Genomic divergence between two sister Medicago species triggered by the quaternary climatic oscillations on the Qinghai-Tibet plateau and northern China

Speciation is a central topic in evolutionary biology. However, how genomic divergence originates and accumulates in the face of gene flow during ecological adaptation remains poorly understood. Closely related species that have adapted to distinct environments but inhabit some overlapping ranges provide an ideal system to evaluate this issue. Here, we combine population genomics and species distribution models (SDMs) to examine genomic divergences between two sister plant species, Medicago ruthenica and M. archiducis-nicolai, that occur in northern China and the northeast Qinghai-Tibet Plateau, respectively, with overlapping distributions in the border of the two regions. M. ruthenica and M. archiducis-nicolai are well-delimited based on population genomic data, although hybrids exist in sympatric sampling locations. Coalescent simulations and SDMs suggest that the two species diverged from each other in the Quaternary but have been in continuous contact with gene flow occurring between the two species since then. We also discovered positive selection signatures associated with genes both outside and within genomic islands in both species that are probably involved in adaptations to arid and high-altitude environments. Our findings provide insights into how natural selection and climatic changes in the Quaternary initiated and maintained interspecific divergence of these two sister species.