Footprints of local adaptation span hundreds of linked genes in the Atlantic silverside genome.

The study of local adaptation in the presence of ongoing gene flow is the study of natural selection in action, revealing the functional genetic diversity most relevant to contemporary pressures. In addition to individual genes, genome-wide architecture can itself evolve to enable adaptation. Distributed across a steep thermal gradient along the east coast of North America, Atlantic silversides () exhibit an extraordinary degree of local adaptation in a suite of traits, and the capacity for rapid adaptation from standing genetic variation, but we know little about the patterns of genomic variation across the species range that enable this remarkable adaptability.

Contrasting genomic shifts underlie parallel phenotypic evolution in response to fishing.

Humans cause widespread evolutionary change in nature, but we still know little about the genomic basis of rapid adaptation in the Anthropocene. We tracked genomic changes across all protein-coding genes in experimental fish populations that evolved pronounced shifts in growth rates due to size-selective harvest over only four generations. Comparisons of replicate lines show parallel allele frequency shifts that recapitulate responses to size-selection gradients in the wild across hundreds of unlinked variants concentrated in growth-related genes.

Mercury bioaccumulation increases with latitude in a coastal marine fish (Atlantic silverside, Menidia menidia).

Human exposure to the neurotoxic methylmercury (MeHg) occurs primarily via the consumption of marine fish, but the processes underlying large-scale spatial variations in fish MeHg concentrations [MeHg], which influence human exposure, are not sufficiently understood. We used the Atlantic silverside (), an extensively studied model species and important forage fish, to examine latitudinal patterns in total Hg [Hg] and [MeHg]. Both [Hg] and [MeHg] significantly increased with latitude (0.

Pattern and scale of geographic variation in environmental sex determination in the Atlantic silverside, Menidia menidia.

The Atlantic silverside, Menidia menidia (Pisces: Atherinidae), exhibits an exceptionally high level of clinal variation in sex determination across its geographic range. Previous work suggested linear changes in the level of temperature-dependent sex determination (TSD) with increasing latitude. Based on comparisons at 31 sites encompassing the entire species' range, we find that the change in level of TSD with latitude is instead highly nonlinear.

Circulating levels of plasma IGF-I during recovery from size-selective harvesting in Menidia menidia.

Selection for growth-related traits in domesticated fishes often results in predictable changes within the growth hormone-insulin-like growth factor (GH-IGF-1) axis. Little is known about the mechanisms controlling changes in growth capacity resulting from fishery-induced evolution. We took advantage of a long-term study where Menidia menidia were selected for size at age over multiple generations to mimic fisheries-induced selection.

The response of correlated traits following cessation of fishery-induced selection.

The application of evolutionary principles to the management of fisheries has gained considerable attention recently. Harvesting of fish may apply directional or disruptive selection to key life-history traits, and evidence for fishery-induced evolution is growing. The traits that are directly selected upon are often correlated (genetically or phenotypically) with a suite of interrelated physiological, behavioral, and morphological characters.

Spatial scale and divergent patterns of variation in adapted traits in the ocean.

The geography of adaptive genetic variation is crucial to species conservation yet poorly understood in marine systems. We analyse the spatial scale of genetic variation in traits that broadly display adaptation throughout the range of a highly dispersive marine species. We conducted common garden experiments on the Atlantic silverside, Menidia menidia, from 39 locations along its 3000 km range thereby mapping genetic variation for growth rate, vertebral number and sex determination.

High connectivity among locally adapted populations of a marine fish (Menidia menidia).

Patterns of connectivity are important in understanding the geographic scale of local adaptation in marine populations. While natural selection can lead to local adaptation, high connectivity can diminish the potential for such adaptation to occur. Connectivity, defined as the exchange of individuals among subpopulations, is presumed to be significant in most marine species due to life histories that include widely dispersive stages.

Adaptation to climate change: contrasting patterns of thermal-reaction-norm evolution in Pacific versus Atlantic silversides.

How organisms may adapt to rising global temperatures is uncertain, but concepts can emerge from studying adaptive physiological trait variations across existing spatial climate gradients. Many ectotherms, particularly fish, have evolved increasing genetic growth capacities with latitude (i.e.

Regional differentiation and post-glacial expansion of the Atlantic silverside, an annual fish with high dispersal potential.

The coastal marine environment of the Northwest Atlantic contains strong environmental gradients that create distinct marine biogeographic provinces by limiting dispersal, recruitment, and survival. This region has also been subjected to numerous Pleistocene glacial cycles, resulting in repeated extirpations and recolonizations in northern populations of marine organisms. In this study, we examined patterns of genetic structure and historical demography in the Atlantic silverside, an annual marine fish with high dispersal potential but with well-documented patterns of clinal phenotypic adaptation along the environmental gradients of the Northwest Atlantic.

Ontogenesis of gonadal aromatase gene expression in atlantic silverside (Menidia menidia) populations with genetic and temperature-dependent sex determination.

Cytochrome P450 aromatase (P450arom), an enzyme that converts testosterone to 17beta-estradiol, is an important mediator of sex determination in teleosts with genetic sex determination (GSD) and temperature-dependent sex determination (TSD). We compared the ontogenetic expression of P450arom in two populations of Atlantic silversides, Menidia menidia, which exhibit TSD (South Carolina) or GSD (Nova Scotia, Canada) using quantitative, real-time polymerase chain reaction (qRT-PCR). Embryos and newly hatched larvae were reared at an intermediate sex ratio-producing temperature (21 degrees C), and older larvae and juveniles were reared at temperatures that feminize (15 degrees C) and masculinize (28 degrees C) to assess the temperature response of P450arom during development.

The role of experiments in understanding fishery-induced evolution.

Evidence of fishery-induced evolution has been accumulating rapidly from various avenues of investigation. Here we review the knowledge gained from experimental approaches. The strength of experiments is in their ability to disentangle genetic from environmental differences.

The covariance between genetic and environmental influences across ecological gradients: reassessing the evolutionary significance of countergradient and cogradient variation.

Patterns of phenotypic change across environmental gradients (e.g., latitude, altitude) have long captivated the interest of evolutionary ecologists.

Reversal of evolutionary downsizing caused by selective harvest of large fish.

Evolutionary responses to the long-term exploitation of individuals from a population may include reduced growth rate, age at maturation, body size and productivity. Theoretical models suggest that these genetic changes may be slow or impossible to reverse but rigorous empirical evidence is lacking. Here, we provide the first empirical demonstration of a genetically based reversal of fishing-induced evolution.

Coevolution of foraging behavior with intrinsic growth rate: risk-taking in naturally and artificially selected growth genotypes of Menidia menidia.

Although there is accumulating evidence of growth-rate optimization by natural selection, the coevolution of growth rate and risk-taking behavior has not been sufficiently documented. The Atlantic silverside fish, Menidia menidia, displays countergradient variation in growth across a latitudinal gradient: genotypes from Nova Scotia (NS), for example, grow in length twofold faster than those from South Carolina (SC). Past work has established that fast growth is adaptive in northern climates, but the trade-off is poorer swimming performance and higher susceptibility to predators.

Maladaptive changes in multiple traits caused by fishing: impediments to population recovery.

Some overharvested fish populations fail to recover even after considerable reductions in fishing pressure. The reasons are unclear but may involve genetic changes in life history traits that are detrimental to population growth when natural environmental factors prevail. We empirically modelled this process by subjecting populations of a harvested marine fish, the Atlantic silverside, to experimental size-biased fishing regimes over five generations and then measured correlated responses across multiple traits.

Tests of bioaccumulation models for polychlorinated biphenyl compounds: a study of young-of-the-year bluefish in the Hudson River estuary, USA.

A field-based study regarding uptake of polychlorinated biphenyl compounds (PCBs) by young-of-the-year (YOY) bluefish (Pomatomus saltatrix) was initiated to test a steady-state model of bioaccumulation and trophic transfer in a rapidly growing fish. Determination of prey composition as well as size-dependent growth and specific consumption rates for YOY bluefish from separate field and laboratory studies enabled the input of these species-specific parameters into the model. Furthermore, the time and duration of the exposure of YOY bluefish to dissolved PCBs from a well-characterized system (Hudson River, USA) was well known.

Evolution of intrinsic growth rate: metabolic costs drive trade-offs between growth and swimming performance in Menidia menidia.

There is strong evidence that genetic capacity for growth evolves toward an optimum rather than an absolute maximum. This implies that fast growth has a cost and that trade-offs occur between growth and other life-history traits, but the fundamental mechanisms are poorly understood. Previous work on the Atlantic silverside fish Menidia menidia has demonstrated a trade-off between growth and swimming performance.

Nonlinear growth cost in Menidia menidia: theory and empirical evidence.

Juvenile growth is submaximal in many species, suggesting that a trade-off with juvenile growth must exist. In support of this, recent studies have demonstrated that rapid growth early in life results in decreased physiological performance. Theory clearly shows that for submaximal growth in juveniles to be optimal, the cost of growth must be nonlinear.

Rapid growth results in increased susceptibility to predation in Menidia menidia.

Several recent studies have demonstrated that rapid growth early in life leads to decreased physiological performance. Nearly all involved experiments over short time periods (<1 day) to control for potentially confounding effects of size. This approach, however, neglects the benefits an individual accrues by growing.

The covariance of routine and compensatory juvenile growth rates over a seasonality gradient in a coastal fish.

Compensatory growth is widespread in juvenile animals; it refers to the ability to grow at faster-than-routine rates, following intervals of slow growth due to reduced food supply or temperature. Whether the ability to grow rapidly under routine conditions is associated with enhanced or reduced compensatory growth performance is unknown. We examine the among-population covariance in routine and compensatory growth ability in juvenile Atlantic silverside, Menidia menidia.

Sustaining fisheries yields over evolutionary time scales.

Fishery management plans ignore the potential for evolutionary change in harvestable biomass. We subjected populations of an exploited fish (Menidia menidia) to large, small, or random size-selective harvest of adults over four generations. Harvested biomass evolved rapidly in directions counter to the size-dependent force of fishing mortality.

The allometry of energy reserve depletion: test of a mechanism for size-dependent winter mortality.

We experimentally tested the hypothesis that energy reserve depletion varies inversely with size in the fish Menidia menidia, an estuarine fish known to exhibit size-dependent winter mortality. Individuals in two size groups were starved at two winter temperatures (4°and 8°C) and sacrificed at a range of time intervals (up to 127 days). Lipid levels and lean tissue were analyzed to estimate somatic energy storage.