Chaotic genetic patchiness in the pelagic teleost fish Sardina pilchardus across the Siculo-Tunisian Strait (original) (raw)
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Heredity, 1997
were studied using electrophoretic methods in order to characterize stocks of sardines from the western Mediterranean Sea. The results showed differentiated groups distributed throughout the area studied. They did not form a panmictic population, but they existed as semi-independent, although not completely isolated, breeding units with an estimation of migration of Nem = 3.1 (number of migrants per generation). In the LDH-1 system a north-south dine was detected. We have also detected a discontinuity between the Alboran and the rest of the Mediterranean populations sampled at the enzymatic level. This result confirms the action of the Almeria-Oran front as a barrier that produces discontinuities as detected in other marine species.
2006
Fishery assessment models meant to determine sustainability of commercial marine fish failed to predict recent stock collapses due to overexploitation. One flaw of assessment models is that they strongly rely on catch and age-composition statistics, but largely ignore the genetic background of studied populations. We examined population genetic structure of sardine (Sardina pilchardus) in the centraleastern and northeastern Atlantic Ocean and Mediterranean Sea in order to aid fishery management of this heavily fished small pelagic species. We found that sardine has a striking mitochondrial control region, and sequenced a fragment of 387 bp of its 5'-end in 261 individuals collected off the coasts of Morocco (Dakhla, Tantan, Safi, Larache, Nador), Portugal (Quarteira), Spain (Pasajes, Barcelona), and Greece (Kavala). High levels of haplotypic diversity rendered a rather unresolved NJ phylogeny. The recovered tree had no phylogeographic structuring except for the clustering of 13 individuals of Safi. In contrast, individuals grouped together according to the presence or absence of a 13-bp insertion in the sequence. Φ st pairwise comparisons and molecular variance analyses supported genetic differentiation between the population of Pasajes (Bay of Biscay), and those of the Mediterranean Sea and Moroccan coast, with a contact zone around the Strait of Gibraltar. This result confirms the existence of two subspecies, S. pilchardus pilchardus and S. pilchardus sardina that were previously identified based on meristics and morphometry. Mismatch distribution analysis showed that sardine populations are expanding since the Pleistocene. Surprisingly, the population of Safi showed strong and statistically significant levels of genetic differentiation that could be related with isolation and genetic drift. Comparative analysis of the Safi population versus the rest including mismatch distributions, and a Bayesian skyline plot suggest that the Safi population likely underwent an early genetic bottleneck. The genetic singularity of the Safi population could have been responsible for the historical collapse of this sardine stock in 1970s.
Signature of an early genetic bottleneck in a population of Moroccan sardines (Sardina pilchardus)
Molecular Phylogenetics and Evolution, 2006
Fishery assessment models meant to determine sustainability of commercial marine Wsh failed to predict recent stock collapses due to overexploitation. One Xaw of assessment models is that they strongly rely on catch and age-composition statistics, but largely ignore the genetic background of the studied populations. We examined population genetic structure of sardine (Sardina pilchardus) in the centraleastern and northeastern Atlantic Ocean and Mediterranean Sea to aid Wshery management of this heavily Wshed small pelagic species. We found that sardine has a striking mitochondrial control region, and sequenced a fragment of 387 bp of its 5Ј-end in 261 individuals collected oV the coasts of Morocco (Dakhla, Tantan, SaW, Larache, and Nador), Portugal (Quarteira), Spain (Pasajes, Barcelona), and Greece (Kavala). High levels of haplotypic diversity rendered a rather unresolved NJ phylogeny. The recovered tree had no phylogeographic structuring except for the clustering of 13 individuals of SaW. In contrast, individuals grouped together according to the presence or absence of a 13-bp insertion in the sequence. ST pairwise comparisons and molecular variance analyses supported genetic diVerentiation between the population of Pasajes (Bay of Biscay), and those of the Mediterranean Sea and Moroccan coast, with a contact zone around the Strait of Gibraltar. This result conWrms the existence of two subspecies, S. pilchardus pilchardus and S. pilchardus sardina that were previously identiWed based on meristics and morphometry. Mismatch distribution analysis showed that sardine populations are expanding since the Pleistocene. Surprisingly, the population of SaW showed strong and statistically signiWcant levels of genetic diVerentiation that could be related with isolation and genetic drift. Comparative analysis of the SaW population versus the rest including mismatch distributions, and a Bayesian skyline plot suggest that the SaW population likely underwent an early genetic bottleneck. The genetic singularity of the SaW population could have been responsible for the historical collapse of this sardine stock in the 1970s.
Temporal genetic variation in populations of Diplodus sargus from the SW Mediterranean Sea
Marine Ecology-progress Series, 2007
Population genetic studies on white sea bream Diplodus sargus have revealed different patterns in the subdivision of populations in the Mediterranean Sea. However, the stability of observed allele frequencies over time remains poorly tested. The aim of this study was to show that the genetic structure of D. sargus could significantly change over time by analysing temporal variations in allozymes. In order to determine temporal variation in the genetic structure of 5 natural D. sargus populations in the SW Mediterranean, we screened 14 allozyme loci. Our main finding was the significant genotypic differentiation among cohorts (year-classes) in the Guardamar (F ST = 0.012; p < 0.001) and Cape of Palos (F ST = 0.008; p < 0.001) populations. The differentiation observed in the present study when considering pair-wise comparisons between cohorts is similar to that of all populations throughout the Mediterranean Sea. Our results suggest that microgeographical variations, also known as 'chaotic genetic patchiness', could occur in D. sargus populations from the SW Mediterranean. The recruitment of genetically variable cohorts at 1 site each year may account for these variations. We also discussed alternative explanations for this genetic pattern. This study confirms the importance of understanding the ecology, behaviour and environment of fish populations when investigating population genetic structure. Our results also highlight the importance of incorporating temporal samples when conducting population structure studies.
Matching genetics with oceanography: directional gene flow in a Mediterranean fish species
2011
Genetic connectivity and geographic fragmentation are two opposing mechanisms determining the population structure of species. While the first homogenizes the genetic background across populations the second one allows their differentiation. Therefore, knowledge of processes affecting dispersal of marine organisms is crucial to understand their genetic distribution patterns and for the effective management of their populations. In this study, we use genetic analyses of eleven microsatellites in combination with oceanographic satellite and dispersal simulation data to determine distribution patterns for Serranus cabrilla, a ubiquitous demersal broadcast spawner, in the Mediterranean Sea. Pairwise population F ST values ranged between )0.003 and 0.135. Two genetically distinct clusters were identified, with a clear division located between the oceanographic discontinuities at the Ibiza Channel (IC) and the Almeria-Oran Front (AOF), revealing an admixed population in between. The Balearic Front (BF) also appeared to dictate population structure. Directional gene flow on the Spanish coast was observed as S. cabrilla dispersed from west to east over the AOF, from north to south on the IC and from south of the IC towards the Balearic Islands. Correlations between genetic and oceanographic data were highly significant. Seasonal changes in current patterns and the relationship between ocean circulation patterns and spawning season may also play an important role in population structure around oceanographic fronts.
Marine Biology, 2002
Determining the origin of genetic structure is of wide interest because of its use in stock discrimination in marine organisms. Schematically, genetic differentiation can result from historical patterns maintained over geological time or from present-day isolation attributable to biological characteristics of the species. We used a comparative approach to population genetic analysis based on allozyme polymorphism to determine the impact of reproductive strategy (i.e. biological origin) and habitat (i.e. historical origin) on the genetic structure of individuals sampled from five isolated islands in French Polynesia. Eight species of coral reef fishes from two families (Chaetodontidae and Pomacentridae) were selected to test the impact of sea-level change (historical origin) and reproductive strategy (biological origin) on genetic structure. Seven of the eight study species showed significant divergence in allelic frequencies computed over all sites. For these seven species, multilocus F stvalues ranged from 0.0114 to 0.0287. None of the eight species showed a significant relationship between genetic divergence and geographical distance between sites. Significant divergence (difference in allozyme frequencies) between some pairs of sites occurred but was unrelated to distances between them. These results suggest that the genetic structure of coral reef fish in French Polynesia is likely to be driven according to an island model in which migrations between populations are rare and random in space and time. Overall, none of the species showed congruent genetic structures between sites sampled. Genetic structure of the eight species did not appear significantly related either to reproductive strategy or habitat preference. Genetic diversity (heterozygosity) was significantly correlated with these two factors, with species laying benthic eggs and/or inhabiting lagoons showing significantly higher multilocus heterozygosity than species laying pelagic eggs and/or living on the outer reef slope. Overall, the absence of differences according to habitat and/or reproductive strategy did not provide any conclusive pattern regarding the origin of the genetic structure, but the limited divergence in allelic frequencies suggests recent differentiations.
Aquaculture Research, 2007
Ten populations of Penaeus kerathurus (ForskÌl), a prawn of high commercial value, were sampled from the eastern and western Mediterranean coastal waters of Tunisia and screened electrophoretically for genetic variation at 13 allozyme loci. Four among the six polymorphic loci were out of Hardy^Weinberg equilibrium (H^WE) in at least one population. In the same way, the multilocus test showed deviation from H^WE in six populations. These populations showed heterozygote de¢ciency whereas the average heterozygosity for the four remaining ones is quite similar to the expected levels. Genetic variability was low. The number of alleles per locus ranged from 1.2 to 1.5 (average 51.3), and the observed heterozygosity varied between 0.010 and 0.048 (average 5 0.021). Signi¢cant population di¡erentiation (F ST 5 0.076, Po0.05) in the total data set re£ected the di¡erentiation of the two populations, which were at the margins of the range sampled, from all the others (Pairwise F ST values ranged from 0.035 to 0.208). Although there was no signi¢cant di¡erentiation among the other populations (pairwise F ST values ranged from À 0.006 to 0.201, P40.05). Our data suggest a population structure consistent with separation by Mediterranean Sea basins that might re£ect di¡erent local biogeographical zones.
African Journal of Marine Science, 2011
To investigate the possible influence of the Siculo-Tunisian Strait on the genetic structure of white seabream Diplodus sargus, 13 polymorphic allozyme loci and a fragment of the cytochrome b mito-chondrial DNA were analysed. Allozyme data indicated a moderate but significant differentiation between some north-eastern (Bizerta, Ghar El Melh Lagoon and Mahdia) and southern (Gabes Gulf and El Biban Lagoon) samples. This heterogeneity was also highlighted after removing PGM* and PGI-1* loci which may be under selection. These results can be explained by the chaotic genetic patchiness hypothesis. In contrast, the mtDNA data indicated genetic homogeneity among localities showing the absence of structure in white seabream populations across the Siculo-Tunisian Strait. Historical demography of this species suggests that it has undergone a recent population expansion as a consequence of a bottleneck event during the Pleistocene glaciations.
Aquatic Living Resources, 2012
The meagre Argyrosomus regius is a large Sciaenid fish known to reproduce in the eastern Atlantic and Mediterranean Sea in just five distinct and restricted geographic areas: along the Mauritanian coast and at estuary openings (Gironde, Tagus, Guadalquivir and Nile). The biological traits of A. regius (high dispersal capabilities, high fecundity, long larval phase, overlapping generations, reproduction until 40 years of age) are, in principle, favourable to high gene flow, which should lead to genetic homogeneity over large geographic scales. Nevertheless, the high geographic distances between the few reproductive areas leads one ask whether there is genetic differentiation in this species. In the present study, the genetic differentiation of the wild A. regius was investigated across most of its natural range from the Atlantic Ocean (France, Portugal, Spain, Mauritania) to the Mediterranean Sea (Egypt, Turkey), using 11 microsatellite markers previously identified in another Sciaenid, the red drum Sciaenops ocellatus. At least two very distinct groups could be identified, separated by the Gibraltar Strait. Genetic divergences (F ST values) were intermediate between the Atlantic samples (0.012-0.041), high between Egypt and the Atlantic (0.06-0.107) or Aegean Sea (0.081) and extremely high between the Aegean Sea and the Atlantic (0.098-0.168). A. regius exhibited a very high level of genetic differentiation rarely reported in marine fishes. These results also demonstrate the existence of a sixth independent spawning area in the Menderes delta (Turkey). Factors potentially involved in this very high genetic fragmentation are discussed, including physical barriers, glaciation pulses and biological traits.
2007
Background: Marine pelagic fishes exhibit rather complex patterns of genetic differentiation, which are the result of both historical processes and present day gene flow. Comparative multilocus analyses based on both nuclear and mitochondrial genetic markers are probably the most efficient and informative approach to discerning the relative role of historical events and life-history traits in shaping genetic heterogeneity. The European sardine (Sardina pilchardus) is a small pelagic fish with a relatively high migratory capability that is expected to show low levels of genetic differentiation among populations. Previous genetic studies based on meristic and mitochondrial control region haplotype frequency data supported the existence of two sardine subspecies (S. p. pilchardus and S. p. sardina). Results: We investigated genetic structure of sardine among nine locations in the Atlantic Ocean and Mediterranean Sea using allelic size variation of eight specific microsatellite loci. Bayesian clustering and assignment tests, maximum likelihood estimates of migration rates, as well as classical genetic-variance-based methods (hierarchical AMOVA test and R ST pairwise comparisons) supported a single evolutionary unit for sardines. These analyses only detected weak but significant genetic differentiation, which followed an isolation-by-distance pattern according to Mantel test. Conclusion: We suggest that the discordant genetic structuring patterns inferred based on mitochondrial and microsatellite data might indicate that the two different classes of molecular markers may be reflecting different and complementary aspects of the evolutionary history of sardine. Mitochondrial data might be reflecting past isolation of sardine populations into two distinct groupings during Pleistocene whereas microsatellite data reveal the existence of present day gene flow among populations, and a pattern of isolation by distance.