The genomic basis of local adaptation in the white x Engelmann spruce hybrid zone

Hybridization between species can occur along repeated zones of contact, providing a powerful natural laboratory for studying the interplay between migration and selection, and for identifying loci involved in adaptation and reproductive isolation. However, interpreting how evolutionary processes shape genomic patterns can be challenging: repeatability of genotype-environment association alone is not strong evidence for selection, as such patterns could also occur at loci that originally differentiated between the parental species due to drift. On the other hand, repeated directional patterns of introgression, where an allele typical of one species moves far into the range of the other, are unlikely to occur due to drift and therefore provide stronger evidence of natural selection. Here we compared hybridization and local adaptation patterns between two replicated regions within the western Canada interior spruce hybrid zone: broad latitudinal transects with gradual environmental variation and narrow elevational transects with steep environmental contrasts. Despite differences in geography and selective pressures, the two regions revealed strikingly similar genome-wide patterns of differentiation and adaptation. Genomic clines analyses confirmed this trend, uncovering consistent patterns of directional introgression between transect types. The strong similarity in differentiation, environmental adaptation and directional introgression between replicated hybrid zones is unlikely under an entirely drift-driven process, suggesting that selection plays an important role in shaping hybridization patterns between Picea glauca and P. engelmannii. Consistent with hybridization theory, we found longer introgressed genomic tracts in the elevational transects, likely because steep environmental gradients over short distances restrict recombination preserving larger ancestry blocks in the genome.