Adaptive introgression from Populus balsamifera (balsam poplar) in P. trichocarpa (black cottonwood)

Hybridization is a widespread phenomenon that has shaped the genome of many lineages. In natural hybrid zones, back-crossing of early-generation hybrids over subsequent generations can result in introgression which can introduce adaptive genetic variation. Populus has emerged as a model genus for population genomic studies of adaptation due to porous species barriers and a wealth of genomic resources available. Populus trichocarpa and P. balsamifera are sibling poplar species ecologically divergent and adapted to strongly contrasting environments. In this thesis, I provide evidence for adaptive introgression in P. trichocarpa and P. balsamifera, by implementing local ancestry analyses together with functional, phenotypic and selection tests. Based on a local ancestry analysis across the whole genome, I detected asymmetric patterns of introgression with stronger signals of introgression from P. balsamifera to P. trichocarpa than vice versa. There was no overlap between the introgressed regions in P. trichocarpa compared to those in P. balsamifera or the enriched GO and Pfam terms in the introgressed regions. In admixed P. trichocarpa individuals, candidate regions for adaptive introgression showed strong signals of selection and were enriched for genes that may play crucial roles for survival and adaptation. These analyses also revealed overrepresentation of P. balsamifera introgression in subtelomeric regions and possible protection of the sex-determining regions from interspecific gene flow. An admixture mapping analysis and phenotypic tests provided strong support that introgressive hybridization is a driver of clinal adaptation in P. trichocarpa and suggest that the northern range extension of P. trichocarpa depends, at least in part, on introgression of alleles from P. balsamifera. A number of genomic regions showing unusually high levels of P. balsamifera introgression were significantly associated with putatively adaptive trait combinations, in particular, regions on chromosome 9 and chromosome 15. Overall, these results contribute to our understanding of introgression as a source of genetic variation associated with adaptive characters that may allow improved survival in new environments. To my knowledge, this is the first fine-scale study on natural hybrids of tree species with such a comprehensive view of the effects of admixture in adaptation.Science, Faculty ofBotany, Department ofGraduat