Differential Introgression Between the Northern and Southern Extents of the Lodgepole x Jack Pine Hybrid Zone Suggests Environmentally-driven Selection and Local Adaptation.

Lodgepole and jack pine form a mosaic hybrid zone in western Canada. Introgression occurs between lodgepole and jack pine through this hybrid zone by repeated backcrossing with advanced generation hybrid progeny. Using environmentally-associated SNPs identified by redundancy analyses, we examined patterns of introgression between the northern and southern extents of this hybrid zone to identify differential introgression. Through genomic cline analyses, we found extensive introgression of these SNPs through the hybrid zone. Twenty-eight SNPs had significantly different patterns of introgression between the northern and southern extents. Fine-scale patterns revealed several SNPs that were introgressing more frequently than expected, suggesting adaptive introgression. We found that adaptive introgression is occurring more frequently in the northern hybrid extent compared to the southern hybrid extent, suggesting different environmental pressures. Using gene annotations and major allele frequency maps, we identified evidence of differing environmental pressures resulting in putative local adaptation within this hybrid zone.