Climate adaptation shaped by subtle to moderate allele frequency shifts in loblolly pine

Understanding the genomic basis of local adaptation is crucial to determine the potential of long-lived woody species to withstand changes in their natural environment. In the past, efforts to dissect the genomic architecture in gymnosperms species have been limited due to the absence of reference genomes. Recently, the genomes of some commercially important conifers, such as loblolly pine, have become available, allowing whole-genome studies of these species. In this study, we test for associations between 87k SNPs, obtained from whole-genome re-sequencing of loblolly pine individuals, and 270 environmental variables and combinations of them. We determine the geographic location of significant alleles and identify their genomic location using our newly constructed ultra-dense 26k SNP linkage map. We found that water availability is the main climatic variable shaping local adaptation of the species, and found 492 SNPs showing significant associations with climatic variables or combinations of them. Our results suggest that adaptation to climate in the species might have occurred by many changes in the allele frequency of alleles with moderate to small effect sizes, and by the smaller contribution of large effect alleles in genes related to moisture deficit, temperature and precipitation. Genomic regions of low recombination and high population differentiation harbored SNPs associated with principal components but not with individual climatic variables, suggesting climate adaptation might have evolved as a result of different selection pressures acting on groups of genes associated with an aspect of climate rather than on individual climatic variables.