BACKGROUND
parallel adaptive changes under replicated environmental conditions have been particularly valuable for understanding evolutionary processes in natural populations.
one of the classical questions in evolutionary biology concerns whether different species and populations within species will adapt to the same agent of selection in the same way or whether the response will involve different traits and genes  <cit> .
parallel genotypic adaptation appears to be frequent and occurs at all taxonomic levels from microbes and plants to humans  <cit>  and is likely to result in changes at a relatively small number of genes  <cit> .
for instance, the study of colosimo et al.
<cit>  demonstrated that selection on a single gene, ectodysplasin , is responsible for the parallel reduction of armor plates in freshwater populations of threespine stickleback gasterosteus aculeatus.
however, more complex physiological processes relevant in the context of parallel freshwater adaptation of threespine sticklebacks are influenced by several genes, each of small effect  <cit> .
using a survey of the published literature on parallel adaptation of independent lineages of natural populations, conte et al.
<cit>  concluded that divergence at loci under selection is most likely to be based on standing genetic variation derived from a common ancestor rather than mutations occurring de novo after divergence.
hence, probability of gene reuse is plausibly higher in closely related species, which are likely to show similar divergence at loci subjected to similar selection pressures  <cit> .
an excellent opportunity to test for genetic parallelism exists in the two north atlantic eel species, the european eel  and the american eel .
both species are morphologically almost indistinguishable, with the number of vertebrae being regarded as the best diagnostic character between species  <cit> .
divergence time between the two species remains largely unresolved, encompassing between  <dig>  and  <dig>  million years  <cit> .
remarkably, although mitochondrial dna lineages of the two species are reciprocally monophyletic  <cit> , differentiation at nuclear loci is surprisingly low , suggestive of ongoing gene flow.
in this sense, it is well established that the spawning grounds of the two species overlap in the sargasso sea and there is also overlap in spawning time  <cit> .
european and american eels are known to hybridize, with hybrids observed almost exclusively in iceland  <cit> .
hence, the sister species status of european and american eel and the low but biologically significant gene flow makes them an adequate system in which to test the occurrence of selection at homologous loci within each species.
north atlantic eels have a catadromous life cycle and after spawning in the sargasso sea, larvae are transported by the gulf stream and other currents to the shores of north america and europe/north africa, respectively.
upon reaching the continental shelf, larvae metamorphose into glass eels, which complete the migration into riverine, estuarine and coastal feeding habitats and grow up as yellow eels.
after a highly variable feeding stage, yellow eels metamorphose into partially mature silver eels that migrate back to the sargasso undertaking a journey of about  <dig>  km for the american eel and  <dig> - <dig>  km for the european eel.
upon arriving in the sargasso sea, eels reproduce and die  <cit> .
during the continental phase, eels occupy a broad range of habitats from the caribbean to greenland in the western atlantic  and from morocco to iceland in the eastern atlantic .
the presence of eels in extremely heterogenous environments in terms of temperature , salinity , substrate, depth or productivity along their geographic distribution makes them ideal species in which to study the consequences of spatially varying selective pressures that often result in local adaptation of ecologically important traits  <cit> .
beginning with levene  <cit> , who introduced the first theoretical model for examining the impact of diversifying selection in space, a number of studies have shown that balancing selection due to spatial heterogeneity is an important mechanism responsible for the maintenance of genetic polymorphism .
genetic variation in a spatially heterogenous environment may be maintained even when dispersal results in complete mixing of the gene pool  <cit> .
however, under such a panmixia scenario, in which offspring are distributed to environments at random independently of the environment experienced by the parents, local selection cannot to lead to local adaptation  <cit> .
in the case of eels, owing to panmixia in both european  <cit>  and american eel  <cit>  and random larval dispersal across habitats, heritable trans-generational local adaptation is not possible although single-generation footprints of selection can still be detected.
in this sense, significant geographic clines at allozyme loci have been detected in both european  <cit>  and american eel  <cit> .
in the most comprehensive study to date, gagnaire et al.
<cit>  found evidence for spatially varying selection at  <dig> coding genes in american eel showing significant correlations between allele frequencies and environmental variables  across the entire species range.
in this study, we tested for single-generation signatures of spatial varying selection in european eel and compared the results to those obtained by gagnaire et al.
<cit> .
we genotyped glass eels from  <dig> sampling locations across the geographic distribution of the species, using the same set of snps analyzed by gagnaire et al.
<cit>  in american eel.
we used two main analytical approaches, one that identifies outliers as those markers with greater differentiation among all snps and a second based on determining positive associations between allele frequencies and environmental factors.
following the positive associations observed by gagnaire et al.
<cit>  in american eel, variables used in our study were degrees north latitude, degrees east/west longitude and sea-surface temperature at river mouth.
we specifically wanted to test whether the same genes were under spatially varying selection in both european and american eel, hence providing evidence for parallel patterns of local selection, or whether the response involved a different set of genes.
considering their sister species status and the existence of gene flow between species, together with the similar environmental conditions they encounter  <cit> , we hypothesize that the two north atlantic eel species show parallel patterns of selection at the same loci.
RESULTS
genetic diversity values for all genotyped individuals at  <dig> snps are summarized in table  <dig>   <dig> out of  <dig> loci were monomorphic and  <dig> were polymorphic in european eel, although frequency of the most common allele was > <dig>  at  <dig> loci.
diversity indices were higher in american eel  than in european eel , suggestive of a strong ascertainment bias effect due to the fact that snps were identified in american eel.
o
e
o
e
three loci deviated significantly from hardy-weinberg expectations after bonferroni correction: locus ugp_2_ <dig>  showing a deficit of heterozygotes, and loci tent_ <dig> and ugp_a_ <dig>  showing an excess of heterozygotes.
however, those loci were not excluded from the analysis as they could reflect selection.
two loci  were genotyped at all locations except tiber, so all tests for selection were conducted considering all  <dig> locations and  <dig> loci  and on a restricted data set with  <dig> locations  and  <dig> loci.
overall genetic differentiation was low .
using structure, a scenario with two clusters  corresponding to the two species was the most likely, with no substructuring within species .
in the same analysis, a total of  <dig> individuals from iceland were identified as admixed individuals with 90% probability intervals that did not overlap with zero.
while the snps used in this study were not species-diagnostic, results were concordant with the study of pujolar et al.
<cit> , in which the same individuals were identified as admixed on the basis of  <dig> species-diagnostic snps, encompassing four f <dig> hybrids and one second generation backcross.
hybrids were only observed in iceland and were absent in the remaining european locations.
all hybrid individuals were removed from further analyses.
the selection detection workbench lositan identified three outlier loci possibly under diversifying selection using the full data set, gapdh_ <dig>  myh_ <dig> and aldh_2_ <dig>  with p <  <dig>  .
when using the restricted data set with  <dig> locations, a further outlier was also identified, ald_r .
using the complete data set with  <dig> locations bayescan identified a single outlier, gapdh_ <dig>  showing a high fst value of  <dig>  relative to the background fst .
at this locus, the alpha coefficient was positive, suggestive of diversifying selection.
when using the restricted data set with  <dig> locations, ald_r was also identified as outlier in addition to gapdh_ <dig>  showing a high fst value of  <dig>  and a positive alpha coefficient.
st
-outlier approach implemented in lositan and bayescan based on the full data set with  <dig> locations  and a reduced data set with  <dig> locations
st
st
heterozygosity and fst values are detailed for all outlier loci detected in lositan.
bayesian posterior probabilities , q values, alpha coefficients and fst values are detailed for all outlier loci detected in bayescan.
a generalized linear model between allelic frequencies and explanatory variables using the full data set revealed significant associations with temperature , latitude  and longitude  .
one locus  showed a significant association with both latitude and longitude.
no interactions with explanatory variables were found at any locus.
locus ald_r showed a positive association with longitude when using the restricted data set with  <dig> locations .
correlation coefficients and p-values are detailed for all loci showing significant associations using glm.
bayes factors  are presented for all positive associations in bayenv.
on the other hand, one positive association was found using bayenv, locus gapdh_ <dig> and latitude, with a bayes factor of  <dig>  , while no associations were found for the rest of loci .
in addition, a positive association was found between locus ald_r and longitude, with a bayes factor of  <dig> , when considering the reduced data set with  <dig> locations.
overall, we identified a total of  <dig> candidate loci,  <dig> from outlier tests plus  <dig> additional loci from the analysis of association of allelic frequencies with explanatory variables.
discussion
signatures of local selection in european eel
the observation of a small set of snps showing significantly high genetic differentiation in comparison with the background fst and significant associations between allele frequencies and environmental variables is consistent with the action of spatially varying selection associated with the highly heterogenous habitats that glass eels colonize throughout their geographic range.
our findings fit the general prediction that selective pressures frequently vary in space, often resulting in local selection of ecologically adaptive traits  <cit> .
overall, we found signatures of selection at a total of  <dig> loci.
we found discordances between the two approaches used , with few snps identified as targets of selection by both methods and with a higher number of candidates identified using a generalized linear model.
this is in agreement with recent studies showing that snps positively correlated with environmental variables were not fst outliers  <cit> .
it has been suggested that snp-environment associations are more sensitive to detect subtle clines in allele frequencies than fst outlier tests and that both approaches might be complementary but not concordant when testing for selection  <cit> .
nevertheless, three loci in our study  showed higher genetic differentiation than the background fst together with significant associations between allele frequencies and environmental variables.
all three are genes with major metabolic functions: gapdh  is part of the glycolysis pathway and catalyzes the conversion of glyceraldehyde 3-phosphate to d-glycerate  <dig> -bisphosphate; aldh <dig>  belongs to the aldehyde dehydrogenase family of enzymes that catalyze acetaldehyde to acetic acid and is the second enzyme of the major oxidative pathway of alcohol metabolism; ald_r  catalyzes the reduction of glucose to sorbitol, the first step in the polyol pathway of glucose metabolism.
besides these genes, a positive environment correlation was observed for pgk , which is a transferase enzyme in glycolysis acting in the first atp-generating step of the glycolytic pathway.
surprisingly, none of the above genes linked to metabolism showed positive associations with temperature, arguably an environmental variable of key importance influencing enzymatic activities and metabolic pathways  <cit> .
in eels, decreased metabolic activities have been observed below certain threshold temperatures in both european  <cit>  and american eel  <cit> , and distinct behaviour patterns such as upstream migration of glass eels have been shown to be temperature-related  <cit> .
since it could be argued that temperature at other time intervals might be more relevant than the 30 day-interval used in our study, we re-conducted a generalized linear model between allelic frequencies at gapdh, aldh <dig> and ald_r with temperature using other time intervals .
no significant associations were found at any of the locus, which suggests that other agents of selection than temperature could underlie the significant associations found.
in the case of aldose reductase, this is an enzyme induced by hyperosmolarity stress  <cit> .
spatially varying selection in european eel regarding osmoregulation seems plausible, since eels occupy highly variable habitats across europe in terms of salinity, including both fresh and salt-water  habitats  <cit> .
besides genes involved in metabolic functions, several genes involved in defense response showed a positive environment correlation, including trim <dig> , cst , psa <dig>  and ubia <dig> , all involved in catalytic activity.
interestingly, trim <dig> is a gene implicated in processes associated with innate immunity  <cit> .
together with other trim family genes, trim <dig> is located on a region of significantly elevated genetic diversity  in the threespine stickleback, which suggests that the polymorphism increase on lg xiii has been likely driven by selection on innate immunity genes  <cit> .
while allele frequencies at trim <dig> were positively correlated with temperature, allele frequencies at cst, ubia <dig> and psa <dig> were associated with geographic coordinates.
as in the case of metabolism, it is possible that other explanatory variables  may play a role in defense response rather than temperature or geographic coordinates.
no apparent parallel patterns of selection in north atlantic eels
the contrasting pattern of spatially varying selection observed in european eel  and american eel  <cit>  using the same panel of candidate snps suggests no common genetic-by-environment associations between north atlantic eels.
using generalized linear models, gagnaire et al.
<cit>  found significant associations with environmental variables at  <dig> loci within glass eels , none of which are common with the  <dig> loci that showed significant associations in our study using the same statistical approach  and also conducted on glass eels.
however, two loci  showed some evidence of selection in both species.
in american eel, trim <dig> showed the highest fst value detected between localities  although no correlation with environmental variables was detected at this locus.
cst showed signatures of selection within cohorts of juveniles but not within glass eels  <cit> .
while most loci under selection in gagnaire et al.
<cit>  represented metabolic genes associated with temperature, those genes with a major role in metabolism in our study  did not show a positive association with temperature despite the similar temperature ranges encountered by both species .
however, the different signatures of selection between species could be due to distinct selective pressures associated with the much longer larval migration for european eel than for american eel, with estimates ranging from 7 months to 2 years for european eel depending on the assumptions and methods used, whereas estimates for american eel range between  <dig> and 12 months  <cit> .
the one extra year that possibly european eel larvae spend in the open sea could impose a different set of selective agents relative to american eel.
in contrast with our findings, the recent survey of conte et al.
<cit>  on published literature of repeated phenotypic evolution in natural populations concluded that the probability of gene reuse was high .
however, the survey was based on candidate gene studies, which might have biased upward the reuse estimates.
the lack of parallel selection patterns in north atlantic eels is unanticipated owing to the sister species status of european and american eel and the permeable barrier to gene flow between species.
a recent study using a rad-sequencing approach to identify diagnostic markers between the two species found a small proportion of fixed snps , while most of the snps showed low non-significant differentiation that suggest that most of the genome is homogenized by gene flow  <cit> .
one possible explanation to the lack of parallel selection is that complex phenotypic traits affected by local selection might have a highly polygenic basis, hence influenced by several genes, each with a small contribution to the ultimate function  <cit> .
parallel selection is more likely to occur when the adaptive response is controlled by a single gene, i.e.
the eda gene and armor plate reduction  <cit>  and the kit ligand kitlg gene and pigmentation  <cit>  in threespine sticklebacks or the melonacortin- <dig> receptor mc1r and colour pattern in beach mice  <cit> .
more complex traits are likely to involve a higher number of genes, thus reducing the likelihood of selection acting on the same genes in multiple species or locations, as it has been argued in the case of osmoregulation in threespine sticklebacks  <cit> .
similarly, partial parallel patterns of genetic differentiation have been observed between two whitefish sympatric species pairs  across lakes, suggestive of polygenic adaptation  <cit> .
CONCLUSIONS
the distinct signatures of selection between north american eels could be attributable to differences in larval migration between species.
alternatively, the fact that many genes of small effect likely shape adaptive pathways  could explain the private signatures of spatially varying selection with no shared genetic-by-environment associations between european and american eel.
as an alternative to candidate loci approaches, high-density genome-wide scans using next-generation sequencing and genotyping-by-sequencing approaches  <cit>  might be more adequate.
a recent study using rad  sequencing generated a snp resource for european eel consisting of  <dig>  loci and over  <dig>  snps  <cit>  that provides a valuable tool for future studies on parallel selection in both north atlantic eels on a genome-wide scale.

 parallel adaptive changes under replicated environmental conditions have been particularly valuable for understanding evolutionary processes in natural populations. one of the classical questions in evolutionary biology concerns whether different species and populations within species will adapt to the same agent of selection in the same way or whether the response will involve different traits and genes. parallel genotypic adaptation appears to be frequent and occurs at all taxonomic levels from microbes and plants to humans and is likely to result in changes at a relatively small number of genes. for instance, the study of colosimo et al. demonstrated that selection on a single gene, ectodysplasin, is responsible for the parallel reduction of armor plates in freshwater populations of threespine stickleback gasterosteus aculeatus. however, more complex physiological processes relevant in the context of parallel freshwater adaptation of threespine sticklebacks are influenced by several genes, each of small effect. using a survey of the published literature on parallel adaptation of independent lineages of natural populations, conte et al. concluded that divergence at loci under selection is most likely to be based on standing genetic variation derived from a common ancestor rather than mutations occurring de novo after divergence. hence, probability of gene reuse is plausibly higher in closely related species, which are likely to show similar divergence at loci subjected to similar selection pressures. an excellent opportunity to test for genetic parallelism exists in the two north atlantic eel species, the european eel and the american eel. both species are morphologically almost indistinguishable, with the number of vertebrae being regarded as the best diagnostic character between species. divergence time between the two species remains largely unresolved, encompassing between and million years. remarkably, although mitochondrial dna lineages of the two species are reciprocally monophyletic, differentiation at nuclear loci is surprisingly low, suggestive of ongoing gene flow. in this sense, it is well established that the spawning grounds of the two species overlap in the sargasso sea and there is also overlap in spawning time. european and american eels are known to hybridize, with hybrids observed almost exclusively in iceland. hence, the sister species status of european and american eel and the low but biologically significant gene flow makes them an adequate system in which to test the occurrence of selection at homologous loci within each species. north atlantic eels have a catadromous life cycle and after spawning in the sargasso sea, larvae are transported by the gulf stream and other currents to the shores of north america and europe/north africa, respectively. upon reaching the continental shelf, larvae metamorphose into glass eels, which complete the migration into riverine, estuarine and coastal feeding habitats and grow up as yellow eels. after a highly variable feeding stage, yellow eels metamorphose into partially mature silver eels that migrate back to the sargasso undertaking a journey of about  km for the american eel and -  km for the european eel. upon arriving in the sargasso sea, eels reproduce and die. during the continental phase, eels occupy a broad range of habitats from the caribbean to greenland in the western atlantic and from morocco to iceland in the eastern atlantic. the presence of eels in extremely heterogenous environments in terms of temperature, salinity, substrate, depth or productivity along their geographic distribution makes them ideal species in which to study the consequences of spatially varying selective pressures that often result in local adaptation of ecologically important traits. beginning with levene, who introduced the first theoretical model for examining the impact of diversifying selection in space, a number of studies have shown that balancing selection due to spatial heterogeneity is an important mechanism responsible for the maintenance of genetic polymorphism. genetic variation in a spatially heterogenous environment may be maintained even when dispersal results in complete mixing of the gene pool. however, under such a panmixia scenario, in which offspring are distributed to environments at random independently of the environment experienced by the parents, local selection cannot to lead to local adaptation. in the case of eels, owing to panmixia in both european and american eel and random larval dispersal across habitats, heritable trans-generational local adaptation is not possible although single-generation footprints of selection can still be detected. in this sense, significant geographic clines at allozyme loci have been detected in both european and american eel. in the most comprehensive study to date, gagnaire et al. found evidence for spatially varying selection at coding genes in american eel showing significant correlations between allele frequencies and environmental variables across the entire species range. in this study, we tested for single-generation signatures of spatial varying selection in european eel and compared the results to those obtained by gagnaire et al.. we genotyped glass eels from sampling locations across the geographic distribution of the species, using the same set of snps analyzed by gagnaire et al. in american eel. we used two main analytical approaches, one that identifies outliers as those markers with greater differentiation among all snps and a second based on determining positive associations between allele frequencies and environmental factors. following the positive associations observed by gagnaire et al. in american eel, variables used in our study were degrees north latitude, degrees east/west longitude and sea-surface temperature at river mouth. we specifically wanted to test whether the same genes were under spatially varying selection in both european and american eel, hence providing evidence for parallel patterns of local selection, or whether the response involved a different set of genes. considering their sister species status and the existence of gene flow between species, together with the similar environmental conditions they encounter, we hypothesize that the two north atlantic eel species show parallel patterns of selection at the same loci.  genetic diversity values for all genotyped individuals at snps are summarized in table  out of loci were monomorphic and were polymorphic in european eel, although frequency of the most common allele was > at loci. diversity indices were higher in american eel than in european eel, suggestive of a strong ascertainment bias effect due to the fact that snps were identified in american eel. o e o e three loci deviated significantly from hardy-weinberg expectations after bonferroni correction: locus ugp_2_ showing a deficit of heterozygotes, and loci tent_ and ugp_a_ showing an excess of heterozygotes. however, those loci were not excluded from the analysis as they could reflect selection. two loci were genotyped at all locations except tiber, so all tests for selection were conducted considering all locations and loci and on a restricted data set with locations and loci. overall genetic differentiation was low. using structure, a scenario with two clusters corresponding to the two species was the most likely, with no substructuring within species. in the same analysis, a total of individuals from iceland were identified as admixed individuals with 90% probability intervals that did not overlap with zero. while the snps used in this study were not species-diagnostic, results were concordant with the study of pujolar et al., in which the same individuals were identified as admixed on the basis of species-diagnostic snps, encompassing four f hybrids and one second generation backcross. hybrids were only observed in iceland and were absent in the remaining european locations. all hybrid individuals were removed from further analyses. the selection detection workbench lositan identified three outlier loci possibly under diversifying selection using the full data set, gapdh_ myh_ and aldh_2_ with p < . when using the restricted data set with locations, a further outlier was also identified, ald_r. using the complete data set with locations bayescan identified a single outlier, gapdh_ showing a high fst value of relative to the background fst. at this locus, the alpha coefficient was positive, suggestive of diversifying selection. when using the restricted data set with locations, ald_r was also identified as outlier in addition to gapdh_ showing a high fst value of and a positive alpha coefficient. st -outlier approach implemented in lositan and bayescan based on the full data set with locations and a reduced data set with locations st st heterozygosity and fst values are detailed for all outlier loci detected in lositan. bayesian posterior probabilities, q values, alpha coefficients and fst values are detailed for all outlier loci detected in bayescan. a generalized linear model between allelic frequencies and explanatory variables using the full data set revealed significant associations with temperature, latitude and longitude. one locus showed a significant association with both latitude and longitude. no interactions with explanatory variables were found at any locus. locus ald_r showed a positive association with longitude when using the restricted data set with locations. correlation coefficients and p-values are detailed for all loci showing significant associations using glm. bayes factors are presented for all positive associations in bayenv. on the other hand, one positive association was found using bayenv, locus gapdh_ and latitude, with a bayes factor of, while no associations were found for the rest of loci. in addition, a positive association was found between locus ald_r and longitude, with a bayes factor of, when considering the reduced data set with locations. overall, we identified a total of candidate loci, from outlier tests plus additional loci from the analysis of association of allelic frequencies with explanatory variables. discussion signatures of local selection in european eel the observation of a small set of snps showing significantly high genetic differentiation in comparison with the background fst and significant associations between allele frequencies and environmental variables is consistent with the action of spatially varying selection associated with the highly heterogenous habitats that glass eels colonize throughout their geographic range. our findings fit the general prediction that selective pressures frequently vary in space, often resulting in local selection of ecologically adaptive traits. overall, we found signatures of selection at a total of loci. we found discordances between the two approaches used, with few snps identified as targets of selection by both methods and with a higher number of candidates identified using a generalized linear model. this is in agreement with recent studies showing that snps positively correlated with environmental variables were not fst outliers. it has been suggested that snp-environment associations are more sensitive to detect subtle clines in allele frequencies than fst outlier tests and that both approaches might be complementary but not concordant when testing for selection. nevertheless, three loci in our study showed higher genetic differentiation than the background fst together with significant associations between allele frequencies and environmental variables. all three are genes with major metabolic functions: gapdh is part of the glycolysis pathway and catalyzes the conversion of glyceraldehyde 3-phosphate to d-glycerate -bisphosphate; aldh belongs to the aldehyde dehydrogenase family of enzymes that catalyze acetaldehyde to acetic acid and is the second enzyme of the major oxidative pathway of alcohol metabolism; ald_r catalyzes the reduction of glucose to sorbitol, the first step in the polyol pathway of glucose metabolism. besides these genes, a positive environment correlation was observed for pgk, which is a transferase enzyme in glycolysis acting in the first atp-generating step of the glycolytic pathway. surprisingly, none of the above genes linked to metabolism showed positive associations with temperature, arguably an environmental variable of key importance influencing enzymatic activities and metabolic pathways. in eels, decreased metabolic activities have been observed below certain threshold temperatures in both european and american eel, and distinct behaviour patterns such as upstream migration of glass eels have been shown to be temperature-related. since it could be argued that temperature at other time intervals might be more relevant than the 30 day-interval used in our study, we re-conducted a generalized linear model between allelic frequencies at gapdh, aldh and ald_r with temperature using other time intervals. no significant associations were found at any of the locus, which suggests that other agents of selection than temperature could underlie the significant associations found. in the case of aldose reductase, this is an enzyme induced by hyperosmolarity stress. spatially varying selection in european eel regarding osmoregulation seems plausible, since eels occupy highly variable habitats across europe in terms of salinity, including both fresh and salt-water habitats. besides genes involved in metabolic functions, several genes involved in defense response showed a positive environment correlation, including trim, cst, psa and ubia, all involved in catalytic activity. interestingly, trim is a gene implicated in processes associated with innate immunity. together with other trim family genes, trim is located on a region of significantly elevated genetic diversity in the threespine stickleback, which suggests that the polymorphism increase on lg xiii has been likely driven by selection on innate immunity genes. while allele frequencies at trim were positively correlated with temperature, allele frequencies at cst, ubia and psa were associated with geographic coordinates. as in the case of metabolism, it is possible that other explanatory variables may play a role in defense response rather than temperature or geographic coordinates. no apparent parallel patterns of selection in north atlantic eels the contrasting pattern of spatially varying selection observed in european eel and american eel using the same panel of candidate snps suggests no common genetic-by-environment associations between north atlantic eels. using generalized linear models, gagnaire et al. found significant associations with environmental variables at loci within glass eels, none of which are common with the loci that showed significant associations in our study using the same statistical approach and also conducted on glass eels. however, two loci showed some evidence of selection in both species. in american eel, trim showed the highest fst value detected between localities although no correlation with environmental variables was detected at this locus. cst showed signatures of selection within cohorts of juveniles but not within glass eels. while most loci under selection in gagnaire et al. represented metabolic genes associated with temperature, those genes with a major role in metabolism in our study did not show a positive association with temperature despite the similar temperature ranges encountered by both species. however, the different signatures of selection between species could be due to distinct selective pressures associated with the much longer larval migration for european eel than for american eel, with estimates ranging from 7 months to 2 years for european eel depending on the assumptions and methods used, whereas estimates for american eel range between and 12 months. the one extra year that possibly european eel larvae spend in the open sea could impose a different set of selective agents relative to american eel. in contrast with our findings, the recent survey of conte et al. on published literature of repeated phenotypic evolution in natural populations concluded that the probability of gene reuse was high. however, the survey was based on candidate gene studies, which might have biased upward the reuse estimates. the lack of parallel selection patterns in north atlantic eels is unanticipated owing to the sister species status of european and american eel and the permeable barrier to gene flow between species. a recent study using a rad-sequencing approach to identify diagnostic markers between the two species found a small proportion of fixed snps, while most of the snps showed low non-significant differentiation that suggest that most of the genome is homogenized by gene flow. one possible explanation to the lack of parallel selection is that complex phenotypic traits affected by local selection might have a highly polygenic basis, hence influenced by several genes, each with a small contribution to the ultimate function. parallel selection is more likely to occur when the adaptive response is controlled by a single gene, i.e. the eda gene and armor plate reduction and the kit ligand kitlg gene and pigmentation in threespine sticklebacks or the melonacortin- receptor mc1r and colour pattern in beach mice. more complex traits are likely to involve a higher number of genes, thus reducing the likelihood of selection acting on the same genes in multiple species or locations, as it has been argued in the case of osmoregulation in threespine sticklebacks. similarly, partial parallel patterns of genetic differentiation have been observed between two whitefish sympatric species pairs across lakes, suggestive of polygenic adaptation.  the distinct signatures of selection between north american eels could be attributable to differences in larval migration between species. alternatively, the fact that many genes of small effect likely shape adaptive pathways could explain the private signatures of spatially varying selection with no shared genetic-by-environment associations between european and american eel. as an alternative to candidate loci approaches, high-density genome-wide scans using next-generation sequencing and genotyping-by-sequencing approaches might be more adequate. a recent study using rad sequencing generated a snp resource for european eel consisting of loci and over snps that provides a valuable tool for future studies on parallel selection in both north atlantic eels on a genome-wide scale. 
we tested for single-generation signatures of spatially varying selection in european eel by searching for elevated genetic differentiation using fst-based outlier tests and by testing for significant associations between allele frequencies and environmental variables. the two north atlantic eel species, the european and the american eel, represent an ideal system in which to study parallel selection patterns due to their sister species status and the presence of ongoing gene flow. candidate genes for local selection constituted mainly genes with a major role in metabolism as well as defense genes. the different signatures of selection between species could be due to distinct selective pressures associated with the much longer larval migration for european eel relative to american eel. contrary to what has been found for american eel, only snps in our study correlated with differences in temperature, which suggests that other explanatory variables may play a role. none of the genes found to be associated with explanatory variables in european eel showed any correlations with environmental factors in the previous study in american eel. a panel of coding-gene snps previously analyzed in american eel was used to genotype european eel individuals from sampling locations across the species distribution. adaptationeuropean eelgenetic-by-environment associationsparallel selectionsingle nucleotide polymorphisms the lack of parallel selection in north atlantic eels could also be due to most phenotypic traits being polygenic, thus reducing the likelihood of selection acting on the same genes in both species.
 the two north atlantic eel species, the european and the american eel, represent an ideal system in which to study parallel selection patterns due to their sister species status and the presence of ongoing gene flow. a panel of coding-gene snps previously analyzed in american eel was used to genotype european eel individuals from sampling locations across the species distribution. we tested for single-generation signatures of spatially varying selection in european eel by searching for elevated genetic differentiation using fst-based outlier tests and by testing for significant associations between allele frequencies and environmental variables.  we found signatures of possible selection at a total of coding-gene snps. candidate genes for local selection constituted mainly genes with a major role in metabolism as well as defense genes. contrary to what has been found for american eel, only snps in our study correlated with differences in temperature, which suggests that other explanatory variables may play a role. none of the genes found to be associated with explanatory variables in european eel showed any correlations with environmental factors in the previous study in american eel.  the different signatures of selection between species could be due to distinct selective pressures associated with the much longer larval migration for european eel relative to american eel. the lack of parallel selection in north atlantic eels could also be due to most phenotypic traits being polygenic, thus reducing the likelihood of selection acting on the same genes in both species. adaptationeuropean eelgenetic-by-environment associationsparallel selectionsingle nucleotide polymorphisms 