Patterns of Population Structure and Hybridization within and between Populus trichocarpa and Populus balsamifera

Abstract

The genus Populus consists of many ecologically and economically important forest tree species. Their rapid growth makes them one of the most productive hardwoods growing in temperate latitudes. Populus spp. frequently hybridize where their ranges overlap, and poplar hybrids are the most frequently planted genotypes for fiber production. To better understand the genomics of hybridization in Populus, we sampled and sequenced the genome of 574 poplar trees from six east-west transects across the hybrid zone between Populus trichocarpa and Populus balsamifera in western North America. I used these data to characterize population structure within and between transects, and hybridization between the species. There was a consistent transition from greater P. balsamifera ancestry in the north and east to greater P. trichocarpa ancestry in the south and west. Hybridization between the species was common across each of the six transects, though more common in colder climates. The results also showed that both latitude and longitude affect the genetic structure of this species complex, and that subtle introgression from P. balsamifera may facilitate adaptation of P. trichocarpa to colder climates.