The 5S rDNA of mussels Mytilus galloprovincialis and M-edulis: sequence variation and chromosomal location

The 5S ribosomal DNA of the mussels Mytilus galloprovincialis and M. edulis was amplified by PCR using contiguous primers. The most general 5S rDNA am plification pattern consisted of several products in both mussels. Two main PCR products of about 250 bp and 760 bp were cloned and sequenced, reveali ng two classes of 5S rDNA units. These were characterized as containing an identical coding region of 119 bp but with highly divergent spacers. Clones of each unit type exhibited minimal differences except those of the large unit of M. edulis. The sequences analysed of the two mussels possess the sa me coding region and only six fixed base changes on the spacers. FISH, carr ied out with specific probes, consistently showed hybridization signals on the largest metacentric pair (two differentiated sites) and with variable f requency on two other metacentric pairs (one site on each pair). Difference s in the 5S rDNA distribution between both mussels were not found. In M. ed ulis, chromosomes carrying 18S-28S rDNA were also identified by FISH. These correspond to two submetacentric-subtelocentric pairs, as was previously r eported in M. galloprovincialis, demonstrating that the two rDNA multigene families are located on different chromosome pairs in these mussels.