Thirteen (GT ) and four (CT ), microsatellite loci (n = 10 or more and n =20 or more, respectively) have been isolated from a partial genomic library of brown trout and sequenced. On average, a (GT ) repeat sequence occurs approximately every 23 kb and a (CT ) repeat sequence every 76 kb in brown trout genome. Primers for DNA amplifications using the polymerase chain reaction (PCR) were synthesized for three single locus microsatellites. Mendelian inheritance of the observed polymorphisms was confirmed in full-sib families. Four brown trout populations (10 unrelated individuals per population) were screened for polymorphism with these three microsatellite loci. The total number of alleles detected in the four populations is five at one locus, six at the other two microsatellite loci and is three, on average, per population. Heterozygosities range from 0.18 to 0.74. The largest differences in allelic frequencies occurred between the Mediterranean and the Atlantic populations: this result is congruent with previous allozymic data. The gene-centromere distances of the three microsatellite markers were determined on gynogenetic lines: post-reduction rates range from 0.17 to 0.60. For all the three microsatellite loci, the primers designed from brown trout sequences can be used in another closely related species of salmonid, the rainbow trout (Oncorhynchus mykiss). This last aspect supports the view that microsatellite markers may have wide application in genetic studies in salmonid species and fishes in general.

A consensus microsatellite-based linkage map of the turbot (Scophthalmus maximus) was constructed from two unrelated families. The mapping panel was derived from a gynogenetic family of 96 haploid embryos and a biparental diploid family of 85 full-sib progeny with known linkage phase. A total of 242 microsatellites were mapped in 26 linkage groups, six markers remaining unlinked. The consensus map length was 1343.2 cM, with an average distance between markers of 6.5 6 0.5 cM. Similar length of female and male maps was evidenced. However, the mean recombination at common intervals throughout the genome revealed significant differences between sexes, $1.6 times higher in the female than in the male. The comparison of turbot microsatellite flanking sequences against the Tetraodon nigroviridis genome revealed 55 significant matches, with a mean length of 102 bp and high sequence similarity (81-100%). The comparative mapping revealed significant syntenic regions among fish species. This study represents the first linkage map in the turbot, one of the most important flatfish in European aquaculture. This map will be suitable for QTL identification of productive traits in this species and for further evolutionary studies in fish and vertebrate species.

The conservation of flanking regions of ten microsatellites has been investigated in three salmonid species. The proportion of conserved microsatellites and the level of polymorphism are high in the three species. This confirms the potential interest of microsatellite loci for interspecific comparisons. 1996 The Fisheries Society of the British Isles

Variation in the internal transcribed spacer (ITS) of rDNA was examined throughout the range of the brown trout (Salmo trutta) to analyze the usefulness of this molecular marker for phylogeographic analysis. The results were compared with those previously obtained with mtDNA, a region exhaustively analyzed along the brown trout distribution. ITS2 was essentially conserved at all populations sampled, no informative characters being detected across the main lineages described in this species. Conversely, ITS1 showed a greater homogenization than other genetic markers at a microgeographic scale, with variation partitioning into several major phylogenetic groups. Phylogeographic patterns were partially congruent between both ITS1 and mtDNA. The main discrepancies were the detection of intra-individual variation and putative recombinant ITS1 sequences in hybridization areas between genetically different, yet historically overlapping, assemblages. Also, the existence of an ancient ITS1 sequence in the Mediterranean-southeastern area (rMEDA), not revealed by mtDNA analysis, was evidenced after rDNA ITS1 analysis.

Twelve microsatellite and 28 allozyme loci were employed to analyse genetic diversity in natural and domestic populations of turbot (Scophthalmus maximus) from northwest Spain in comparison with other flatfish species with similar habitat, life history, and geographic distribution—the brill (Scophthalmus rhombus) and the flounder (Platichthys flesus). These species had shown much higher allozyme diversity than turbot in previous studies, and were used as a reference to check for putative historical bottlenecks in turbot. Significantly lower genetic variability in turbot than in brill and flounder was confirmed with allozymes, but not with the highly variable microsatellite loci. This intermarker discrepancy could be explained by different mutation rates in relation with historical bottlenecks along turbot evolution. A significantly lower genetic diversity was observed in a domestic strain of turbot than in natural populations of this species. This sample evidenced a strong family structure from microsatellite data, which suggests caution against the use of commercial batches for broodstock foundation in turbot farming. A strong concordance was found across the two categories of markers used when analysing the pattern of genetic subdivision at a local scale within the three species analysed, low and nonsignificant genetic differentiation being observed between Atlantic and Cantabric areas.