Common-garden study of introgression at loci associated with traits adaptive to coastal environment from Quercus dentata into Quercus mongolica var. crispula

Adaptive introgression, which can provide genetic diversity resulting in the expansion to novel habitats and the creation of different ecotypes, has been proposed in oaks (genus Quercus). In northern Japan, Quercus mongolica var. crispula (Qc) is common in inland habitats, and Quercus dentata (Qd) occurs in coastal habitats. At the northern distributional limit of Qd, Quercus x angustilepidota (Qa), a hybrid taxon between Qc and Qd, occurs in coastal habitats. These taxa, Qc, Qa, and Qd, were transplanted to inland and coastal common gardens (sites). Genotypes at 27 495 loci, phenotypes of eight leaf and shoot traits, and the size of 30-year-old trees were measured for 224 individuals in both sites. Genotypic variation revealed a hybrid zone between Qc and Qd, including both northern-edge Qd admixed with Qc and coastal Qa backcrossed to Qc. Phenotypes of Qa trees were intermediate between those of Qc and Qd trees. The size of Qa and Qd trees was smaller than that of Qc trees in the inland site but was larger in the coastal site, suggesting adaptation of Qa and Qd to coastal environment. Local ancestry was estimated from phased genotypes of admixed trees using reference genotypes of 47 Qc and 25 Qd trees, indicating heterogeneous ancestry along chromosomes. Admixture mapping of local ancestry detected loci significantly associated with a leaf trait, which were located in an introgressed genomic region with genes related to stress response. Further studies are necessary to elucidate the genetic basis of adaptive introgression resulting in Qd-like phenotypes of Qa.