Abstract
In boreal forests, autumn frost tolerance in seedlings is a critical fitness component because it determines survival rates during regeneration. To understand the forces that drive local adaptation in this trait, we conducted freezing tests in a common garden setting for 54 Pinus sylvestris (Scots pine) populations (>5000 seedlings) collected across Scandinavia into western Russia, and genotyped 24 of these populations (>900 seedlings) at >10 000 SNPs. Variation in cold hardiness among populations, as measured by QST, was above 80% and followed a distinct cline along latitude and longitude, demonstrating significant adaptation to climate at origin. In contrast, the genetic differentiation was very weak (mean FST 0.37%). Despite even allele frequency distribution in the vast majority of SNPs among all populations, a few rare alleles appeared at very high or at fixation in marginal populations restricted to northwestern Fennoscandia. Genotype–environment associations showed that climate variables explained 2.9% of the genetic differentiation, while genotype–phenotype associations revealed a high marker-estimated heritability of frost hardiness of 0.56, but identified no major loci. Very extensive gene flow, strong local adaptation, and signals of complex demographic history across markers are interesting topics of forthcoming studies on this species to better clarify signatures of selection and demography.
Highlights
Most boreal forest species have wide distribution ranges within which ecotype-specific variation and adaptive clines potentially arise across heterogeneous environments
For local adaptation to occur in species with large distribution ranges and extensive gene flow, such as Scots pine (Pinus sylvestris L.), selection has to be strong enough to overcome the homogenizing force of gene flow (Savolainen et al, 2007)
Neutral forces have certainly acted on boreal conifer species when they tracked the receding ice sheets after the Last Glacial Maximum (LGM) and recolonized northern regions from their southern ice age refugia
Summary
Most boreal forest species have wide distribution ranges within which ecotype-specific variation and adaptive clines potentially arise across heterogeneous environments. Neutral forces have certainly acted on boreal conifer species when they tracked the receding ice sheets after the Last Glacial Maximum (LGM) and recolonized northern regions from their southern ice age refugia. Such migration routes expose migrating individuals to environmental clines that change with latitude. Selection on standing variation in quantitative polygenic traits may require only minute allele frequency shifts to facilitate local adaptation (Csillery et al, 2018). This means that even if selection on a trait is strong, the identification of adaptive changes in minor allele
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