Anonymous Microsatellite Research Articles

Towards a Genetic Linkage Map of the California Condor, an Endangered New World Vulture Species.

The development of a linkage map is an important component for promoting genetic and genomic studies in California condors, an endangered New World vulture species. Using a set of designed anonymous microsatellite markers, we genotyped a reference condor population involving 121 individuals. After marker validation and genotype filtering, the genetic linkage analysis was performed using 123 microsatellite loci. This resulted in the identification of 15 linkage groups/subgroups that formed a first-generation condor genetic map, while no markers linked to a lethal chondrodystrophy mutation were found. A panel of polymorphic markers that is instrumental in molecular parentage diagnostics and other genetic studies in the California condor was selected. Further condor conservation genomics research will be focused on updating the linkage map and integrating it with cytogenetic and BAC-based physical maps and ultimately with the genome sequence assembly.

A comparative study of population genetic structure reveals patterns consistent with selection at functional microsatellites in common sunflower.

Microsatellites, also known as short tandem repeats (STRs), have long been considered non-functional, neutrally evolving regions of the genome. Recent findings suggest that they can function as drivers of rapid adaptive evolution. Previous work on the common sunflower identified 479 transcribed microsatellites where allele length significantly correlates with gene expression (eSTRs) in a stepwise manner. Here, a population genetic approach is used to test whether eSTR allele length variation is under selection. Genotypic variation among and within populations at 13 eSTRs was compared with that at 19 anonymous microsatellites in 672 individuals from 17 natural populations of sunflower from across a cline running from Saskatchewan to Oklahoma (distance of approximately 1600km). Expected heterozygosity, allelic richness, and allelic diversity were significantly lower at eSTRs, a pattern consistent with higher relative rates of purifying selection. Further, an analysis of variation in microsatellite allele lengths (lnRV), and heterozygosities (lnRH), indicate recent selective sweeps at the eSTRs. Mean microsatellite allele lengths at four eSTRs within populations are significantly correlated with latitude consistent with the predictions of the tuning-knob model which predicts stepwise relationships between microsatellite allele length and phenotypes. This finding suggests that shorter or longer alleles at eSTRs may be favored in climatic extremes. Collectively, our results imply that eSTRs are likely under selection and that they may be playing a role in facilitating local adaptation across a well-defined cline in the common sunflower.

Development of microsatellite markers using next-generation sequencing for the fish Colossoma macropomum.

Tambaqui (Colossoma macropomum) is a fish species from the Amazon and Orinoco Rivers, with favorable characteristics to the cultivation system and great market acceptance in South America. However, the construction of a genetic map for the genetic improvement of this species is limited by the low number of molecular markers currently described. Thus, this study aimed to validate gene-associated and anonymous (non-genic) microsatellites obtained by next generation sequencing (RNA-seq and whole genome shotgun-WGS, respectively), for future construction of a genetic map and search for quantitative trait loci (QTL) in this species. In the RNA-seq data, the observed and expected heterozygosity (Ho and He) ranged from 0.09 to 0.73, and 0.09 to 0.85, respectively. In the WGS data, Ho and He ranged from 0.33 to 0.95, and 0.28 to 0.92, respectively. In general, the evaluation of 200 markers resulted in 45 polymorphic loci, of which 14 were gene-associated (RNA-Seq) and 31 were anonymous (WGS). Moreover, some markers were related to genes of the immune system, biological regulation/control and biogenesis. This study contributes to increase the number of molecular markers available for genetic studies in C. macropomum, which will allow the development of breeding programs assisted by molecular markers.

Characterisation and transferability of transcriptomic microsatellite markers for Nothofagus species

ABSTRACTDiscriminant molecular markers are required for research on population genetics, as well as evolutionary studies involving identification of hybrids and parental species, or detection of the genome regions under selection. We provide a set of 27 transcriptomic microsatellite markers (SSRs) for South American Nothofagus species, derived from 73 Nothofagus alpina (=N. nervosa) annotated unigenes. Rates of cross-amplification ranged from 22% to 37%. Genetic characterisation of 22 transcriptomic SSRs for N. alpina and N. obliqua reveals low genetic variability, due to the general occurrence of one major allele at each locus, and high specificity, with few alleles shared between species (14%). At inter-species level 95% of loci were discriminant, with a total G''st over loci of 0.9, indicating that alleles were mostly fixed for all loci in both species. At intra-species level the number of markers with significant differentiation was 2.5 times higher for N. obliqua than for N. alpina populations. Moreover, transcriptomic SSRs showed higher performance compared with published anonymous microsatellites isolated from genome sequences without annotation. This set of transcriptomic microsatellites will be useful to the scientific community working on conservation and evolutionary aspects of Nothofagus species.

Subtle genetic population structure in Pacific halibut Hippoglossus stenolepis.

Pacific halibut Hippoglossus stenolepis from 10 sampling locations throughout their range were investigated for signs of population structure. Two genetic data sets were created: (1) all individuals (n = 828) at few anonymous microsatellite markers (number of loci = 16); (2) fewer individuals (n = 435) genotyped at anonymous as well as expressed sequence-tag linked microsatellites (number of loci = 61). A combination of multidimensional scaling plots, discriminant analysis of principal components and pairwise differentiation estimates suggested that samples from the Aleutian Islands, particularly the western Aleutian Islands, were genetically distinct from samples collected in other regions. In addition, outlier analyses found that two markers linked to expressed sequence tags may be under directional selection and could explain the differentiation among samples. These results confirm findings from previous research and suggest that population structure may exist within a current management unit (i.e. International Pacific Halibut Commission Regulatory Area 4B).

Genetic barcoding facilitates captive and wild management of three cryptic African crocodile species complexes

AbstractManagement of threatened species with conserved morphology is hindered by misidentification and potential hybridization in captivity. African crocodiles of the genera Osteolaemus, Crocodylus and Mecistops were previously considered monotypic, but have recently been identified as consisting of three, two and two cryptic species, respectively. Mitochondrial cytochrome c oxidase subunit I (COI) new and published sequences from 310 wild‐caught specimens were compared with 71 US zoological specimens to identify (barcode) species. We also sequenced either nuclear lactate dehydrogenase A or an anonymous microsatellite flanking sequence in both wild and captive specimens to identify potential first‐generation intra‐generic hybridization in captive populations. The Osteolaemus COI barcoding gap was 7–9% sequence divergence. In contrast, the two African Crocodylus species had a 4.7% barcoding gap. These results, in addition to a previously published Mecistops barcoding gap (4.9%), support the use of barcoding as a tool for diagnosing African crocodile species in captive populations. Captive individuals from 14 collections in the US represented five of the seven putative African taxa. Nuclear heterozygosity at fixed polymorphisms differentiating species within genera suggested that 23% of the captive Osteolaemus samples were admixed O. tetraspis × O. sp. nov. cf. tetraspis – West Africa, while no ex situ hybrids were found for Crocodylus or Mecistops. Our results underscore the need to formally recognize divergent crocodile taxa, especially in light of regional threats, and for proper captive breeding management.

The genome sequence of a widespread apex predator, the golden eagle (Aquila chrysaetos).

Biologists routinely use molecular markers to identify conservation units, to quantify genetic connectivity, to estimate population sizes, and to identify targets of selection. Many imperiled eagle populations require such efforts and would benefit from enhanced genomic resources. We sequenced, assembled, and annotated the first eagle genome using DNA from a male golden eagle (Aquila chrysaetos) captured in western North America. We constructed genomic libraries that were sequenced using Illumina technology and assembled the high-quality data to a depth of ∼40x coverage. The genome assembly includes 2,552 scaffolds >10 Kb and 415 scaffolds >1.2 Mb. We annotated 16,571 genes that are involved in myriad biological processes, including such disparate traits as beak formation and color vision. We also identified repetitive regions spanning 92 Mb (∼6% of the assembly), including LINES, SINES, LTR-RTs and DNA transposons. The mitochondrial genome encompasses 17,332 bp and is ∼91% identical to the Mountain Hawk-Eagle (Nisaetus nipalensis). Finally, the data reveal that several anonymous microsatellites commonly used for population studies are embedded within protein-coding genes and thus may not have evolved in a neutral fashion. Because the genome sequence includes ∼800,000 novel polymorphisms, markers can now be chosen based on their proximity to functional genes involved in migration, carnivory, and other biological processes.

Stronger transferability but lower variability in transcriptomic‐ than in anonymous microsatellites: evidence from Hylid frogs

A simple way to quickly optimize microsatellites in nonmodel organisms is to reuse loci available in closely related taxa; however, this approach can be limited by the stochastic and low cross-amplification success experienced in some groups (e.g. amphibians). An efficient alternative is to develop loci from transcriptome sequences. Transcriptomic microsatellites have been found to vary in their levels of cross-species amplification and variability, but this has to date never been tested in amphibians. Here, we compare the patterns of cross-amplification and levels of polymorphism of 18 published anonymous microsatellites isolated from genomic DNA vs. 17 loci derived from a transcriptome, across nine species of tree frogs (Hyla arborea and Hyla cinerea group). We established a clear negative relationship between divergence time and amplification success, which was much steeper for anonymous than transcriptomic markers, with half-lives (time at which 50% of the markers still amplify) of 1.1 and 37 My, respectively. Transcriptomic markers are significantly less polymorphic than anonymous loci, but remain variable across diverged taxa. We conclude that the exploitation of amphibian transcriptomes for developing microsatellites seems an optimal approach for multispecies surveys (e.g. analyses of hybrid zones, comparative linkage mapping), whereas anonymous microsatellites may be more informative for fine-scale analyses of intraspecific variation. Moreover, our results confirm the pattern that microsatellite cross-amplification is greatly variable among amphibians and should be assessed independently within target lineages. Finally, we provide a bank of microsatellites for Palaearctic tree frogs (so far only available for H. arborea), which will be useful for conservation and evolutionary studies in this radiation.

Lack of parallel genetic patterns underlying the repeated ecological divergence of beach and stream‐spawning kokanee salmon

Recent progress in methods for detecting adaptive population divergence in situ shows promise for elucidating the conditions under which selection acts to generate intraspecific diversity. Rapid ecological diversification is common in fishes; however, the role of phenotypic plasticity and adaptation to local environments is poorly understood. It is now possible to investigate genetic patterns to make inferences regarding phenotypic traits under selection and possible mechanisms underlying ecotype divergence, particularly where similar novel phenotypes have arisen in multiple independent populations. Here, we employed a bottom-up approach to test for signatures of directional selection associated with divergence of beach- and stream-spawning kokanee, the obligate freshwater form of sockeye salmon (Oncorhynchus nerka). Beach- and stream-spawners co-exist in many post-glacial lakes and exhibit distinct reproductive behaviours, life-history traits and spawning habitat preferences. Replicate ecotype pairs across five lakes in British Columbia, Canada were genotyped at 57 expressed sequence tag-linked and anonymous microsatellite loci identified in a previous genome scan. Fifteen loci exhibited signatures of directional selection (high FST outliers), four of which were identified in multiple lakes. However, the lack of parallel genetic patterns across all lakes may be a result of: 1) an inability to detect loci truly under selection; 2) alternative genetic pathways underlying ecotype divergence in this system; and/or 3) phenotypic plasticity playing a formative role in driving kokanee spawning habitat differences. Gene annotations for detected outliers suggest pathogen resistance and energy metabolism as potential mechanisms contributing to the divergence of beach- and stream-spawning kokanee, but further study is required.

Informative microsatellites for freshwater and marine shrimp species

Marine and freshwater shrimp are overexploited worldwide due to their high commercial value being usually fished in a predatory manner. The concern about conserving the stocks for economical or ecological reasons has been threatened by the absence of molecular markers for the majority of these species. In this work, we described ten anonymous microsatellites isolated from an enriched-library with magnetic beads technology of Penaeus (Litopenaeus) vannamei genome, considered the most economically important marine shrimp in the world. In addition, we tested these loci and a panel of 21 Simple Sequence Repeat from Expressed Sequence Tag (EST-SSR) loci, also isolated from P. vannamei, in ten other marine and two freshwater shrimp species. The cross species amplification success rate ranged from 32 to 77% in Macrobrachium jelskii and Penaeus (Litopenaeus) schmitti, respectively. These microsatellites appear conservative in a wide spectrum of shrimp groups, facilitating genetic studies in those species in which no microsatellites have been yet described. Key words: Conservative, Simple Sequence Repeat (SSR), shrimp genetic, penaeid, Macrobrachium.

Molecular signatures of lineage‐specific adaptive evolution in a unique sea basin: the example of an anadromous goby Leucopsarion petersii

Climate changes on various time scales often shape genetic novelty and adaptive variation in many biotas. We explored molecular signatures of directional selection in populations of the ice goby Leucopsarion petersii inhabiting a unique sea basin, the Sea of Japan, where a wide variety of environments existed in the Pleistocene in relation to shifts in sea level by repeated glaciations. This species consisted of two historically allopatric lineages, the Japan Sea (JS) and Pacific Ocean (PO) lineages, and these have lived under contrasting marine environments that are expected to have imposed different selection regimes caused by past climatic and current oceanographic factors. We applied a limited genome-scan approach using seven candidate genes for phenotypic differences between two lineages in combination with 100 anonymous microsatellite loci. Neuropeptide Y (NPY) gene, which is an important regulator of food intake and potent orexigenic agent, and three anonymous microsatellites were identified as robust outliers, that is, candidate loci potentially under directional selection, by multiple divergence- and diversity-based outlier tests in comparisons focused on multiple populations of the JS vs. PO lineages. For these outlier loci, populations of the JS lineage had putative signals of selective sweeps. Additionally, real-time quantitative PCR analysis using fish reared in a common environment showed a higher expression level for NPY gene in the JS lineage. Thus, this study succeeded in identifying candidate genomic regions under selection across populations of the JS lineage and provided evidence for lineage-specific adaptive evolution in this unique sea basin.

Microsatellite length variation in candidate genes correlates with habitat in the gilthead sea bream Sparus aurata

The genetic basis and evolutionary implications of local adaptation in high gene flow marine organisms are still poorly understood. In several Mediterranean fish species, alternative migration patterns exist between individuals entering coastal lagoons that offer favourable conditions for growth and those staying in the sea where environmental conditions are less subject to rapid and stressful change. Whether these coexisting strategies are phenotypically plastic or include a role for local adaptation through differential survival needs to be determined. Here, we explore the genetic basis of alternate habitat use in western Mediterranean populations of the gilthead sea bream (Sparus aurata). Samples from lagoonal and open-sea habitats were typed for three candidate gene microsatellite loci, seven anonymous microsatellites and 44 amplified fragment length polymorphism markers to test for genotype-environment associations. While anonymous markers globally indicated high levels of gene flow across geographic locations and habitats, non-neutral differentiation patterns correlated with habitat type were found at two candidate microsatellite loci located in the promoter region of the growth hormone and prolactin genes. Further analysis of these two genes revealed that a mechanism based on habitat choice alone could not explain the distribution of genotype frequencies at a regional scale, thus implying a role for differential survival between habitats. We also found an association between allele size and habitat type, which, in the light of previous studies, suggests that polymorphisms in the proximal promoter region could influence gene expression by modulating transcription factor binding, thus providing a potential explanatory link between genotype and growth phenotype in nature.

Second-Generation Genetic Linkage Map of Catfish and Its Integration with the BAC-Based Physical Map

Construction of high-density genetic linkage maps is crucially important for quantitative trait loci (QTL) studies, and they are more useful when integrated with physical maps. Such integrated maps are valuable genome resources for fine mapping of QTL, comparative genomics, and accurate and efficient whole-genome assembly. Previously, we established both linkage maps and a physical map for channel catfish, Ictalurus punctatus, the dominant aquaculture species in the United States. Here we added 2030 BAC end sequence (BES)-derived microsatellites from 1481 physical map contigs, as well as markers from singleton BES, ESTs, anonymous microsatellites, and SNPs, to construct a second-generation linkage map. Average marker density across the 29 linkage groups reached 1.4 cM/marker. The increased marker density highlighted variations in recombination rates within and among catfish chromosomes. This work effectively anchored 44.8% of the catfish BAC physical map contigs, covering ∼52.8% of the genome. The genome size was estimated to be 2546 cM on the linkage map, and the calculated physical distance per centimorgan was 393 Kb. This integrated map should enable comparative studies with teleost model species as well as provide a framework for ordering and assembling whole-genome scaffolds.

Genetic Evidence for the Uncoupling of Local Aquaculture Activities and a Population of an Invasive Species—A Case Study of Pacific Oysters (Crassostrea gigas)

Human-mediated introduction of nonnative species into coastal areas via aquaculture is one of the main pathways that can lead to biological invasions. To develop strategies to counteract invasions, it is critical to determine whether populations establishing in the wild are self-sustaining or based on repeated introductions. Invasions by the Pacific oyster (Crassostrea gigas) have been associated with the growing oyster aquaculture industry worldwide. In this study, temporal genetic variability of farmed and wild oysters from the largest enclosed bay in Ireland was assessed to reconstruct the recent biological history of the feral populations using 7 anonymous microsatellites and 7 microsatellites linked to expressed sequence tags (ESTs). There was no evidence of EST-linked markers showing footprints of selection. Allelic richness was higher in feral than in aquaculture samples (P = 0.003, paired t-test). Significant deviations from Hardy-Weinberg equilibrium due to heterozygote deficiencies were detected for almost all loci and samples, most likely explained by the presence of null alleles. Relatively high genetic differentiation was found between aquaculture and feral oysters (largest pairwise multilocus F(ST) 0.074, P < 0.01) and between year classes of oysters from aquaculture (largest pairwise multilocus F(ST) 0.073, P < 0.01), which was also confirmed by the strong separation of aquaculture and wild samples using Bayesian clustering approaches. A 10-fold higher effective population size (N(e)) and a high number of private alleles in wild oysters suggest an established self-sustaining feral population. The wild oyster population studied appears demographically independent from the current aquaculture activities in the estuary and alternative scenarios of introduction pathways are discussed.

A Microsatellite Genome Screen Identifies Chromosomal Regions under Differential Selection in Steelhead and Rainbow Trout

AbstractNovel genetic resources now make it possible to directly assess the effects of natural selection on specific regions of the salmonid genome. Natural selection on specific genetic loci will also affect the variation in linked flanking regions through “hitchhiking” effects, causing greater differentiation between populations than would be expected from purely neutral processes. Here we identify candidate genomic regions for the effects of differential natural selection in two closely related populations of steelhead–rainbow trout Oncorhynchus mykiss separated for the last century following anthropogenic introduction above a barrier waterfall in a coastal California stream. A set of 110 expressed sequence tag (EST)‐linked and 188 anonymous microsatellite loci, most of which are mapped, were chosen to provide distributed coverage of all linkage groups in the O. mykiss genome. A genome screen was then performed to identify genomic regions under divergent selection. Multiple complementary statistical methods were used to detect outlier loci, and loci identified by more than one method were considered strong candidates for genomic regions affected by recent natural selection and genomic adaptation to the environment above the waterfall. We identified six strongly supported outlier loci, including one linked to an EST and two located in a genomic region that contains quantitative trait loci associated with egg development, spawning time, and other life history variation. Identification of regions in the genome that are potentially under selection provides valuable information about the genetic basis of selection in novel habitats and the genetic architecture of salmonid life history variation.

Heterozygosity‐fitness correlations of conserved microsatellite markers in Kentish plovers Charadrius alexandrinus

Heterozygosity-fitness correlations (HFCs) are frequently used to examine the relationship between genetic diversity and fitness. Most studies have reported positive HFCs, although there is a strong bias towards investigating HFCs in genetically impoverished populations. We investigated HFCs in a large genetically diverse breeding population of Kentish plovers Charadrius alexandrinus in southern Turkey. This small shorebird exhibits highly variable mating and care systems, and it is becoming an ecological model species to understand breeding system evolution. Using 11 conserved and six anonymous microsatellite markers, we tested whether and how heterozygosity was associated with chick survival, tarsus and body mass growth controlling for nongenetic effects (chick sex, hatching date, length of biparental care and site quality) that influence survival and growth. There was no genome-wide effect of heterozygosity on fitness, and we did not find any significant effects of heterozygosity on growth rates. However, two of the 11 conserved markers displayed an association with offspring survival: one marker showed a positive HFC, whereas the other marker showed a negative HFC. Heterozygosity at three further conserved loci showed significant interaction with nongenetic variables. In contrast, heterozygosity based on anonymous microsatellite loci was not associated with fitness or growth. Markers that were correlated with chick survival were not more likely to be located in exons or introns than other markers that lacked this association.

Localization of genes for V+LDL plasma cholesterol levels on two diets in the opossum Monodelphis domestica

Plasma cholesterol levels among individuals vary considerably in response to diet. However, the genes that influence this response are largely unknown. Non-HDL (V+LDL) cholesterol levels vary dramatically among gray, short-tailed opossums fed an atherogenic diet, and we previously reported that two quantitative trait loci (QTLs) influenced V+LDL cholesterol on two diets. We used hypothesis-free, genome-wide linkage analyses on data from 325 pedigreed opossums and located one QTL for V+LDL cholesterol on the basal diet on opossum chromosome 1q [logarithm of the odds (LOD) = 3.11, genomic P = 0.019] and another QTL for V+LDL on the atherogenic diet (i.e., high levels of cholesterol and fat) on chromosome 8 (LOD = 9.88, genomic P = 5 x 10(-9)). We then employed a novel strategy involving combined analyses of genomic resources, expression analysis, sequencing, and genotyping to identify candidate genes for the chromosome 8 QTL. A polymorphism in ABCB4 was strongly associated (P = 9 x 10(-14)) with the plasma V+LDL cholesterol concentrations on the high-cholesterol, high-fat diet. The results of this study indicate that genetic variation in ABCB4, or closely linked genes, is responsible for the dramatic differences among opossums in their V+LDL cholesterol response to an atherogenic diet.

Microsatellite-centromere mapping in walking catfish Clarias macrocephalus (Günther, 1864) using gynogenetic diploids

Thirty new microsatellite loci were isolated from a microsatellite-enriched genomic library of walking catfish Clarias macrocephalus. The CA motif was the most abundant, although several other motifs were also isolated. Two gynogenetic diploid families, each consisting of a female and 50 offspring, were genotyped at 56 microsatellite loci, including 30 loci developed in the present study and 26 loci reported in the previous study. Overall, 33 anonymous microsatellites derived from genomic library and 11 EST-linked microsatellites were mapped to their centromeres. High levels of chiasma interference were apparent in the walking catfish genome as indicated by an average frequency of second-division segregation (y) of 0.643 ± 0.248. Twenty-six loci (59%) showed a high microsatellite–centromere recombination, with a frequency >0.67, and three loci had recombination frequencies >0.9. This study demonstrated that gene–centromere mapping provided a rapid method for the expansion of the initial linkage map of walking catfish.

Genomic sequences and genetic differentiation at associated tandem repeat markers in growth hormone, somatolactin and insulin-like growth factor-1 genes of the sea bass,Dicentrarchus labrax

The completion of genomic sequences of physiologically important genes frequently reveals non-coding genetic elements such as tandem repeats (micro- and minisatellites) that are often more polymorphic than nearby coding sequences. We obtained the complete genomic sequences of three hormone genes in sea bass Dicentrarchus labrax : growth hormone (dlGH ), somatolactin (dlSL ) and insulin-like growth factor-1 (dlIGF-1 ), including 5′- and 3′-untranslated regions. Mini- and microsatellites were discovered in both flanking and intron regions. Some were partially conserved across Perciformes. To assess the usefulness and relevance of these gene-associated markers for understanding population structure, an investigation was made on genetic diversity and differentiation at four of them in (i ) five wild populations from the North Sea, the Bay of Biscay and the Western Mediterranean, and (ii ) two samples of hatchery-bred individuals from a freshwater-acclimation experiment. Gene and allelic diversities were lower in cultured individuals than in wild ones. Significant genetic differentiation was demonstrated between Bay of Biscay + North Sea and Mediterranean populations (F st > 0.06, p -associated markers. Significant genetic differentiation was also detected among the Atlantic and North Sea samples, but restricted to the locus associated with dlSL . Significant genetic differentiation was also found among experimental individuals before and after a salinity challenge (F st ≈ 0.05, p and dlIGF-1 loci. Gene-associated markers proved to be more efficient than formerly used anonymous microsatellite markers in providing a clear picture of genetic differentiation.

Selection footprint at the first intron of the Prl gene in natural populations of the flathead mullet ( Mugil cephalus, L. 1758)

The knowledge about mechanisms of adaptive evolution in highly dispersive species such as marine fishes largely improved over the past years. However, studies reporting consistent phenotype–genotype relationship that aim to understand eco-physiological performance of fish in natural environments still concerned only few species and few candidate genes. In order to understand how salinity may shape genetic variation in fish, we analysed six anonymous microsatellite loci and the first intron of the candidate prolactin-A ( Prl) gene in environmentally-contrasted populations of the euryhaline flathead mullet ( Mugil cephalus; Mugilidae). Prolactin is commonly reported as “the” freshwater adaptive hormone in marine fishes, and differentiation was expected among populations from marine, estuarine, and lagoon environments and individuals transferred from marine to freshwater conditions (reservoir) after their first year. Indeed, no genetic structure was found at microsatellite loci, consistent with the dispersion ability of this species. In contrast, significant genetic differentiation was observed at locus Prl ( θ̂ = 0.022, P = 0.013). “Neutrality tests” reported significant support ( P < 0.01) for putative selection at locus Prl when comparing allele frequency distributions of the translocated freshwater population to other populations; the shortest Prl allele was found more frequent in the reservoir than in other populations. To further investigate this selective process, correlation among individual otolith growth rates of reservoir's fish and Prl genotypes was investigated. While just below the conventional level of significance, individuals homozygous for the shortest Prl allele presented lower then higher growth rates during their first ( P = 0.060) and second ( P = 0.087) years than other genotypes, respectively. Results suggest that local selective advantage for growth, and differential energy costs are associated to Prl's genotypes depending on the fish habitat.