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.