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Thermal reaction norms
The significant population x temperature interactions revealed genetically-based thermal reactions norms. We had initially hypothesized that the study populations would be adapted to the thermal regime that was closest to their local environments. Thus, we expected the two populations from warmer locations (Geneva and Constance) to outperform the two smaller populations from colder environments (Allos and Pavin) under warm conditions, e.g. by producing larger individuals and showing higher survival rates. Although this was not the case, the four populations showed contrasted responses at both timepoints. Intriguingly, the cold-originating population of Allos exhibited distinct reaction norms with higher survival rates, earlier hatching and bigger individuals than the other populations, clearly corresponding to a better performance than other populations. At T2, in the warm treatment, Allos alevins were bigger with smaller yolk sacs than alevins of other origins. This morphology may indicate that Allos individuals can convert yolk reserves into larval tissue more efficiently at warm than individuals of other origin ; and may reveal a strategy to cope with a warm environment that differs from the three other populations as emerging bigger with smaller yolk allows maximizing movement and predator escape (Skoglund, Einum and Robertsen, 2011).
Neutral and adaptive population divergence
Levels of pairwise population differentiation based on neutral markers were moderate to high and revealed two pairs of weakly differentiated populations, consistently with the information available on introduction events in study populations (. Overall QST estimates revealed divergent selection on all traits at hatching except survival; in all other traits, QST did not significantly differ from FST, hence divergence for those traits can be ascribed to drift. Divergent selection as seen from QST-FST comparisons in salmonids has been well documented by previous investigators (Perry, Audet and Bernatchez, 2005; Jensen et al., 2008; Rogell et al., 2013; McKinney et al., 2014; Côte et al., 2016), but evidence of adaptive divergence among recently founded populations (e.g. less than 100 years for Allos) has been more rarely reported (Koskinen, Haugen and Primmer, 2002; Whitney, Hinch and Patterson, 2013). In contrast, pairwise QST showed a different pattern with evidence of uniform selection in many cases. Uniform selection for survival can be expected for such a trait highly related to fitness, which should be maximized in every habitat. In contrast, such a pattern for morphological traits like yolk volume at hatching and embryo length is more surprising but could be expected between similar environments like the Geneva and Constance lakes. Other studies have documented QST < FST results (Andersen et al., 2007; Chapuis et al., 2007; Lamy et al., 2011), but there might be a general bias toward a discreet reporting of such patterns compared to QST > FST (Lamy et al., 2012). Trait conservatism can be a consequence of uniform selection that always leads to QST lower than FST. However, QST < FST patterns may emerge even under diversifying selection in small populations with reduced gene flow, where the among-population variance due to drift can become larger than among-population variance due to differences in local optima (Lamy et al., 2012; Moore et al., 2017). In particular, the divergence among recently founded populations (Allos and Pavin) may be low due to a stronger role of drift relative to selection in such small populations. Accordingly, the fact that many overall and pairwise comparisons showed a pattern of QST not differing from FST further supports this hypothesis. Alternatively, canalization could be another explanation to QST < FST, as traits could share similar genetic constraints rather than identical selective optima (Flatt, 2005; Lamy et al., 2012).
Finally, it is important to consider that the different patterns we observed between overall and pairwise QST-FST comparisons, may be explained by statistical reasons. First, it has been acknowledged that it is more difficult to detect significant differences between QST and FST in pairwise comparisons due to a loss of statistical power (Gilbert and Whitlock, 2015). Second, the significance of QST can be estimated with several approaches that can reveal contrasted results (e.g. (Leinonen et al., 2013)), hence it may be interesting to compare QST estimates and their confidence intervals using several methods, e.g. bootstraping versus Bayesian approaches.
Within population diversity
Local adaptation also depends on the adaptive potential of populations, which relates to the amount of genetic variance, itself linked to trait heritability. We observed very low heritability values for survival at T1, and almost null heritability for overall survival. Such fitness related traits are expected to have a lower heritability than morphological traits because they are supposedly under stronger selective pressures that tend to reduce additive genetic variance (Merila and Sheldon, 1999). In addition, we observed lower heritability estimates in the warm treatment for most traits, which is consistent with conclusions from Charmantier and Garant (2005) who reported a trend for lower heritability values for traits measured under stressful conditions in wild populations.
Table of contents :
1 1. Introduction and context
Biodiversity in a changing environment
Environmental changes in freshwater systems
Multiple stressor interactions
Evolutionary concepts: responses of populations to environmental variations
Relationship between phenotype and environment
Local adaptation
Study model and research questions
Alpine populations of a cold-water specialist, the arctic charr Salvelinus alpinus in a warming context
Local threats: fine sediment impacts on salmonids and potential for interactions with temperature
Research aims
2 2. Temperature as a selective pressure – investigating local adaptation patterns in native and introduced populations
Foreword
Article information
Abstract
Introduction
Material and Methods
Experimental design
Life history traits measurements
Data analysis
Results
Thermal reaction norms
Neutral genetic diversity
Quantitative trait variation
Discussion
Thermal reaction norms
Neutral and adaptive population divergence
Within population diversity
Impacts of stocking
Conclusion
Supplementary files
3 3. Temperature as a constraint on tolerance to other stressors – the example of fine sediments
Foreword
Article information
Abstract
Introduction
Material and methods
Study species
Experimental design
Sediments
Temperature
Life-history traits
Statistical analyses
Results
Discussion
Supplementary file
4 4. Response of wild populations in a multiple stressors context
Foreword
Article information
Abstract
Introduction
Material & Methods
Study system and populations
Rearing experiment
Results
Discussion
Separate effects of temperature and sediment
Interactive effects of temperature and sediment
Conservation implications for arctic charr populations at the Southern edge
Conclusions
Supplementary files
5 5. General discussion
Adaptation and plasticity in response to temperature and considerations on thermal tolerance
Response to sediment exposure and assessment of the sediment risk
Life history trade-offs in early life: growth, development, and maintenance
Implications of this work for Southernmost arctic charr populations
Multiple stressors affecting arctic charr and management of multiple stressors
Genetic structure of populations and influence of stocking practices
Concluding remarks
6 6. References
7 Appendix
