Evidence That Allelic Introgression from Populus balsamifera (balsam poplar) Underlies Local Adaptation in P. trichocarpa (black cottonwood)

Introgression between closely related species can provide allelic variation underlying adaptation; however only few studies have addressed adaptive introgression in plant species. Populus trichocarpa and P. balsamifera are attractive sister species to study the effects of natural hybridization on local adaptation. We selected 25 pure P. trichocarpa, 25 pure P. balsamifera, and 68 admixed individuals from BC (Canada), and used genome resequencing data from chromosomes 6, 12 and 15 to study patterns of trichocarpa- balsamifera introgression. Previous studies revealed candidate genes for local adaptation on chromosomes 6 and 15, while chromosome 12 contains a paralog block from a candidate region in chromosome 15 but did not show signatures of selection. Our ancestry analysis revealed that a candidate region for local adaptation at the first 1 Mb of chromosome 15 introgressed from balsamifera into trichocarpa, and that these balsamifera alleles were restricted to interior and northern populations of trichocarpa. We did not find signals of introgression in the paralog block of this region in chromosome 12. In chromosome 6, no signal of balsamifera introgression was detected in the previously identified candidate region for adaptation. Finally, we found relatively uniform introgression levels from trichocarpa into balsamifera. Our results show that (i) introgression of a ~1Mb genomic region in chromosome 15 from balsamifera could be adaptive in trichocarpa, (ii) in chromosome 6, processes other than introgression underlie variation in the region associated with local adaptation, and (iii) syntenic paralog blocks such as those in chromosome 15 and 12 can undergo different evolutionary paths.