Population genetic studies reveal biodiversity patterns and inform about drivers of evolutionary differentiation and adaptation, including gene flow, drift and selection. This can advance our understanding and aid decision making regarding management and conservation efforts. Microsatellites have long been used in population genetic studies. Thanks to the development of newer techniques, sequencing approaches such as restriction site associated DNA sequencing (RADseq) are on their way to replace microsatellites for some applications. However, the performance of these two marker types in population genetics have rarely been systematically compared. We utilized three neutrally and adaptively differentiated populations of anadromous pike (Esox lucius) to assess the relative performance of microsatellites and RADseq with respect to resolution and conclusiveness of estimates of population differentiation and genetic structure. To this end, the same set of individuals (N = 64) were genotyped with both RADseq and microsatellite markers. To assess effects of sample size, the same subset of 10 randomly chosen individuals from each population (N = 30 in total) were also genotyped with both methods. Comparisons of estimated genetic diversity and structure showed that both markers were able to uncover genetic structuring. The full RADseq dataset provided the clearest detection of the finer scaled genetic structuring, and the other three datasets (full and subset microsatellite, and subset RADseq) provided comparable results. A search for outlier loci performed on the full SNP dataset pointed to signs of selection potentially associated with salinity and temperature, exemplifying the utility of RADseq to inform about the importance of different environmental factors. To evaluate whether performance differences between the markers are general or context specific, the results of previous studies that have investigated population structure using both marker types were synthesized. The synthesis revealed that RADseq performed as well as, or better than microsatellites in detecting genetic structuring in the included studies. The differences in the ability to detect population structure, both in the present and the previous studies, are likely explained by the higher number of loci typically utilized in RADseq compared to microsatellite analysis, as increasing the number of markers will (regardless of the marker type) increase power and allow for clearer detection and higher resolution of genetic structure.

Intraspecific genetic admixture occurs when previously separated populations withina species start interbreeding, and it can have either positive, negative, or neutral effectson reproductive performance. As there currently is no reliable predictor for theoutcome of admixture, an increased knowledge about admixture effects in differentspecies and populations is important to increase the understanding about what determinesthe response to admixture. We tested for effects of admixture on F1 offspringquality in three subpopulations of pike (Esox lucius). Gametes were collected inthe field, and eggs from each female were experimentally fertilized with milt from amale from each population (one “pure” and two “admixed” treatments). Three offspringquality measures (hatching success, fry survival, and fry length) were determinedand compared between (a) pure and admixed population combinations and (b)the sex-specifictreatments within each admixed population combination (based onthe origin of the male and female, respectively). The results suggested that althoughthere were no overall effects of admixture on offspring quality, the consequences fora given population combination could be sex-specificand thus differ depending onwhich of the parents originated from one or the other population. All offspring qualitytraits were influenced by both maternal ID and paternal ID. Sex-andindividual-specificeffects can have implications for dispersal behavior and gene flow betweennatural populations, and are important to consider in conservation efforts.

Environmental heterogeneity is a key determinant of genetic and phenotypic diversity. Stable andhomogenous environments tends to result in evolution of specialism and local adaptations, whiletemporally unpredictable environments may maintain a diversity of specialists, promote generaliststrategies, or favour diversified bet hedging strategies. We compared salinity tolerance between twoanadromous subpopulations of pike (Esox Lucius) that utilize freshwater spawning sites with differentsalinity regimes. Eggs from each population were artificially fertilized and incubated in a salinitygradient (0, 3, 5, 7, and 9 psu) using a split-brood design. Effects on embryonic development, hatchingsuccess, survival of larvae, and fry body length were compared between populations and families.The population naturally spawning in the stable freshwater habitat showed signs of specialization forfreshwater spawning. The population exposed to fluctuating selective pressure in a spawning area withoccasional brackish water intrusions tolerated higher salinities and displayed considerable variation inreaction norms. Genetic differences and plasticity of salinity tolerance may enable populations to copewith changes in salinity regimes associated with future climate change. That geographically adjacentsubpopulations can constitute separate units with different genetic characteristics must be consideredin management and conservation efforts to avoid potentially negative effects of genetic admixture onpopulation fitness and persistence.

Background: In the wake of climate change many environments will be exposed to increased and more variable temperatures. Knowledge about how species and populations respond to altered temperature regimes is therefore important to improve projections of how ecosystems will be affected by global warming, and to aid management. We conducted a common garden, split-brood temperature gradient (4.5 degrees C, 9.7 degrees C and 12.3 degrees C) experiment to study the effects of temperature in two populations (10 families from each population) of anadromous pike (Esox lucius) that normally experience different temperatures during spawning. Four offspring performance measures (hatching success, day degrees until hatching, fry survival, and fry body length) were compared between populations and among families. Results: Temperature affected all performance measures in a population-specific manner. Low temperature had a positive effect on the Harfjarden population and a negative effect on the Lervik population. Further, the effects of temperature differed among families within populations. Conclusions: The population-specific responses to temperature indicate genetic differentiation in developmental plasticity between populations, and may reflect an adaptation to low temperature during early fry development in Harfjarden, where the stream leading up to the wetland dries out relatively early in the spring, forcing individuals to spawn early. The family-specific responses to temperature treatment indicate presence of genetic variation for developmental plasticity (G x E) within both populations. Protecting between- and within-population genetic variation for developmental plasticity and high temperature-related adaptive potential of early life history traits will be key to long-term viability and persistence in the face of continued climate change.