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Keywords:

  • AMOVA;
  • cytochrome b;
  • haplotype network;
  • mitochondrial lineage;
  • phylogeography;
  • transoceanic barrier

Abstract

  1. Top of page
  2. Abstract
  3. References

Nucleotide variation of partial cytochrome b sequences was analysed in the bluefish Pomatomus saltatrix to investigate the population-structuring roles of climate change and oceanic barriers. Western and eastern North Atlantic Ocean populations appeared to be totally isolated, with the latter connected to the Mediterranean Sea within which further structuring occurred.

Highly migratory fishes exhibit great diversity in population structuring. For example, within the family Scombridae some species have been revealed to be globally distributed exhibiting high connectivity across oceans (Theisen et al., 2008), whereas others have shown significant population genetic structuring in their geographic range with boundaries within the Atlantic Ocean (Alvarado Bremer et al., 2005). Population structuring of a highly migratory marine fish can be promoted by behavioural traits such as homing, but can also be caused by oceanic barriers to gene flow, such as currents (Machado-Schiaffino et al., 2010), salinity (Nielsen et al., 2004) or temperature boundaries (Crow et al., 2007). For example, the distribution of the bluefish Pomatomus saltatrix (L.) coincides with sea surface temperatures of 18–27° C (Juanes et al., 1996), and it has been suggested that shifts in ranges and contacts between populations have resulted from historical changes in water temperature (Goodbred & Graves, 1996). These shifts could also be affected by current climate change, and so correct management of fish communities needs an assessment of this issue (Steele, 1998). In turn, this requires a knowledge of population genetic structure (Bowen et al., 2005).

The bluefish Pomatomus saltatrix, the only living representative of its genus and family, exhibits a worldwide tropical and subtropical distribution that comprises the western North and South Atlantic Ocean, the Azores Islands, the south European Atlantic Ocean, the Mediterranean Sea coasts, the Black and Marmara Seas, the Atlantic Ocean African coasts, the Indian Ocean, Australia and New Zealand (Randall, 1995; Juanes et al., 1996). Information about its population genetic structure, however, is still scarce. Available data comprise regional studies of variation at polymorphic allozyme markers (Nurthen et al., 1992) and microsatellite loci (Dos Santos et al., 2008), and one large-scale analysis based on mitochondrial restriction fragment length polymorphism (RFLP; Goodbred & Graves, 1996) of samples from Australia, South Africa, Portugal, North America and Brazil. The lowest net nucleotide sequence divergence was obtained when comparing North American and Portuguese samples, suggesting that the eastern and western Atlantic populations were the last in diverging; interspersion of RFLP haplotypes from the two sides of the North Atlantic Ocean suggested that either recent or historical gene flow might have been promoted by corridors formed during global distribution changes of temperate and warm marine waters (Goodbred & Graves, 1996). This kind of event has occurred in past glacial and interglacial periods, strongly shaping species and populations distributed along American and European coasts (Carr & Marshall, 2008).

In the present exploratory study, variation in mitochondrial sequences was analysed using P. saltatrix samples collected on both sides of the North Atlantic Ocean and the Mediterranean Sea. The aim was to assess genetic structuring in the Mediterranean Sea and Atlantic Ocean basins and to determine the possible existence of transatlantic barriers to gene flow in this worldwide species.

Samples were collected between 2006 and 2008 from seven different locations from western (New York and Florida, U.S.A.) and eastern (Cadiz, southern Spain and the Canary Islands) Atlantic Ocean coasts, and from western (Alicante, Spain) and eastern (Istanbul and Cannakale, Turkey) Mediterranean Sea. DNA was extracted by a Chelex-based methodology (Estoup et al., 1996). Universal L14841 and H15149 primers (Kocher et al., 1989) were used to obtain a partial sequence from the mitochondrial gene coding for cytochrome b (cyt b) from every individual following standard methodology described in Campo et al. (2007). This particular gene was chosen as it has been widely used to clarify population and subpopulation structure of marine and terrestrial species (Kyle & Wilson, 2007; Hwa et al., 2009). Two additional sequences from the Canary Islands were retrieved from GenBank (http://www.ncbi.nlm.nih.gov/Genbank) for inclusion in the analyses (accession numbers DQ080341 and DQ197982).

The jModeltest software version 0.11 (Posada, 2009) was employed to determine the best evolution model fit to the data using the corrected Akaike information criterion (AIC; Akaike, 1974). A haplotype network was constructed with Network version 4.05 (Forster et al., 2008) using the median-joining (MJ) procedure (Bandelt et al., 1999). The programme Arlequin version 3.0 (Excoffier et al., 2005) was used to perform an analysis of molecular variance (AMOVA; Excoffier et al., 1992) and to estimate pair-wise FST values (Wright, 1969). Significance tests were performed through 100 000 permutations. An isolation by distance model (IBD; Wright, 1943) was tested in the R statistical framework using package ecodist (Goslee & Urban, 2007). In addition, the programmes SAMOVA version 1.0 (Dupanloup et al., 2002) and BARRIER version 2.2 (Manni et al., 2004) were used to test for the presence of geographical barriers to dispersal.

A total of 82 original 300 base pair (bp) long sequences were obtained from the seven sampling locations (Table I). Ten polymorphic sites were found, yielding 11 different haplotypes that are available under the GenBank accession numbers GQ367552–GQ367562. The best fit to the data was assigned to the Kimura two parameter model (Kimura, 1980). Haplotype network revealed two groups of haplotypes with strong geographical affiliation (Fig. 1). Divergence between haplotypes within each Atlantic coast (including the Mediterranean Sea within the eastern Atlantic Ocean) was not greater than one mutational step, but the lineage found in the eastern Atlantic Ocean (haplotypes 1–7) was separated from that found in the western Atlantic Ocean (haplotypes 8–11) by three mutational events. Populations from different continents did not share any haplotypes. The AMOVA confirmed significant between-continent divergence (d.f., P < 0·05) accounting for 87·66% of the total variance. Interspersion of haplotypes among continents which had been found by Goodbred & Graves (1996) for RFLPs was not found in this study. It must be noted, however, that their sample size was larger.

Table I.  Sample size (n) and values of haplotypic diversity (h) and nucleotide diversity (π) by location. Data are presented as mean ±s.d.
Locationnhπ
Mediterranean Sea
 Cannakale100·200 ± 0·1540·001 ± 0·001
 Istanbul110·327 ± 0·1530·001 ± 0·001
 Alicante380·588 ± 0·0730·002 ± 0·002
Eastern North Atlantic
 Cadiz40·500 ± 0·2650·002 ± 0·002
 Canary Islands40·000 ± 0·0000·000 ± 0·000
Western North Atlantic
 New York90·833 ± 0·0800·004 ± 0·003
 Florida60·733 ± 0·1550·003 ± 0·002
image

Figure 1. Median-joining network diagram of cyt b haplotypes. Each circle represents a haplotype with area proportional to its frequency. Mutations between two haplotypes are marked as the corresponding nucleotide position (inline image, New York; inline image, Florida; inline image, east Atlantic Ocean (Canary Islands, Cadiz); inline image, Mediterranean Sea (Alicante, Cannakale and Istanbul).

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Pair-wise FST analysis provides a measure of genetic divergence between populations (Table II). Location pair-wise FST for cyt b were high and significant when samples from the Mediterranean Sea and the western North Atlantic Ocean were compared, as expected from the significant AMOVA. The pair-wise FST matrix was used to test the IBD model employing the Mantel test approach described in Puebla et al. (2009). A significant result was obtained after 100 000 permutations (r = 0·889, P < 0·05). It has to be taken into account that when long-distance samplings have been made, statistical significance of IBD can be obtained even if geographic distance is not the main factor in shaping genetic differences (Templeton et al., 1995; Templeton, 2001). Thus, the presence of barriers to dispersal of P. saltatrix was further tested using the simulation approach of SAMOVA and the computational geometry approach of BARRIER. Both programmes identified a main barrier to dispersal in the middle of the Atlantic Ocean.

Table II.  Population pair-wise FST (below diagonal) and their P-values (above diagonal)
LocationCannakaleIstanbulAlicanteCadizCanary IslandsNew YorkFlorida
Cannakale>0·05>0·05>0·05>0·05<0·001<0·001
Istanbul0·062<0·05>0·05>0·05<0·001<0·001
Alicante0·0770·108>0·05>0·05<0·001<0·001
Cadiz0·0810·083−0·093>0·05<0·001<0·01
Canary Islands−0·121−0·040−0·0170<0·001<0·01
New York0·8710·8640·8460·7590·833>0·05
Florida0·9140·9040·8620·8510·8930

A significant FST was also found when comparing Istanbul v. Alicante samples (FST = 0·108, P < 0·05), which are the Mediterranean Sea easternmost and westernmost samples, respectively. This result suggests the existence of some kind of impairments to migration inside the Mediterranean Sea basin and indicates some level of regional population structuring for this species, limited so far to eastern and western Australian waters (Nurthen et al., 1992). The complexity of the current patterns inside the Mediterranean Sea can contribute to this structuring (Juanes et al., 1996). Significant differences between eastern Atlantic Ocean (Cadiz and Canary Islands) and Mediterranean Sea samples were not found, suggesting that P. saltatrix can migrate across well-known biogeographic barriers such as the Strait of Gibraltar and the Oran–Almeria front as do other species (Patarnello et al., 2007).

In conclusion, although obtained from a limited number of samples, the present cyt b sequence data reveal a complete genetic isolation between the two sides of the North Atlantic Ocean, without apparent evidence of current or recent gene flow. Further studies on much larger sample sizes will support or refute the existence of oceanic barriers to gene flow in this species and help obtain a more accurate estimate of the time of divergence between P. saltatrix lineages, including the role of past climatic events in such separations.

This study was funded by the Spanish National project CGL2009-08279. A.F.P. holds a Foundation for the promotion in Asturias of the Applied Scientific Research and Technology, Plan for Science, Technology and Innovation (FICYT-PCTI) grant from the Asturias Regional Government (Ref. BP09028). Sample collection in North America was funded through the Bluefish–Striped Bass Dynamics Program. We are grateful to P. Sanchez-Jerez, T. Ceyhan, P. Clarke, J. Murt and D. Stormer for sampling. Special thanks to I. J. Winfield and two anonymous JFB reviewers for help with improving the manuscript.

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