Conclusions

We utilized RADseq to study neutral and adaptive genetic variation and structuring in allopatric and sympatric populations of pike belonging to freshwater, anadromous and brackish water resident spawning ecotypes. To our knowledge, this is the first population genetic study of pike that has included populations of all three ecotypes. We utilized a genetic marker that can inform on both neutral and functional diversity and structure, and provide insights about which processes and environmental factors that influence neutral and adaptive diversity and differentiation.
The populations in lakes formed separate distinct genetic clusters, indicating an important effect of allopatry. For the sympatric Baltic Sea populations, we found that most anadromous populations were assigned to a shared genetic cluster, and that the two populations from Denmark, despite belonging to different ecotypes, were assigned to a separate cluster; pointing to important roles of geographical separation, IBE, IBA and spatial sorting (Berggren et al., 2012; Shine et al., 2011). The differentiation among anadromous populations may also in part reflect an effect of IBT (Sunde et al., 2019). Unlike previous studies (Laikre et al., 2005; Wennerström et al., 2016), the population genetic structure detected in the present study could not be adequately explained by IBD. We also found markedly higher levels of differentiation than previous large-scaled studies in the Baltic Sea, which indicates that the populations are more isolated than previously believed, and suggests that each population should be considered as a separate unit.
Besides the importance of within population diversity for variuous aspects of ecological success (Des Roches et al., 2018; Forsman, 2014; Forsman & Wennersten, 2016; Hughes et al., 2008; Waldman et al., 2016), there is evidence to suggest that portfolio effects associated with variation among populations may increase the stability, productivity, and resilience of species in variable environments (Hui, Fox, & Gurevitch, 2017; Schindler et al., 2010; Waldman et al., 2016). Given ongoing climate change and other anthropogenic environmental makeovers, it is therefore imperative that the genetic structure of sympatric anadromous populations of pike is maintained and that management actions are not implemented that contribute to increased homogenization of populations.
The analysis of adaptive variation revealed different patterns of structuring than did the neutral variation, and indicated adaptive similarity between some reproductively isolated populations - a sign of convergent evolution. Adaptive structuring was associated with latitude, indicating that salinity and temperature drive adaptive differentiation. The importance of salinity and temperature was also evidenced by the outlier analyses, which identified candidate genes that have previously been shown to be associated with these two environmental factors.
Utilization of neutral and fuctional markers together has potential to provide a comprehensive understanding of the eco-evolutionary processes that jointly influence genetic diversity and shape genetic structure of natural populations. Advances in next-generation sequencing has enabled the use of molecular methods that yield thousands of loci including functional regions. Adaptive variation has a pivotal role for the evolvability of populations, their ability to cope with environmental changes, and how they respond to admixture, management and conservation efforts. The finding that neutral genetic variation was not reflective of adaptive variation thus points to the need for management and conservation efforts to be based not only on netural, but also functional, genetic diversity, to achieve desirable outcomes and ensure successful protection of biodiversity.