Length variation in mtDNA control region in hatchery stocks of European sea bass subjected to acclimation experiments

Length polymorphism of the mtDNA control region was analysed in samples from the sea bass Dicentrarchus labrax before and after acclimation to freshwater. Acclimation trials were repeated twice for two samples originating from the same broodstock. DNA amplification of 221 individuals made it possible to detect 40 different D-loop length variants and a mean level of gene diversity (k(c)) of 0.850. Out of 52 scored distinct genotypes, 18 were homoplasmic and the remaining ones were heteroplasmic for up to four variants. Shifts in frequencies between starting and acclimated samples in the same direction of the same genotypes in both years were not observed. Patterns of mtDNA control region variation were compared to previous allozymic and RAPD studies carried out on the same samples. While variation found in the allozyme alleles and the RAPD markers in replicate experiments revealed differential survival of the genotypes, stochastic processes, such as genetic drift, explained the observed variation of mtDNA control region. Very high levels of genotype diversity were observed within each sample, ranging from 0.77 to 0.97. This high diversity associated with the maternal inheritance of mtDNA makes this molecular marker especially suitable for studies in aquaculture genetics, when the allozyme studies fail to reveal genetic variation or when the identification of progeny based on the mother's contribution is relevant. (C) Inra/Elsevier, Paris.