Environment-dependent introgression from Quercus dentata to a coastal ecotype of Quercus mongolica var. crispula in northern Japan

• Introgression from one species in a specific environment to another may facilitate colonization of the environment by the recipient species. However, such environment-dependent introgression has been clarified in limited plant taxa. • In northern Japan, there are two interfertile oak species: Quercus dentata (Qd) in coastal areas and Q. mongolica var. crispula (Qc) in inland areas. However, at higher latitudes where Qd is rare, a coastal Qc ecotype with Qd-like traits is distributed in the coastal areas. We distinguished inland Qc, coastal Qc, and coastal Qd populations based on genome-wide genotypes and multitrait phenotypes and verified introgression from coastal Qd to coastal Qc using reduced library sequencing. • Genotypes and phenotypes differed among the populations, and coastal Qc was intermediate between inland Qc and coastal Qd. The ABBA–BABA test showed introgression from coastal Qd to coastal Qc. In coastal Qc, we found various stages of introgression after the first generation of backcross but detected no genomic regions where introgression was enhanced. • Overall, we show evidence for introgression from a coastal species to an ecotype of an inland species, which has colonized the coastal environment. It remains unclear whether introgressed alleles are selected in the coastal environment.