Scale and direction of adaptive introgression between black cottonwood (Populus trichocarpa) and balsam poplar (P. balsamifera)

Introgression can introduce novel genetic variation at a faster rate than mutation alone, and result in adaptive introgression when adaptive alleles are maintained in the recipient genome over time by natural selection. A previous study from our group demonstrated adaptive introgression from Populus balsamifera into P. trichocarpa in a target genomic region. Here we expanded our local ancestry analysis to the whole genome of both parents to provide a comprehensive, unbiased view of introgression patterns and to identify additional candidate regions for adaptive introgression genome-wide. Populus trichocarpa is a large, fast-growing tree of mild coastal regions of the Pacific northwest, whereas P. balsamifera is a smaller stature tree of continental and boreal regions with intense winter-cold. The species are parapatric with extensive hybridization. Here, using local ancestry analysis, we detected asymmetric patterns of introgression across the whole genome of these two species of poplar...