Maintenance of genetic differentiation across a transition zone in the sea: discordance between nuclear and cytoplasmic markers.

To investigate the origin and maintenance of the genetic discontinuity between Atlantic and Mediterranean populations of the common sea bass (Dicentrarchus labrax) we analysed the genetic variation at a fragment of mitochondrial cytochrome b sequence for 18 population samples. The result were also compared with new or previously published microsatellite data. Seven mitochondrial haplotypes and an average nucleotidic divergence of 0.02 between Atlantic and Mediterranean populations that matches a Pleistocene allopatric isolation were found. The frequency variation at the cytochrome b locus was many times greater between Atlantic and Mediterranean populations (theta(C) = 0.67) than at microsatellite loci (theta(N)= 0.02). The examination of the different evolutionary forces at play suggests that a sex-biased hybrid breakdown is a likely explanation for part of the observed discrepancy between mitochondrial and nuclear loci. In addition, an analysis is made of the correlation between microsatellite loci points towards the possible existence of a hybrid zone in samples from the Alboran Sea.

Development and use of microsatellite markers in sea bass, Dicentrarchus labrax (Linnaeus, 1758) (Perciformes: Serrandidae).

The first microsatellite markers in sea bass, Dicentrarchus labrax, are described. Microsatellite-containing clones were isolated from a genomic library, their DNA sequences determined, and polymerase chain reaction primers designed for seven dinucleotide microsatellite loci. All microsatellites were shown to be highly polymorphic: six unrelated individuals displayed 86% observed heterozygosity and an average of 7.6 alleles per locus. Inheritance was shown to be Mendelian by analysis of genotypic ratios in F1 offspring. An analysis of 97 mass-reared offspring of a complete diallel cross between three males and three females showed that the parentage of all individuals could be determined unambiguously after scoring one locus in all individuals, followed by scoring a second locus in 24 individuals. The results suggest that microsatellites will be valuable tools in studies of sea bass for genetic tagging, marker-assisted selection programs, and identification of natural stocks.