Sequence characterization of the 5S ribosomal DNA and the internal transcribed spacer (ITS) region in four European Donax species (Bivalvia: Donacidae).

BACKGROUND

The whole repeat unit of 5S rDNA and the internal transcribed spacer (ITS) of four European Donax species were analysed. After amplifying, cloning and sequencing several 5S and ITS units, their basic features and their variation were described. The phylogenetic usefulness of 5S and ITS sequences in the inference of evolutionary relationships among these wedge clams was also investigated.

RESULTS

The length of the 5S repeat presented little variation among species, except D. trunculus that differed from the rest of the Donax species in 170-210 bp. The deduced coding region covered 120 bp, and showed recognizable internal control regions (ICRs) involved in the transcription. The length of non-transcribed spacer region (NTS) ranged from 157 bp to 165 bp in Donax trunculus and from 335 bp to 367 bp in the other three species. The conservation degree of transcriptional regulatory regions was analysed revealing a conserved TATA-like box in the upstream region. Regarding ITS sequences, the four Donax species showed slight size differences among clones due to the variation occurring in the ITS1 and ITS2, except Donax variegatus did not display size differences in the ITS2. The total length of the ITS sequence ranged between 814 and 1014 bp. Resulting phylogenetic trees display that the two ribosomal DNA regions provide well-resolved phylogenies where the four European Donax species form a single clade receiving high support in nodes. The topology obtained with 5S sequences was in agreement with Donax evolutionary relationships inferred from several sequences of different nature in previous studies.

CONCLUSIONS

This is not only a basic research work, where new data and new knowledge is provided about Donax species, but also have allowed the authentication of these wedge clams and offers future applications to provide other genetic resources.