The effects of medieval dams on genetic divergence and demographic history in brown trout populations

BACKGROUND: Habitat fragmentation has accelerated within the last century, but may have been ongoing over longer time scales. We analyzed the timing and genetic consequences of fragmentation in two isolated lake-dwelling brown trout populations. They are from the same river system (the Gudenå River, Denmark) and have been isolated from downstream anadromous trout by dams established ca. 600–800 years ago. For reference, we included ten other anadromous populations and two hatchery strains. Based on analysis of 44 microsatellite loci we investigated if the lake populations have been naturally genetically differentiated from anadromous trout for thousands of years, or have diverged recently due to the establishment of dams. RESULTS: Divergence time estimates were based on 1) Approximate Bayesian Computation and 2) a coalescent-based isolation-with-gene-flow model. Both methods suggested divergence times ca. 600–800 years bp, providing strong evidence for establishment of dams in the Medieval as the factor causing divergence. Bayesian cluster analysis showed influence of stocked trout in several reference populations, but not in the focal lake and anadromous populations. Estimates of effective population size using a linkage disequilibrium method ranged from 244 to > 1,000 in all but one anadromous population, but were lower (153 and 252) in the lake populations. CONCLUSIONS: We show that genetic divergence of lake-dwelling trout in two Danish lakes reflects establishment of water mills and impassable dams ca. 600–800 years ago rather than a natural genetic population structure. Although effective population sizes of the two lake populations are not critically low they may ultimately limit response to selection and thereby future adaptation. Our results demonstrate that populations may have been affected by anthropogenic disturbance over longer time scales than normally assumed

Captive Breeding Programs Based on Family Groups in Polyploid Sturgeons

In species with long life cycles and discontinuous availability of individuals to reproduction, implementing a long-term captive breeding program can be difficult or impossible. In such cases, managing diversity among familiar groups instead of individuals could become a suitable approach to avoid inbreeding and increase the possibility to accomplish a breeding scheme. This is the case of several sturgeon species including the Adriatic sturgeon, whose recovery depends on the management of a few captive stocks directly descended from the same group of wild parents. In the present study, relatedness among 445 potential breeders was inferred with a novel software for pedigree reconstruction in tetraploids ("BreedingSturgeons"). This information was used to plan a breeding scheme considering familiar groups as breeding units and identifying mating priorities. A two-step strategy is proposed: a short-term breeding program, relying on the 13 remaining F0 individuals of certain wild origin; and a long-term plan based on F1 families. Simulations to evaluate the loss of alleles in the F2 generation under different pairing strategies and assess the number of individuals to breed, costs and logistical aquaculture constraints were performed. The strategy proposed is transferable to the several other tetraploid sturgeon species on the brink of extinction

Genetic and morphometric heterogeneity among recruits of the European eel, Anguilla anguilla

The recognition of the vulnerability of marine species to environmental conditions throughout their life-cycle has broadened the scope of investigations on factors influencing their population dynamics and demographics. The European eel Anguilla anguilla (Linnaeus, 1758) stock is declining rapidly due to overfishing, pollution, habitat degradation, diseases, and oceanic conditions. We analyzed arrival waves of glass eels collected in the Bay of Biscay and the western Mediterranean area during the period 2001-2003. We observed significant differences in length, weight, and condition between Atlantic and Mediterranean samples, and among arrival waves within sites. All samples were screened for genetic variation using ten allozyme and six microsatellite loci. We observed a pattern of genetic patchiness among arrival waves, namely a highly significant genetic differentiation without a temporal grouping of samples. Although natural selection and gene flow could also play a role, we suggest that the pattern observed results from the high variance in reproductive success in each spawning season. A low effective population size might have contributed to the current decline in the abundance of European eel. A precautionary approach to fisheries should be implemented in order to preserve maximal genetic potential to cope with changing anthropogenic and environment pressures

Distribution of genetic variation in farmed and natural stocks of European eel

[Extract] The aim of the study is to provide more detailed knowledge on the genetic variability, demography and population substructuring of European eel by analysing and comparing natural and farmed individuals. Natural eel samples have been obtained in two geographical sites (Netherlands, France) including temporal samples in a short-scale (within years) and a long-scale (between years). Simultaneously, farmed glass eels have been grown in two separate batches during one year. Batches have been monitored and genetic samples have been obtained during the year

Genetic patchiness among recruits in the European eel <i>Anguilla anguilla</i>

Heterogeneity in genetic composition among recruits of marine species is mostly due to a large variance in reproductive success mediated by oceanographic processes. Temporal genetic variation in a population of the European eel was quantified over 2 time scales among glass eel (1) interannual samples (cohorts), and (2) intra-annual samples within cohorts ('arrival waves'). A total of 789 glass eels comprising 11 different arrival waves were collected at Den Oever in The Netherlands over the period 2001 to 2003. All samples were screened for genetic variation using 10 allozyme and 6 microsatellite loci. The main result from this study is the highly significant genetic differentiation among arrival waves, despite the low FST values (FST = 0.0036). Heterogeneity in genetic composition was observed both among cohorts and among samples within cohorts. Genetic differentiation partitioned within cohorts was more than 10-fold the differences among cohorts. Genetic heterogeneity is likely to result from a large variance in the contribution of individuals to each cohort determined by genetic drift. Although natural selection and gene flow could also play a role in the observed genetic pattern, we suggest that large variances in reproductive success are a contributing factor to the recruit differentiation. If only a subset of the adults contribute to the new recruits, effective population size in European eel might be much lower than the census size. A low effective population size combined with fluctuating oceanic conditions might have contributed to the current dramatic decline in abundance of European eel

Genetic analysis of tuna populations, <i>Thunnus thynnus thynnus</i> and <i>T. alalunga</i>

The genetic population structures of Atlantic northern bluefin tuna (Thunnus thynnus thynnus) and albacore (T. alalunga) were examined using allozyme analysis. A total of 822 Atlantic northern bluefin tuna from 18 different samples (16 Mediterranean, 1 East Atlantic, 1 West Atlantic) and 188 albacore from 5 samples (4 Mediterranean, 1 East Atlantic) were surveyed for genetic variation in 37 loci. Polymorphism and heterozygosity reveal a moderate level of genetic variability, with only two highly polymorphic loci in both Atlantic northern bluefin tuna (FH* and SOD-1*) and albacore (GPI-3* and XDH*). The level of population differentiation found for Atlantic northern bluefin tuna and albacore fits the pattern that has generally been observed in tunas, with genetic differences on a broad rather than a more local scale. For Atlantic northern bluefin tuna, no spatial or temporal genetic heterogeneity was observed within the Mediterranean Sea or between the East Atlantic and Mediterranean, indicating the existence of a single genetic grouping on the eastern side of the Atlantic Ocean. Very limited genetic differentiation was found between West Atlantic and East Atlantic/Mediterranean northern bluefin tuna, mainly due to an inversion of SOD-1* allele frequencies. Regarding albacore, no genetic heterogeneity was observed within the Mediterranean Sea or between Mediterranean and Azores samples, suggesting the existence of a single gene pool in this area

Genetic patchiness among recruits in the European eel Anguilla anguilla

Heterogeneity in genetic composition among recruits of marine species is mostly due to a large variance in reproductive success mediated by oceanographic processes. Temporal genetic variation in a population of the European eel was quantified over 2 time scales among glass eel (1) interannual samples (cohorts), and (2) intra-annual samples within cohorts ('arrival waves'). A total of 789 glass eels comprising 11 different arrival waves were collected at Den Oever in The Netherlands over the period 2001 to 2003. All samples were screened for genetic variation using 10 allozyme and 6 microsatellite loci. The main result from this study is the highly significant genetic differentiation among arrival waves, despite the low FST values (FST = 0.0036). Heterogeneity in genetic composition was observed both among cohorts and among samples within cohorts. Genetic differentiation partitioned within cohorts was more than 10-fold the differences among cohorts. Genetic heterogeneity is likely to result from a large variance in the contribution of individuals to each cohort determined by genetic drift. Although natural selection and gene flow could also play a role in the observed genetic pattern, we suggest that large variances in reproductive success are a contributing factor to the recruit differentiation. If only a subset of the adults contribute to the new recruits, effective population size in European eel might be much lower than the census size. A low effective population size combined with fluctuating oceanic conditions might have contributed to the current dramatic decline in abundance of European eel

Growth rate correlates to individual heterozygosity in the european eel, Anguilla anguilla L.

Heterozygosity-fitness correlations (HFCs) have been reported in populations of many species. We provide evidence for a positive correlation between genetic variability and growth rate at 12 allozyme loci in a catadromous marine fish species. the European eel (Anguilla anguilla L.). More heterozygous individuals show a significantly higher length and weight increase and an above average condition index in comparison with more homozygous individuals. To a lesser extent. six microsatellite loci show a similar pattern, with positive but not significant correlations between heterozygosity and growth rate. The HFCs observed could be explained by an effect of either direct allozyme overdominance or associative overdominance. Selection affecting some of the allozyme loci would explain the greater strength of the HFCs found at allozymes in comparison with microsatellites and the lack of correlation between MLH at allozymes and MLH at microsatellites. Associative overdominance (where allozyme loci are merely acting as neutral markers of closely linked fitness loci) might provide an explanation for the HFCs if we consider that allozyme loci have a higher chance than microsatellites to be in linkage disequilibrium with fitness loci

Environmental stress and life-stage dependence on the detection of heterozygosity–fitness correlations in the European eel, Anguilla anguilla

Heterozygosity–fitness correlations (HFCs) have been reported in populations of many species, although HFCs can clearly vary across species, conspecific populations, temporal samples, and sexes. We studied (i) the temporal stability of the association between genetic variation and growth rate (length and mass increase) and (ii) the influence of genetic variability on survival in the European eel (Anguilla anguilla L). HFCs were assessed using genotypes from 10 allozyme and 6 microsatellite markers in 22-month-old experimental individuals. The results were compared with those of a previous study carried out in 12-month-old individuals, in which more heterozygous individuals showed a significantly faster growth rate. In contrast, 22-month-old individuals showed no evidence that genetic variability was correlated with growth rate. Additionally, heterozygous individuals did not show a higher survival rate compared with more homozygous individuals after either handling stress or parasite infection. The decrease in HFCs over time is consistent with the general prediction that differences in growth and survival among individuals are maximal early in life and in our case most likely due to the relaxation of environmental conditions related to population-density effects. Alternatively, the decline in HFCs could be attributed to either ontogenetic variance in gene activity between 12- and 22-month-old individuals or differential mortality leaving only the largest individuals