Climate and geography shape hybrid zone dynamics across space and time: insights fromPopulus trichocarpaxP. balsamifera

Summary Quantifying the mechanisms underlying hybrid zone formation requires an evaluation of both neutral and nonneutral evolutionary processes. Population demographic changes, alongside landscape heterogeneity and climate adaptation, can influence the timing, stability, and extent of introgression where species hybridize. Thus, quantifying interactions across diverged lineages, and the relative contributions of interspecific genetic exchange and selection to divergence at the genome-wide level is needed. We quantified the contributions of climate and geography to the genetic structure across replicate contact zones between Populus trichocarpa and P. balsamifera, evaluating the demographic context of hybridization and the role of genetic connectivity over time. We found genomic structure differed among the seven latitudinally arrayed transects. While ancestry was strongly structured by climate, geography appears to have restricted gene flow in certain regions of the hybrid zone. The best fitting demographic model indicated high rates of gene flow alongside selection have likely influenced the establishment and maintenance of the hybrid zone. Our results point towards the importance of climate in structuring the contact zones between P. trichocarpa and P. balsamifera and emphasize the value of sampling replicated zones of contact to understand how neutral and non-neutral processes influence hybrid zone formation across space and time.