Unique features in the mitochondrial D-loop region of the European seabass Dicentrarchus labrax

We have cloned and sequenced the displacement-loop (D-loop) region of the mitochondrial DNA (mtDNA) from the European seabass Dicentrarchus labrax (Dl). This sequencing revealed the presence of four tandemly repeated elements (R1, R2, R3 and R4); the individual variation in mtDNA total length is entirely accounted for by their variable number. The individuals examined also possessed an imperfect copy of one of the tandem repeats (ΨR2). At least one termination-associated sequence (TAS) is present in each of the repeats and in two copies 5′ upstream from the tandem array as well. The alignment of the Dl D-loop region with D-loop sequences from four other Teleosts and one Chondrosteus showed the Dl sequence to be larger than that of other fish. The extraordinary length of the D1 D-loop sequence is also due to the 5′ and 3′ regions that are flanking the tandem array, the largest ones to date analyzed in fish. In this study, we also report the unique organization and localization of putative TAS and conserved-sequence block (CSB) elements, and the presence of a conserved 218-bp sequence in the D1 D-loop region.