was ampli fied by PCR and several clones were sequenced. In addition, the PCR product from several individuals was digested with restriction enzymes. The result s obtained indicate that 5S rDNA is organized in tandem repeats of 544-546 bp, 120 of which could represent the coding region and 424-426 the spacer r egion. Minimal intra- and inter-individual variation was detected, always w ithin the spacer region. In comparison to the published 5S rRNA sequences o f three other bivalves, C. edule displays a maximum of four different nucle otide positions. A specific probe of C. edule 5S rDNA was generated by PCR and used for FISH. Five chromosome pairs were identified that carried a clu ster of 5S rDNA at the telomere of the long arm. After performing FISH with a heterologous 18S-28S rDNA probe and C-banding, absence of linkage betwee n 5S and 18S-28S rDNA was demonstrated. (C) Inra/Elsevier, Paris.