Mitochondrial DNA length variation and heteroplasmy in natural populations of the European sea bass, Dicentrarchus labrax

Mitochondrial DNA length variation in Dicentrarchus labrax is the outcome of the simultaneous variation in the copy number of two tandem repeat arrays located in the D-loop region. The repeated sequences, named R17 and R48, are 17 and 48 bp long, respectively. On the basis of 209 individuals collected in eight Mediterranean localities, length variation and its partitioning between the two repeat arrays have been characterized and quantified by means of frequency distributions of repeat copy number and genetic diversity estimates. The median copy numbers are 4 and 11 for the R17 and R48 repeat arrays, respectively. For both repeat arrays, continuity in step variation and the occurrence of both frequent and rare arrangements was observed. More than 50% of the individuals are heteroplasmic for up to four variants. Within populations, heteroplasmy ranges between 33.3% and 70.0% of the individuals. A large proportion of the total gene diversity (29.1%) occurs within individuals and the greatest proportion (66.6%) is found between individuals within populations, while only 4.3% is due to variation between populations. Although the interpopulation component of the diversity is rather low, the significant differentiation of the two Eastern populations from all the other Western samples suggests the occurrence of a phylogeographic component of the genetic divergence between the Mediterranean populations, which is only recognizable on a large geographic scale.