Search

Previous attempts to test for small-scale stock structuring within Atlantic herring (Clupea harengus L.) with molecular markers have been hampered by uninformative levels of genetic variation. Here we report the first application of microsatellite DNA markers to investigate population subdivision in Atlantic herring from Norwegian waters and the Barents Sea, and also examine microsatellite differentiation between C. harengus and Pacific herring (C. pallasi). Results from four microsatellite loci indicate high, and informative, variation compared to molecular markers used previously: number of alleles per locus=18-41; mean expected heterozygosity within samples=0.90-0.93. Significant genetic differences were detected between almost all samples representing postulated Icelandic summer-spawner, Norwegian spring-spawner and Norwegian fjord stocks, using Fisher's exact test, FST and RST values. Levels of allele frequency differentiation between Atlantic and Pacific herring overlapped the range seen among Atlantic herring samples, indicating that microsatellites are poor indicators of the degree of species differentiation. Comparison with allozyme and mitochondrial DNA restriction fragment length polymorphism (RFLP) datasets from the same samples suggests that microsatellites may detect structuring at a finer scale, but are less informative at larger scales of divergence.

Despite the importance of larval biology in the life histories of many marine animals, relatively little information exists on the dynamics and genetic composition of larval cohorts. The supply of megalopae larvae of the shore crab, Carcinus maenas, was measured on a daily basis during 8 months spread along two larval periods (2006 and 2007) at the Ria de Aveiro estuary, on the Portuguese northwest coast. A total of 10 microsatellite DNA loci were employed to explore the genetic structure, variability and relatedness of temporally distinct megalopal events, selected from the major pulses of supply. Larval variation was also compared genetically with that of a previously studied adult crabs sample, at the same loci, collected in 2006 and 2007 along the Iberian Peninsula. Results revealed a lack of genetic differentiation and identical diversity levels among larval events over time. No evidence of reduced genetic diversity between megalopae relative to the diversity assessed from the pooled sample of adults was found. Moreover, there was no evidence of any family relatedness among larvae from temporal events. The results obtained for C. maenas contradict predictions made by the sweepstakes reproduction hypothesis, in which large variance in reproductive success is expected, which is presumably detectable as sharp genetic discontinuities among separate larval events. Data here indicate conversely a high degree of temporal genetic stability among larval supply to a given estuary under variable oceanographic conditions, consistent with the hypothesis that sampled larvae were drawn from a large number of adults that do not differ in reproductive success.

Two regions of the mtDNA of Atlantic herring (Clupea harengus), ND3/4 and ND5/6, were surveyed by RFLP analysis to assess the effects of marker variability on the power of statistical tests. Although haplotype diversity was similar, marked differences in the number of common haplotypes were observed between the two regions, with ND3/4 having a more even distribution of haplotype frequencies. ND3/4 also revealed higher differentiation among samples than ND5/6, similar to previously published microsatellite data of the same samples. The data suggested that the highly skewed haplotype distribution of many mtDNA markers may be one of the reasons for the less frequent detection of population differentiation compared to microsatellites. Pooling (`binning') of related haplotypes did not increase differentiation revealed by each mtDNA region individually, but greatly strengthened geographical patterns shown by both regions combined. The data provided evidence for genetic differentiation of Icelandic herring from other north-eastern Atlantic stocks, and indicated genetic differences between Baltic and Celtic Sea herring. However, mtDNA data failed to confirm previously reported differences between Barents Sea herring, Norwegian fjord populations and Norwegian spring spawners.

Chondrostoma lusitanicum is a Portuguese endemic cyprinid with a restricted distribution and reduced numbers in some basins, justifying its status as a threatened species. We examined genetic population structure using samples from throughout its geographical range in Portugal, using sequencing of b cytochrome and restriction fragment length polymorphism analysis of the NADH subunits 5 and 6. There was reduced within-population genetic variability but considerable among-population differentiation, particularly marked between both the Mira and Arade basins in the extreme south and other populations. These results confirm phylogeographic relationships suggested by previous fragmentary allozyme studies for C. lusitanicum, and are in accordance with allozyme and mitochondrial DNA data on phylogeography of coexisting cyprinid species of the genus Leuciscus. The levels of genetic divergence revealed by sequence and RFLP data showed strongly concordant patterns: geographical genetic structuring, with the definition of three distinct groups, was observed. The high values of nucleotide divergence and pairwise sequence divergence of the Mira and Arade groups, when compared with all other samples, support a distinct taxonomic status probably at the species level. Results are also discussed in relation to conservation of this highly fragmented species, in terms of Evolutionary Significant Units and Management Units.