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Keywords:

  • adaptation;
  • ecological gradient;
  • genomic cline;
  • geographic cline;
  • hybrid zone;
  • selection;
  • single nucleotide polymorphisms (SNPs);
  • spruce

Summary

  • Differential patterns of introgression between species across ecological gradients provide a fine-scale depiction of extrinsic and intrinsic factors that contribute to the maintenance of species barriers and adaptation across heterogeneous environments.
  • Introgression was examined for 721 individuals collected from the ecological transition zone spanning maritime to continental climates within the Picea sitchensis–Picea glauca contact zone using a panel of 268 candidate gene single nucleotide polymorphisms.
  • Geographic clines showed a strong spatial relationship between allele frequencies and both distance from the ocean along major rivers and mean annual precipitation, indicating a strong role for environmental selection. Interspecific patterns of differentiation using outlier tests revealed three candidate genes that may be targets of long-term divergent selection between the parental species, although contemporary genomic clines within the hybrid zone suggested neutral patterns of introgression for these genes.
  • This study provides a fine-scale analysis of locus-specific introgression, identifying a suite of candidate loci that may be targets of extrinsic or intrinsic selection, with broad application in understanding local adaptation to climate.