Differential Genetic Adaptation to Climate within and Between Two Picea species (P. mariana and P. glauca)

Identifying the genetic basis of adaptation is an important goal in evolutionary biology. In forest trees, it can also assist in breeding programs and in the conservation of adaptive genetic resources. Here we aimed at discovering genes involved in adaptation to climate in two widespread and largely sympatric spruce species of the boreal forest, Picea mariana and P. glauca. Using SNPs located in  around 5,000 genes, we scanned the genome of P.mariana for polymorphisms associated with climatic factors in two postglacial lineages of the species (in western and eastern Canada). We also tested around 8,000 genes in P. glauca in eastern Canada, involving 2,700 homologues with P. mariana. Within P. mariana, our results showed that adaptive genes were mostly lineage-specific and that the overlap between lineages was not different from what was expected under chance alone. At the inter-specific level in eastern Canada, we observed a similar pattern, with most of the adaptive genes being species-specific. In both cases, the proportion of overlap (between lineages or between species) increased when considering more integrative levels, from genes to gene families to metabolic pathways, but the overlaps were not greater than expected under chance alone. The specificity of adaptive genes to lineages or species is likely due to differences in demographic and evolutionary histories, as well as differences in present ecological niches. Still, some of the identified shared genes, families and pathways may be indicative of important genetic adaptation mechanisms across lineages and species.