Genetic boundary and gene flow between 2 parapatric subspecies of brown rats

Two parapatric Rattus norvegicus subspecies, R. n. humiliatus (RNH) and R. n. caraco (RNC), are classified according to morphological divergence and are mainly distributed in North and Northeast China. Here, we aimed to explore the population genetic structure, genetic boundary, and gene flow in these rats using 16 microsatellite loci. Structure analysis and principal component analysis revealed 3 ancestral clusters. We found that the intermediate cluster exhibited higher genetic diversity and a lower inbreeding coefficient than the other 2 clusters. The genetic differentiation between the 3 clusters was significant but weak, with a higher F_ST value being observed between the clusters on both sides. The subspecies boundary inferred from microsatellite markers may indicate the existence of an admixture or hybridization area covering Liaoning, Inner Mongolia, and Jilin Provinces, rather than corresponding to the administrative provincial boundaries between Liaoning and Jilin. The RNH and RNC subspecies presented moderate gene exchange and an asymmetric bidirectional gene flow pattern, with higher gene flow from the RNH subspecies to the RNC subspecies, constraining speciation. Such genetic characteristics might be explained by biological processes such as dispersal ability, mate choice, and dynamic lineage boundaries.